Download Electricity and Why it Moves

Name: _____________________
Unit 9:1
Electricity and Why it Moves
Period: _____________________
Electricity
Electricity is moving electrons;
Protons can’t move.
Electrons can move,
but protons are
held together in
the nucleus by the
strong nuclear force,
the strongest force
in nature.
–
–
+
+
–
–
Objects can be positive, negative, or neutral.
The unit of charge is the coulomb (C).
Charge
Negative
Electrons
+
–
+
–
–
+– +
Positive
Protons
–
A neutral object
has an equal
amount of protons
and electrons.
Electricity comes from electrons moving between atoms.
+
–
–
–+ –+
– – –
+ +
– –
+
+
+
A positive object has
lost electrons, so it
has more protons
than electrons.
Any two charges feel a force between them. Electric force depends on the types of charges,
the distance between the charges, and the amounts of the two charges.
Electric Force
Attracting Force
–
+
+
Positive hydrogen
atoms in water are
attracted to the negatively charged balloon, causing the water
stream to bend slightly
toward the balloon.
Separating Charges
Less force
Repelling Force
Opposites attract
Just like gravity, electric
force increases as distance decreases. Closer
charges: more force.
+
Likes repel
+
+
More force
+
+
++
++
Naturally, objects are neutral. Work must be done to separate charges. Separating charges
cause a charge difference and the electric force tries to move the charges back to neutral.
If there is a difference of charge electricity can move between
objects. A big enough difference can cause electricity to arc
(jump a gap). Bigger differences of charge allow bigger arcs.
No difference in charge—electricity can not move.
arcing
rubber rod
–
–
–
neutral
–
–
– – – –
–
– – – –
Afterwards the rod is charged
and it can exert an electric force.
To ground something
you can often touch
it to a pipe. Metal
pipes are good electrical
conductors and usually
connected to ground
somewhere in the
building.
+
Electric force increases
if either of the charges
increases. More charge:
more force.
More force
Two charged balloons repel
each other, so they must be
the same charge (negative).
fur or hair
Ground
+
Less force
When two insulators rub, charges
move between them, causing a
separation of charge (static
electricity) and an electric force.
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A negative object
has gained electrons,
so it has more electrons
than protons.
–
A Van de Graaff
generator produces a charge
difference.
A big charge difference
can push thru big gaps,
like lightening arcing all
the way to the ground.
Batteries
–
+
Batteries cause
electrons to move by
separating charges
in chemicals. When
connected by wires
the charge can flow,
eventually neutralizing the battery.
.
Ground (the earth) can take or give an infinite number of electrons. Ground is electrically
neutral. Both positive and negative charges will neutralize when grounded.
–
+
+
–
+
+
e’s
– – – –
–
A positive object
will take electrons
from ground.
Symbol for
ground
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–
–+ –+
– – –
+ +
– –
e’s
– – – –
–
A negative object
will give electrons
to ground.
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:1
Period: _____________________
1. Positive
2. Negative
3. Neutral
A. A push or pull caused by charges.
1. Ground
A. The symbol for ground.
B. The units for charge.
2. Arcing
B. Moving electrons.
3. Charge
Difference
C. When a spark jumps between two objects.
4. Van de Graff
D. Can accept or give an infinite amount of
electrons. Will neutralize charge.
C. When an object has more protons than
electrons.
4. Coulombs
D. When an object has an equal number
of electrons and protons.
5. Electric force
E. What keeps protons bound in the
nucleus of an atom.
5. Electricity
E. A machine that separates charge.
6. Strong Nuclear
Force
F.
6.
F.
When an object has more electrons
than protons.
Attract or Repel?
___ Two positive charges.
What Charge: Positive (+), Negative (–), or Neutral (0)?
+
+
___ A positive and negative
charge.
____
–
–+ –+
– – –
+ +
– –
___
–
+
+
–
+
+
+
–
___ Two balloons on a string
pushing apart.
Causes an electric force and charges to
move.
___ 2 protons and 4 electrons
–
___ A 3 C charge and a
–4C charge.
–
–
____
+
–
+
–
+– +
___ 18 protons and 16 electrons
___ A piece of rubber after rubbing
it with fur.
What are the charges of the second objects?
–
+
Does the Electric Force increase or decrease?
_____ If the distance between the charges increases?
A balloon is rubbed against hair. Afterwards it sticks to a wall.
A) Is the balloon attracted or repelled by the wall?
B) Are the balloon and wall oppositely charged or like charged?
You walk across a carpet. When you try to touch a door knob a
spark jumps between you and the door knob. Why?
_____ If one of the charges is bigger (increases)?
_____ If both of the charges decrease (gets smaller)?
_____ If the charges get closer?
Two objects are charged, but do not arc. Give two ways to make
them arc.
A negatively charged
rubber rod is brought close
to the metal top of an
electroscope.
A) Will the electrons in the
metal stay near the rod or
move away from the rod?
Rubber rod
– –
– – –– –
– –
Metal
B) Why?
C) On the diagram, draw where
the electrons will go.
leaves
D) What will the metal leaves at
the bottom do?
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Electroscope
An object has a charge of 4.5 C.
A) Is the object positive or negative?
B) Did it gain or lose electrons?
C) If you touch it to ground, will it lose electrons to ground
or gain electrons from ground?
D) What will its charge be after it is grounded?
