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Part 5
Electricity
141
Electric Charge
There are two types of electric charges called positive and negative (+,–). The origin of this charge
is the atom.
–
+
The nucleus of the atom has the positive charge. The particle that the positive charge is on is
called a proton. It is very heavy.
The negative charge surrounds the nucleus. The particle that the negative charge is on is called an
 1 
electron. Electrons are very light compared to protons, about  2000  of the mass of a proton.
Sometimes the electrons can be "rubbed" off the atom and then one object has excess (–) charge
and one has excess (+) charge. This is sometimes called static electricity. You feel the results
when you get shocked after crossing dry carpet!
The spark is a result of excess charge being neutralized by leaving or entering your body.
Electric Forces
Opposite charges attract each other and negative ones repel.
How can a comb pick up paper?
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Although paper was neutral, the comb repelled
the (–) charge and attracted the (+) charge. Since
the (–) charge is farther away, the paper is
attracted more than repelled.
Sometimes you see a piece of paper fly off the comb. What causes this?
Batteries, generators, solar cells, etc. "produce" electricity because they all have the ability
to separate +, and – charge. It is the tendency for the + and – charges to unite, which is used in
electric circuits.
Some objects have the ability to allow electrons to pass through them. These objects are
called conductors. Others are called insulators.
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Circuits
Electric Circuits vs Plumbing Circuits
1.
The amount of electric charge that
flows per unit time is the current (rate of
flow of charge). The unit is called
amperes.
2.
There is a "potential" drop between
A and B at R. The unit for this drop is
volts.
3.
The battery provides the "potential
difference" for the circuit. This unit is
volts.
4.
The load, R, provides a resistance
to the current. The unit is ohm., the symbol
for which is 
1.
The amount of water that flows per
unit time is the current (rate of flow of
water). The unit is gallons per min.
2.
There is a pressure drop between A
and B at R. The unit for this is pressure
drop is lbs./in2.
3.
The pump provides the pressure
difference for the plumbing. This unit is
lbs./in2.
4.
The pipe, R, provides a resistance
to the flow of current (no unit).
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Ohm's Law
V
I
R
V=IR
Ohm's Law is a very useful relationship between the potential difference (Voltage), V and
current, I, and the resistance, R of a circuit. If the resistance of a circuit remains constant then if
the voltage of a circuit increases the current will increase. If the voltage stays constant then an
increase in the resistance will decrease the current. Some examples follow
Example 1
The resistance of a light bulb is 100 ohms. What current will it draw at 120 V?
Solution
The current will be I 
V 120V

 12
. A
R 100
Example 2
A toaster draws 12 A on a 120 V circuit. What is its resistance?
Solution
The resistance is R 
V 120V

 10
I
12 A
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Power
Electric power is also useful, There are three formulas to use depending on the
circumstances.
P
I
V
P=IV
P
P=I2R
I2
R
V2
P
V2
R
P
R
Example 3
A light bulb is rated at 60 Watts at 120 Volts. What current will it draw, and how many of
them could you put on a 15 Amp circuit breaker?
Solution
The current can be found using I  P  60W  0.5 A , which is the amount of current to run
V
120V
one bulb.
Then 15 A  30bulbs
 0.5 A 


 bulb 
Example 4
A motor draws 8 A at 240 V. What power does it draw in Watts and horsepower?
Solution
The power can be found using P = I V = 8 A x 240 V = 1920W
Then 1920W 
1hp 
  2.57hp
 746W 
Example 5
You have two 150 W bulbs, one is rated at 120 V and the other is rated at 12 V. How
much current does each draw?
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Solution
P 150W

 125
. A
V 120V
P 150W
The current for the 12 V bulb is I  
 12.5 A
V
12V
The current is for the 120 V bulb is I 
Two Types of Circuits
Parallel – Same voltage at each light as at the
battery.
Series – Voltage at lights less than at battery.
If one light goes out all of the lights go out.
Parallel Circuits:
Same voltage at each resistor:
Vtotal  V1  V2  V3 
Current going though whole circuit is the total of the current going though each resistor:
I total  I 1  I 2  I 3 
The total resistance of the whole circuit is less than the resistance of any of the resistors:
1
1
1
1




Rtotal R1 R2 R3
Series Circuits
The sum of all the voltage drops at each resistor is the total voltage for the whole circuits:
Vtotal  V1  V2  V3 
The current through any resistor is the same as the current in the whole circuit:
I total  I 1  I 2  I 3 
The total resistance of the whole circuit is the sum of all the resistors in the series circuit:
Rtotal  R1  R2  R3 
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Trouble Shooting
Voltmeters – These check for voltage, for instance, in the circuit below assume you have 1.5
volts at E–F, E–B, E–A, S closed
Note – D–E, F–B, C–A. Should be 0.0
volts with S closed
Ohm Meters - Ohm meters and continuity testers check for complete circuits. There should be 0
ohms from DE, FB, AC, FB. NOTE Only use an ohmmeter with circuit off! DC should read
1.5 ohms. If you get (infinite) ohms at DC, then the light is burned out.
Do not put the ohmmeter between EF, the battery will burn it out. All meters need to be
set correctly before you hook them up otherwise you may damage the meter!
______________________________________________________________________________
V
I
R
P
I
V
P=IV
Parallel circuits :
Series Circuits
V2
P
1
Rtotal
P=I2R