Using the object at the right answer
the following questions.
A) Did it gain or lose electrons?
B) When grounded, will it gain or lose
electrons from ground?
C) Draw a wire grounding it.
D) What will its charge be after grounding?
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– 8C
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:
Period: _____________________
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Legal copying of this worksheet requires written permission.
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:2
Circuits and Symbols
Period: _____________________
Electricity works a lot like
water. Often imagining how
water would work in an
circuit will tell you how
electricity will work as well.
Electricity flows through circuits: paths of conductors (usually wires). Any break in
the circuit will cause the circuit to stop, just like a break in a pipe lets water leak out.
A break in a circuit is anywhere an insulator is in the
way of electricity’s flow.
Paper, plastic, or even an
air gap can keep electrons
from flowing.
+
+
A closed circuit has no break:
electricity can flow.
An open circuit has a break somewhere: electricity cannot flow.
Circuit diagrams
Circuit diagrams are a short-cut method of
drawing circuits. They don’t need to be
perfect, but they can be drawn wrong.
battery
Electrical Symbols
Electrical
Device
Function
Water
Equivalents
path for electricity to flow
pipes
pushes electricity
through circuit
pump
light bulb
lights up; resists
electricity
water wheel
switch
turns electricity
on and off
valve
resistor
resists flow of
electricity.
restriction
in a pipe
Symbol
capacitor
wire
These components look similar, but are
very different and have different functions.
+
battery
light bulb
resistor
battery
–
switch
The diagram on the right is a faster way of
drawing the circuit on the left. (Notice the
direction of the battery, which is important.)
A short-circuit (also called a “short”) is a wire that by-passes a device in a circuit.
Short-circuit of battery
This light
Electricity always chooses
turns off: it
is “shorted”
the path of least resistance.
Both lights
Shortout of the
Since wires have virtually no
stay off.
circuit.
circuit
resistance, electricity will go
thru a wire instead of a
device. This is known
When a device is short-circuited the
Short-circuiting a battery drains the battery
as a short-circuit.
current by-passes it. It is easier for the
and can be dangerous. Wires could overheat,
current to go thru the wire than the resistor.
melting the insulation, and even cause a fire.
Short Circuits
Conventional Current
When studying electricity, early scientists guessed that protons
(+ charges) were flowing. We know now that it is the electrons
that move, but it is most common to use conventional current,
which follows the movement of positive charges.
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What’s really happening.
– –
–
–
6V
Conventional current.
+ +
+
+
6V
–
–
–
–
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+
+
+ +
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:2
Period: _____________________
2. Closed circuit B. A short-hand way of drawing electrical
circuits.
3. Circuit
C. A circuit with a break in it; no electricity
diagram
will flow.
4. Voltage
D. Pushes electricity through a circuit.
5. Current
E. Electricity can flow through this.
6. Resistance
F.
The flow of electricity through a circuit.
+
Label the diagram:
C.
+
D.
–
A. Resistor
a.
2.
Pipes
B. Battery
3.
Water wheel
C. Switch
4.
Resists flow
D. Wire
5.
Pump
E. Light bulb
+
b.
–
c.
d.
e.
A. Used to create radiant energy.
2. Battery
B. Pushes electricity through the circuit.
3. Resistor
C. Can turn the electricity on and off.
–
+
–
B.
Valve
+
–
1.
1. Wires
4. Light bulb
D. Allows electricity to flow.
5. Switch
E. Slows down the flow of electricity.
Which of the following are correct?
A.
Match the electrical component with the
water component and diagram symbol
A. Slows down the flow of electricity.
1. Open circuit
A. ___________
Do all of the light bulbs
light up?
1
2
C. ____________
B. ____________
Why or why not?
D. ____________
3
E. ____________
In the Lab
1) Build the following circuit, being sure to connect all
components (parts) with wires. Make sure it works.
Using only a battery, two wires, and a light bulb (no holder),
make the light bulb light. Which of the following diagrams
will light up the light bulb?
+
2) Reverse the direction of the battery. Does the light still
work?
3) Remove the switch. Touch the two wires between the
light bulb and battery together to be sure the light still
turns on.
4) Use these two wires as probes to test which of the
following are conductors (C) or insulators (I).
Paper _________
Plastic ________
Water_________
A Penny_______
Cloth _________
Salt Water _____
Paperclip ______
Wood ________
Sugar Water ___
Glass _________
A Dime _______
Rubber________
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A.
B.
–
–
+
C.
D.
+
+
–
–
What two parts of a light bulb must be touched to make the
light bulb light up?
When the light bulb lights up, is this a closed or open circuit?
What does this mean about the inside of the light bulb?
When the light bulb lights up, what types of energy are used
and created?
Legal copying of this worksheet requires written permission.
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:
Period: _____________________
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Legal copying of this worksheet requires written permission.
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:3
Current, Voltage, and Resistance
Period: _____________________
Current flows through closed circuits. Current is the amount charges that flow each second . In a wire
current never changes. Current can only change if there is a junction: a split or a join.
Current
– – –
– – –
– – –
Devices that use more energy,
use more current.
More current means
more electrons flowing,
which is more electricity
flowing (like more
water flowing).
A lot of
current.
A light bulb is brighter
when it has more
current going thru it.
More current
Electrical current is
measured in Amps,
which is coulombs/sec
(just like gallons per sec).
– – –
Very little
current.
Voltage pushes electricity.
More voltage = more current.
Voltage
Less current
A device that uses
more energy uses
more current. A
louder radio pulls
(uses) more amps.
Resistance slows down electricity.
More resistance = less current.
Resistance
Water falls due to gravitational potential energy (Ep). Likewise, current
moves because of electrical potential
energy given to electrons by voltage.
There must be a change (difference)
of voltage for current to move.
Dams hold back water. Resistors hold
back electrons. Both reduce current.
– –
– – –
– –
High potential energy
Adding Energy (Work)
High voltage
Using Energy
Voltage pumps electricity,
doing work to give potential
electric energy (Ep) to the
electrons. Batteries give
voltage. The resistors use
all of the electrical potential
energy. This is why the
voltage always equals
0 volts just before it reenters
the battery.
Current
flows
1.5V
No voltage (0 V)
Zero potential energy
Batteries can add together,
3 V (increasing voltage) or subtract
(canceling each other out) if they
are put in the circuit wrong.
1.5V
Light is
off
1.5 V
No
current
1.5V
0V
6V
+
Ohm’s Law
Current
(in amps [A])
V
I=
R
6V
+
6V
+
Resistors use up voltage.
With the same voltage
(2 batteries), more resistance
(2 bulbs) = less current
(dimmer bulbs).
6V
6V
Ohm’s Law can tell us the current, voltage, or resistance if the other two of them are known.
Voltage
(in volts [V])
Current is dependent on
voltage and resistance.
Resistance
(in ohms [Ω])
Current can never change
voltage or current, but both
voltage and resistance can
change current.
Current equals the voltage
divided by the resistance.
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With the same resistance
(1 bulb), more voltage
(2 batteries) causes
more current (brighter).
6V
Just as a waterwheel slows
down the falling water, resistors
use the electrical potential
energy to do work (something
useful). Anything that uses
electricity has resistance: light
bulbs, speakers, etc. Regardless
of the amount of resistance,
all the voltage is always used
up in any circuit.
Increasing voltage increases current.
Increasing resistance decreases current.
Decreasing voltage decreases current.
Decreasing resistance increases current.
6V
0 volts total
– –
– –
Ex. How much current does a 12 V
battery push through a 3 Ω resistor?
V = 12 v
R=3Ω
I=?
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I=
V
R
=
12 v
= 4A
3Ω
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:3
Period: _____________________
1. Voltage
A. Units for voltage.
2. Current
B. Slows electricity down in a circuit.
3. Resistance
C. Units for current.
4. Amps
D. Pushes electrons thru a circuit.
5. Ohms (Ω)
E. Units for Resistance.
F.
6. Volts
Voltage (V), Current (I), or Resistance (R)?
Amount of electricity flowing in a
circuit.
Which has more current flowing thru it?
A loud radio or a quiet radio?
A dim light bulb or a bright light bulb?
A fast toy car or a slow toy car?
A cold wire or a hot wire?
A) ____ Flowing electrons.
B) ____ Pushes electricity in
circuits.
C) ____ Like a water pump.
D) ____ Measured in Ohms.
E) ____ Measured in Amps.
F) ____ A battery gives this.
G) ____ Measured in volts.
Which will have more resistance: an insulator or a conductor?
Which resistor is the better conductor: 150Ω or 600Ω?
Which resistor is the better insulator: 24Ω or 6Ω?
Voltage (V), Current (I), or Resistance (R)?
A _____ If you increase resistance what decreases?
B. ____ If you increases voltage what increases?
C. ____ If the current decreased what increased?
D. _____ If current increased what increased?
E. _____ If current increased what decreased?
F. _____ If resistance is decreased, what increases?
G. _____ More batteries will increase these two quantities.
How much current does a 9V battery push thru a 3Ω resistor?
Variables
Equation
Solve
A 4 Ω resistor has 3A running thru it. Find the battery’s voltage.
Equation
Solve
Variables
6V
6V
Total voltage
(VT) =
6V
Total voltage
(VT) =
6V
An 18V battery produces 2A in the circuit. How much resistance
is in the circuit?
Variables
Equation
Solve
Which of these
two water towers
would give the
most voltage to
a water circuit?
Add (A) or reduce (R) voltage?
When the switch is closed
will the light turn on?
Why or why not?
Resistors?
A
Which light bulbs will light? (All are in closed circuits.)
If it does light, draw an arrow to show the direction of current.
3V
H) ___ Slows down current
in a circuit.
I) ___ Does work in an
electric circuit.
J) ___ Gives electric energy.
K) ___ 12 ohms
L) ___ 36 volts.
M) ___ 5 amps.
3V
6V
3V
9V
0V
9V
9V
With the same resistance, which battery will cause more
current to flow: a 12V or a 24V battery?
B
H
G
Wires?
C
D
F
E
Batteries?
High, Medium, or Low voltage?
Point A______
Point E______
Point D______
Point H _____
Point F ______
Point B______
Point C ______
Point G _____
A bird lands on a 20,000 volt wire. Both feet are touching.
A) What voltage is the bird’s left foot?
B) What voltage is the bird’s right foot?
Voltage give what kind of energy to electricity?
C) What is the difference of voltage between the bird’s feet?
How much potential energy does water have after it falls to the
ground?
D) So, why can a bird land on an electrical wire and not get
electrocuted?
How much voltage does a circuit have just before going back
thru the batteries?
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Legal copying of this worksheet requires written permission.
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:
Period: _____________________
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Legal copying of this worksheet requires written permission.
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:4
Types of Circuits
Period: _____________________
Wires and Voltage
Since wires are conductors,
they have no resistance,
so they have no
voltage difference.
Current joins
Junction
Series circuits have all only
one path for the electricity to flow.
There are no junctions.
Current can split
apart or join together.
IT
IT
Total current in and out
of a junction is equal.
Parallel circuits have multiple
paths for the electricity to flow.
Junction
(current splits)
IT
The branches (paths)
of a parallel circuit are
independent: if one
light is unscrewed, the
other will stay on.
The total current equals
the sum of the two
branch currents.
Objects in parallel have the same voltage
because they share wires (same wire: same voltage).
3V
3V
2A
6V
2Ω
2A
0V
9V
Vtotal = 3 V
Rtotal = 8 Ω
Resistors and batteries
in series add together.
Vtotal = 6 V
Rtotal = 1 Ω
Batteries in parallel
share the same voltage
(they just last longer).
Resistors in parallel reduce total
resistance because there are more
paths for the current to flow.
Example 1: Find the total current in this series circuit.
Example 2: Find the total current in this circuit.
12 volts
Step 2. Rtotal = 1 + 2 + 3
= 6 Ω (in series)
R1 = 1 Ω
Step 1: 12 V
Vtotal =
24 volts 12 V
R2 = 2 Ω
Step 3: V = IR
V
Itotal = total
Rtotal
R3 = 3 Ω
Itotal =
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3Ω
0V
1.5 V
0V
6V
6V
6V
1.5 V
6V
1.5 V
4V
2A
Junction
(current joins)
When adding together batteries or resistors, it depends whether they are in series or in parallel.
8V
2A
I2
I1
IT = I1 + I2
Objects in series have the same current
because there is only one path.
4Ω
Junction
Current splits
Your house is wired in parallel, so that each light and
appliance can be turned on and off independently.
The current is the
same everywhere in
a series circuit.
4A
2A
Series versus Parallel
If any part of a series circuit
is broken, the circuit fails.
The lights are dependent on
one another: if either light
is unscrewed both lights
will turn off.
IT
4A
2A
wires don’t change voltage at all.
4Ω
6A
12V 12V 12V
The voltage change 12V
over any wire is zero.
0V 0V
0V
OR
The voltage is the
Batteries add voltage,
same everywhere
resistors
subtract voltage, and
in the same wire.
Multiple Devices
Just like in a river, current can only change
if there is a junction: a split or a join.
Junctions
0V
24
= 4 amps
6
Step 1:
Vtotal =
12 volts
Step 2 :
I branch 2 =
6V
6Ω
3Ω
12v
= 4a
3Ω
6V
0 volts
0 volts
Step 2 : I branch1 =
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12v
= 2a
6Ω
Step 3 :
I total = I 1 + I 2
= 2 + 4 = 6a
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:4
Period: _____________________
1. Series
A. The voltage across any wire.
1. R1
A. Voltage between A and B.
2. Short Circuit
B. A circuit with multiple paths for current
to flow.
2. VT
B. The total voltage in the circuit.
3. Parallel
C. Where current splits or joins.
3. IT
C. The resistance of resistor 1.
D. An independent path for electricity in a
parallel circuit.
4. V2
D. The current in branch 2 of the circuit.
4. Branch
5. I2
E. Total current in the circuit.
E. When a wire by-passes a part of a circuit. 6. RT
F. A circuit with only one path for the
7. VAB
electricity.
5. Junction
6. Zero
F.
The voltage of battery 2.
G. The total resistance of the circuit.
Are these devices in Series or Parallel?
Series or Parallel Circuit?
A. ______
___ Only one path for the electricity to flow.
B. ______
C. ______
D. ______
___ Paths are dependent on each other (one affects the other).
2V
___ How your house is wired.
9V
2V
6Ω
___ Paths are independent of each other.
5Ω
4Ω
6Ω
9V
___ If one light turns off, the others stay on.
VT = _____
___ If you turn off one light, all the lights turn off.
RT = _____
VT = _____
RT = _____
___ Has more than one path for the electricity to flow.
B
A
___ Two devices have the same current.
C
___ Two devices have the same voltage.
Understanding current.
Fill in the missing information on the following graphics.
6V
4Ω
D
4Ω
6V
4A
3A
IT = 18 A
6A
ITotal = _____
I2 = _____
I = _____
1A
F
G
VT = ______
VFG = ______
VED = ______
VAB = ______
VFB = _______
VAC = _______
VEA = ______
VCE = ______
VDC = _______
A) What is the total voltage above?
B) What is the voltage from B to F?
C) What is the resistance from B to F?
D) Find the current flowing from B to F
(label it on the diagram).
I = _____
1A
E
3A
E) How much current flows from C to G (label it)?
ITotal = _____
Which resistor will have
more current running thru
it?
F) So, using D and E above, what is the total current
going thru point E (this is the total current [IT])?
3V
20 Ω
5Ω
G) Using VT and IT, find the total resistance of the circuit [RT].
3V
Why?
Which light bulb will be brighter?
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:
Period: _____________________
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:5
Power and Voltage Drops
Period: _____________________
Electrical power tells us how fast electricity is being used in a circuit or resistor.
A more powerful device uses the same amount of electricity, just faster.
Electrical Power
Electrical Power:
P = VI
Power
(in watts)
Ex. A 12 V battery pushes 3 A
thru a light bulb. How much
power does the light bulb use?
Voltage
(in volts)
Current
(in amps)
V = 12 v
I=3A
P = ____
Electrical Power equals
the voltage times the current.
A brighter bulb uses more power. Since
P= VI, a bulb could be brighter because it uses
more voltage with the same current OR because it
uses more current with the same voltage.
P = VI =
P = 12(3) = 36 w
24 V
16V
4Ω
8V
4Ω
4A
V1 = I1R1
= 4(4) = 16 V
R1 uses 16 V,
leaving 8 V.
V2 = I2R2
= 4(2) = 8 V
8V
4A
2Ω
2Ω
PT = VTIT
PT = 24(4)
PT = 96 w
R2 uses 8 V,
leaving 0 V.
0V
Rtotal = 6 Ω
Finding the total current and power in a parallel
circuit is much easier if you think of it as two
independent series circuits.
Simplifying Parallel Circuits
Semi-complicated
For objects in parallel, the one
smallest resistor uses the most
current and the most power.
Branch 2
Simplified
Branch 1
P2 = V2I2
P2 = 8(4)
P2 = 32 w
2Ω
8V
Both light bulbs have the same
current, but the one with more
resistance is brighter because it uses
more voltage and uses more power.
Notice V = 8 V between the resistors. Also, the
total voltage used by the resistors must equal the
total voltage given by the batteries: 24 volts.
V 24
IT = T = = 4A
RT 6
P1 = V1I1
P1 = 16(4)
P1 = 64 w
4Ω
16 V
4A
24 V
8V
8V
For objects in series the
biggest resistor uses the most
voltage and the most power.
2. Use V = IR for each resistor.
1. Find the total current (IT).
Vtotal =24V
40
W
Batteries add voltage, resistors subtract voltage, and wire don’t change voltage at all. To find
the voltage used by a particular resistor, you have to know the current running thru it.
Voltage Drops
8V
100
W
12V
2A
6V
+
4A
6V
4A
6Ω
3Ω
P2 = V2I2
= 12(4) = 48 w
P1 = V1I1
= 12(2) = 24 w
R2 = 3 Ω
Both light bulbs have the same
voltage, but the one with less
resistance is brighter because it has
more current and uses more power
(this is how your house works).
V 12
I 2 = 2 = = 4A
R2 3
IT = 2 + 4 = 6A
+
– –
–
–
–
3. One electron
enters the
battery
1. Battery pushes
out one electron
–
2. Electrons repel each
other throughout the wire
–
25A
–
–
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–
–
–
–
The electrons that move
to make electricity do
not come from the
battery: they are already
in the wires of the
circuit. Metals are good
conductors because their
electrons can move.
1 electron in
–
Fuse
Electricity causes
heat. Fuse melt
(or break) when too much current passes through
them, protecting expensive electronic equipment.
Like fuses, circuit breakers protect against too much
current, also, but can be reset, instead of replaced.
1 electron out
–
–
Electrons
Too much current causes the
thin strip of metal to melt.
–
V 12
I1 = 1 = = 2A
R1 6
Fuses
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Vtotal = 12V
–
R1 = 6 Ω
–
Vtotal = 12V
Remember that devices
in parallel have the
same voltage, but they
have different current.
–
=
2Α
3Ω
–
6V
6Ω
–
–
–
–
3Ω
6V
–
6Ω
6V
–
6V
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:5
Period: _____________________
A. An independent path in a parallel circuit. Which is brighter: a 60 W or a 100 W bulb?
Which one uses more power?
B. A device that breaks to protect against
2. Circuit breaker
excessive current. Must be replaced.
A 4 Ω and a 10 Ω light bulb are in series. Which one is brighter?
C. Where branches joint or split.
3. Wire
1. Fuse
4. Branch
D. Protects against high current, but can be
reset.
Why?
5. Power
E. Where most of the electrons in a circuit
come from.
A 4 Ω and a 10 Ω light bulb are in parallel. Which is brighter?
6. Junction
F.
Why?
The product of voltage and current.
The electricity in your house is 120 volts. How many amps does
a 60 watt light bulb use?
Label the voltages at the letters.
B
A
V at A =
1Ω
1V
V at B =
1Ω
1V
V at C =
1V
V at D =
1Ω
C
D
R1 = 1 Ω
9V
R2 = 2 Ω
How much power is used by a 120 V circuit using 6 amps?
A 240 volt circuit (like your dryer) has 20 amps flowing through
it. How much power is it using?
VT=
________________
R T=
________________
IT =
________________
35 amps of current goes through a 40 amp fuse. What happens?
V over R1 = _____________
3V
R3 = 3 Ω
V over R2 = _____________
25 amps of current goes through a 15 amp fuse. What happens?
V over R3 = _____________
Series or parallel?
Which resistor uses the
most power?
P2Ω = ___________________
P3Ω = ___________________
B1
PT = ___________________
Why?
6V
Split the following parallel circuit into two independent series
circuits.
B2
I1
I2
R1 =
R2 =
6Ω
3Ω
6V
IT
3V
I1 = ________________
I2 = ________________
RT = _______________
Series or parallel?
6Ω
VBranches = ___________
IT = ________________
3V
3Ω
VT = _______________
P6Ω = ______________
P3Ω = ______________
Which resistor will use more
power?
PT = _______________
VT = ________________
12v
12v
VBranches = ____________
R1 =
R2 =
12 Ω
8Ω
R3 =
8Ω
I1
I2
I3
I2 = _________________
Find the current in each circuit.
Find the total current.
Find the total resistance.
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I1 = _________________
IT
Which resistor uses the most
power?
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I3 = _________________
IT = ________________
RT = ________________
PT = ________________
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:
Period: _____________________
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:6
Meters
Period: _____________________
As the name suggests, a multimeter can act like a multitude of meters. It can be a voltmeter,
ammeter, or ohmmeter, but you have to know how to use them in a circuit.
Multimeters
A multimeter allows you to
diagnose troubleshoot)
circuits or broken equipment quickly. Many people
throw out items that can
be easily fixed.
16V
By moving the
dial you select
which meter it is.
V
A
Ω
Circuit
Symbols
for
Meters
Use the probes
to test the circuit.
V
Voltmeter
A
Ammeter
2Ω
4Ω
--
4Ω
Ω
4Ω
6Ω
2Ω
4Ω
Measure resistance
outside a circuit: with no
battery at all or your measurements will be wrong.
Must have
a break
An ohmmeter can measure individual
resistors or multiple resistors.
If a wire or resistor is
broken (bad) the ohmmeter will read infinite
resistance or error.
Test each part of a
circuit to find which
part is broken.
A voltmeter measures voltage (volts). A voltmeter must be in parallel like an ohmmeter,
but with the circuit on.
Must be a
voltmeter
because it is
in a circuit.
Circuit
symbol
Ohmmeter
An ohmmeter measures resistance (ohms). Measure resistors outside of the circuit.
Ohmmeters
Voltmeters
Ω
4Ω
V 8v
12V
A car battery
only provides
12V, but a lot
of amps.
0V
12V V
4V
2Ω
AA, AAA, and D
cell batteries read
1.5 V when new.
After time the voltage drops some.
Batteries also read
lower when on.
drained
closed switch
A voltmeter needs the circuit on. A voltmeter can read
the voltage given by a battery or used by a resistor.
Ammeters
An ammeter measures current (amps). An ammeter must be in series with the circuit on.
CAUTION! An ammeter
is a very delicate device.
Incorrect usage can badly
damage the ammeter.
Incorrect
A voltmeter can measure a battery in the circuit or out of the
circuit, while a resistor has no voltage outside of a circuit.
Correct
1Α
3V
1Ω
6Ω
3V
1A
3Α
4Α
cstephenmurray.com
3Ω
1.5 V
A
A
A1
1.5 V
To correctly hook up an ammeter,
break the circuit where you want
to read the current and place the
ammeter into the break.
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A2
A3
In this diagram there is only one
ammeter. It has been moved to
different parts of the circuit.
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:6
Period: _____________________
A. Used to measure current.
1. Voltmeter
Ohmmeter, Voltmeter, or Ammeter
B. Used to measure voltage.
____ Must be in series.
____ The circuit cannot connected.
3. Probes
C. The parts of the meter that touches the
metal of the circuit device.
4. Ammeter
D. A device that can measure voltage,
current, or resistance.
____ Used to tell the amount of current in the circuit.
5. Ohmmeter
E. Used to measure a resistor.
2. Multimeter
____ Must be in parallel with the device being measured.
____ The circuit must be on.
____ Delicate. Can be damaged if hooked up wrong.
How do you hook up an ohmmeter?
____ Can measure if a battery is worn out.
____ Can measure a resistor only in a circuit.
____ Can measure a resistor out of the circuit.
How do you hook up a voltmeter?
What is the resistance for a good wire?
How do you hook up an ammeter?
1
M1:
M2:
M3:
M4:
M5:
3
1Ω
9Ω
6V
2Ω
Draw meters that will
measure the following:
Identify the meters as
voltmeters or ammeters.
2
3V
What is the resistance for a broken wire?
4
5
______
______
______
______
______
Figure out what each meter reads.
R1 = 4 Ω
M1: Total voltage
M2: Total current
M3: Voltage over R2
9V
R2 = 2 Ω
9V
R3 = 6 Ω
Figure out what each meter reads.
In Lab:
Measure the three resistors you are given:
R1 = _________
R1 = _________;
R2 = _________
VFA = ________
1.5 V
Put the above resistors in series. RT = _________.
How does RT compare with the individual resistors?
VBE = ________
VCD = ________
1.5 V
Put the above resistors in parallel. RT = _________.
How does RT compare with the individual resistors?
VED = ________
R2 = _________;
Build the following circuit:
A
VAB = ________
1.5V
VDE = ________
VEF = ________
1.5V
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C
F
R2 = ____
0
G
R1 =
E
C
R2 =
D
F
I1 = __________
Using VT and IT, calculate RT.
I2 = __________
IT = __________
R3 = ____
E
B
WITH THE MULTIMETER OFF: set up your circuit to read
the current in the first branch (at B or E). Have the teacher
check your setup before you turn it on.
B
R1 = ____ 0
VBC = ________
VCD = ________
VEA = ________
R3 = _________.
A
Check it with your ohmmeter.
D
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:
Period: _____________________
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:7
Magnets
Period: _____________________
A magnet is anything that can attract or repel another magnet.
Magnet Basics
Donut magnets
Horseshoe magnet
Bar magnets
repel
attract
Aluminum
1) All magnets
have two
poles: a north
and a south.
2) You can’t separate
a N pole from a S pole
(you just make
smaller magnets).
Opposite attract
3) Magnets exert magnetic forces
of attraction and repulsion.
N
e-
N
Temporary magnets become magnets only when near
a permanent magnet. The spinning electrons line up
together when a magnet is near, but will eventually
fall back after the magnet leaves. Only ferrous materials can become temporary magnets. Often bumping
them (like dropping) can cause the electrons to fall
back quickly.
S
An electromagnet is a magnet made by moving electricity.
Electromagnets
Ways to strengthen
an electromagnet:
Electromagnets are particularly
useful because they allow us
to create forces that we can turn
on and off at will.
2) More coils
(the easiest way
to add electricity).
1) More electricity
(more current thru
more batteries, etc).
A toaster holds the
toast down with an
electromagnet. When
the electricity turns
off, the electromagnet
releases the toast.
3) Add a ferrous core,
especially iron,
which becomes a
temporary magnet.
An electromagnet does not have to
have a core: any loops of electricity
will make an electromagnet.
Generators and Motors
Moving electricity creates magnetic fields. Moving magnets make electricity.
Because electricity and magnetism are linked we can make motors and generators.
Generators generate electricity.
Something turns the
generator (does work)
causing magnets to
move, which creates
electricity. Dams,
cars, and power plants
all produce electricity
in this way.
Motors use electricity.
Electricity causes
magnetic forces thru
electromagnets. The
electromagnets
cause the object to
turn (do work).
Work in (air):
electricity out.
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4) Magnets only attract ferrous
metals: Iron, Cobalt, and
Nickel (steel is an alloy of
mostly Iron and Nickel).
Permanent magnets do not lose their magnetism.
Many of the electrons in a permanent magnet are
spinning the same way and the all of the little electromagnets add up. Lodestone and Magnetite are the
only two natural permanent magnetic materials.
How do Magnets Work?
Moving or spinning
electrons in atoms
cause magnetism.
If electrons are
paired and spinning
in opposite directions,
the magnets cancel
each other out. When a substance
is magnetic many of the electrons
are spinning in the same direction.
Likes repel
el
Ste
Electricity in: work
out (moving air).
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Motor or Generator?
It could be either one,
depending on how it is used.
Any motor can create electricity and any generator will turn
if electricity is applied.
Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:7
Period: _____________________
Permanent
magnet
A. Turns when electricity is turned on.
2.
Temporary
magnet
3.
Motor
C. Becomes a magnet when electricity
moves in wire coils.
4.
Generator
D. Uses work to make electricity.
5.
Core
6.
Electromagnet
E. Does not lose its magnetism: lodestone
and magnetite are only natural types.
F.
Center of the electromagnet coils.
N
S
___
___
___
If the three
magnets are
attracting
each other,
label N and
S on the
second
magnet.
N
S
___
___
If the two
magnets are
repelling
each other,
label N and S
on the second magnet.
___
Attract or repel?
Attract or repel?
The diagram show magnetic levitation
(Maglev). The top bar magnet is suspended (floating) above the bottom
magnet while in a graduated cylinder
to keep it from falling to the side.
1) Two keep the upper magnet levitated, are they attracting or repelling each other?
2) Label N and S on the upper
magnet.
___
___
N
B. Becomes a magnet near a magnet, then
loses its magnetism when moved away.
S
1.
True or false (and why):
“A magnet will pick up any piece of metal.”
1) Label the north and south poles of
the nail magnet.
2) Will the nail stay a magnet when
removed from the bar magnet?
3) Is the nail a temporary
or permanent magnet?
What makes a magnet on the atomic level?
As a magnet gets closer to another magnet, does the magnetic
force increase or decrease?
Draw a simple electromagnet:
Motor, Generator, or Both?
_____ Creates electricity.
_____ Has loops of wire in it.
_____ Creates motion.
_____ Is turned by a force.
_____ Can make electricity.
_____
Work
_____Used in a hydroelectric
dam.
_____Used in open or close
windows in a car.
_____Turns when electricity
is applied to it.
Electricity
Name three ways you could increase
the strength of an electromagnet:
Which electromagnet is stronger?
A or B?
A. 9V
B. 9V
C. 9V
D. 3V
B or C?
C or D?
A or C?
_____
Electricity
Work
A or D?
Which is the strongest of the four?
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:
Period: _____________________
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Period: _____________________
Unit 9:8
Electricity and Magnetism Review
1.
Voltage
A. 8 amps
1.
Current
A. A path for electricity to flow.
2.
Resistance
B. 8 volts
2.
Voltage
B. A material that allows electricity to
flow.
3.
Power
C. 8 coulombs
3.
Resistance
4.
Charge
D. 8 watts
4.
Insulator
5.
Current
E. 8 ohms
5.
Conductor
E. What pushes electricity in a circuit.
6.
Circuit
F.
What moves: electrons or protons?
Why?
D. Slows down electricity
The flow of electrons thru wires.
A _______________ allows electricity or heat to pass through it.
An object has a charge of –8 C.
A) Is the object positive or negative?
B) Did it gain or lose electrons?
C) If you touch it to ground, will it lose electrons to ground
or gain electrons from ground?
D) What will its charge be after it is grounded?
What are the charges of the second objects?
Electricity flows through paths called ___________. A
_________ circuit has no breaks in it, while an ___________
circuit has a break and stops the flow of electricity.
An ________________ will not allow electricity to pass.
Electricity is made up of flowing ______________.
Like electric charges attract/repel. Opposite charges attract/
repel.
–
+
Draw a circuit of two batteries, a light bulb, a resistor, and a
switch, all in series.
Repel
Attract
C. A material that resists electricity.
What is electricity?
Will electrons flow between the two objects?
B.
A.
5C
0C
-3C
-3C
-2C
-6C
Why does electricity move?
You have two light bulbs and a battery in a circuit. If you add
another battery, do the light bulbs get brighter or dimmer?
Why?
What is arcing?
You have two light bulbs and a battery in a circuit. If you add
another light bulb, do the light bulbs get brighter or dimmer?
Why?
When and why does arcing occur?
If a 12 v battery is connected to a 24 Ω resistor, how much
current is flowing?
How much voltage is needed to produce 2 amps through a 4 ohm
light bulb?
Increases (I)
Or
Decreases (D)
Increasing resistance _______ current
Decreasing resistance ______ current
Increasing voltage ______ current
Decreasing voltage ______ current
How can you tell if two light bulbs are in parallel?
How can you tell if two light bulbs are in series?
If a light bulb in your house (120 V) draws 0.5 amps, how much
power does it use?
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Is your house wired in series or in parallel?
Why?
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:8
Period: _____________________
What is the change of voltage across a wire?
What happens when you short circuit one light bulb in a two light
bulb circuit?
Why can a bird sit on a wire and not be electrocuted?
What happens if you short circuit a battery?
Fill in the missing information on the following graphics.
Where do most of the electrons come from that run thru a circuit?
2Ω
5A
6V
3A
4Ω
3V
B
VT =
R1 = 10 Ω
3V
VT = _____
RT = _____
IT = _____
R2 =
5Ω
6V
R3 = 3 Ω
Series or parallel?
___________________
RT =
___________________
IT =
___________________
IR3 =
___________________
VR2 =
___________________
PT =
___________________
N
___
___
___
___
The three
magnets are
repelling
each other.
Fill in the
missing
information.
A
S
___
___
___
___
The two
magnets are
attracting
each other.
Fill in the
missing
information.
Series
or
parallel?
I1=
C
D
I2=
I3=
6V
R1 =
2Ω
6V
6V
G
H
R3 =
1Ω
R2 =
3Ω
F
E
VT = _________________
I2 = __________________
VDF = _________________
I3 = _________________
VHG = ________________
IT = __________________
Vat F = ________________
RT = _________________
I1 = ___________________
PT = _________________
Someone asks you how what a circuit is and how it works.
Tell them.
What will a magnet attract?
What are fuses and circuit breakers?
What will a magnet repel?
How are they different?
What does moving electricity cause?
What do moving magnets cause?
What is a motor?
What is an electromagnet?
How does it work?
How do you strengthen an electromagnet?
What is a generator?
How does it work?
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Name: _____________________
Unit 9:
Period: _____________________
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Name: _____________________
Unit 9:
Period: _____________________
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:S1
Lab: How Bright is the Light?
Period: _____________________
Purpose:
To investigate how voltage and resistance affect the brightness of a light bulb in a circuit.
Background:
Students should know that a battery is a source of voltage. More batteries = more voltage
A light bulb causes resistance in a circuit. More light bulbs = more resistance
The brightness of the light bulb increases as the current through the light bulb increases.
Materials and Preparation:
Have 2 batteries, 2 light bulbs, wires/alligator clamps or a spring board per set-up.
Electrical tape and Christmas lights offer an easy and inexpensive alternative to regular light bulbs.
Effect of changing voltage on light bulb brightness:
Circuit
# light bulbs
# batteries
1
1
1
2
1
2
3
1
3
relative brightness
Effect of changing resistance on light bulb brightness:
Circuit
# light bulbs
# batteries
1
1
2
2
2
2
3
3
2
relative brightness
Student OutcomeWrite a scientific statement to explain the effect of voltage and resistance on the brightness of a bulb.
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Copyright © 2007, C. Stephen Murray
Name: _____________________
Unit 9:
Period: _____________________
Curriculum Notes:
1.
MOST IMPORTANT DEMO. I have tried numerous ways for students to understand how voltage, resistance, and current
relate. I have had them take measurements for themselves. The problem is that they take the measurement at different moments in time. They do not accept that the current is the same everywhere in a series circuit. So, make the following circuit:
Any resistor
that allows
enough current.
Variable
power
supply
2.
Variable
resistor
By putting multiple multimeters in the circuit you
can PROVE that the current is the same everywhere because they will all read same amount!!!
You can change the voltage and the resistance,
but all multimeters will always read the same
current. It is also a good demo of how voltage
and resistance affect current.
V
Concept Key—Series circuits. Spend more time on series circuits before moving onto parallel circuits. For years my
students have struggled with calculating voltage drops around a series circuit. This last year I used the above demo
and spent more time with just series circuits and they got it. And the just to parallel circuits was easy. I had almost
100% comprehension with both circuit types and even with compound circuits.
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Copyright © 2007, C. Stephen Murray