I2
R
V2
P
R
P
R
1
1
1


 Same voltage across each branch, current splits.
R1 R2 R3
Rtotal  R1  R2  R3  Same current I flows through whole circuit
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Exercises
Electrostatics Review
1.
In each picture below show the direction of the force between the two charges.
a.
b.
c.
d.
2.
Which of the following is a unit of charge?
a.
gram
b.
coulomb
c.
farad
d.
henry
e.
joule
f.
ft–lb
3.
Two charged particles held close to each other are released. As they move, the force on each
particle increases. Therefore, the particles have
a.
the same sign.
b.
opposite sign.
c.
not enough information given.
4.
An electron is between two plates that are side by side. If the electron is attracted to the plate on
the left and repelled from the plate on the right, which plate is the positive plate and which plate is the
negative plate?
Left ________
Right ________
5.
A metal sphere that is negatively charged is brought near a metal can that rests on a wooden table.
A thin piece of fishing line is connected between the can and the metal object. The can is now
a.
positively charged.
b.
negatively charged.
c.
uncharged.
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6.
A metal sphere that is negatively charged is brought near a metal can that rests on a wooden table.
You touch the can with your finger momentarily. The can is now
a.
positively charged.
b.
negatively charged.
c.
uncharged.
7.
A metal sphere that is negatively charged is brought near a metal can that rests on a wooden table.
A thin piece of copper wire is connected between the can and the metal object. The can is now
a.
positively charged.
b.
negatively charged.
c.
uncharged.
8.
Whenever there is a voltage difference between two points in space
a.
+ charges will move from the high voltage to the low.
b.
– charges will move from the low to the high.
c.
there will be an electric field between the two points.
d.
both a and b but not c.
e.
b, and c provided the two points are connected by a metal wire.
9.
When an electron is in an electric field it
a.
feels a force from the field.
b.
accelerates.
c.
gains more charge.
d.
it acquires a velocity.
e.
all of the above.
10.
Electrons move in an electrical circuit
a.
by being pushed by other electrons.
b.
by colliding with molecules.
c.
by interacting with an established electric field in the wires.
d.
because the wires are so thin.
e.
none of these.
11.
T/F
Charges can move from on point in space to another even though there isn't a conducting
wire present, provided the charges are not bound too tightly to the material they are in, and that there is a
strong enough electric field to set them in motion.
150
Electric Power Review
1.
A 60 Watt bulb is connected to a 110 Volt outlet. What is the current running through the bulb?
a.
25 A
b.
0.5 A
c.
2A
d.
4A
e.
more than 4 A
2.
When a light bulb is plugged into a 120 volt outlet, 0.8 A of current flows through it. What is the
power rating for the bulb?
a.
12 W
b.
15 W
c.
60 W
d.
96 W
e.
120 W
3.
A 60 Watt bulb and a 100 Watt bulb are each rated at 120 V. Which light bulb has a larger
resistance?
a.
The 60 W bulb.
b.
The 100 W bulb.
c.
Both have the same resistance.
4.
For the bulbs in question 3 which has the larger filament?
a.
The 60 W bulb.
b.
The 100 W bulb.
c.
Both have the same resistance.
5.
For the bulbs in question 3 which has the largest current flowing through it?
a.
The 60 W bulb.
b.
The 100 W bulb.
c.
Both have the same resistance.
6.
A power line with a resistance of 4.0  has a current of 200 amps flowing through it. The power
lost in the line is
a.
50 Watts.
b.
800 Watts.
c.
160000 Watts.
d.
3200 Watts.
7.
Where does the energy of question 6 go and what does it do?
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Circuit Review
1.
A wire carrying a current is normally charged
a.
negatively.
b.
positively.
c.
not at all.
2.
The current through a 20 ohm resistor connected to a 240 V power source is
a.
1 A.
b.
10 A.
c.
12 A.
d.
120 A.
e.
4800 A.
3.
A 30 ohm resistor has 3 amps flowing through it. The voltage drop across the resistor is
a.
10 V.
b.
33 V.
c.
9 V.
d.
90 V.
e.
270 V.
4.
Sketch a circuit with one resistor, one battery, and a voltmeter connected to measure the voltage
across the resistor. Remember to indicate the + and – terminals on the voltmeter.
5.
Draw a sketch of a circuit with one resistor, one battery, and an ammeter connected to measure the
current through the resistor. Remember to indicate the + and – terminals on the ammeter.
6.
When a 12 volt battery is connected to a circuit the current leaving the battery is 4 amps. The total
resistance of the circuit is
a.
48 ohms.
b.
16 ohms.
c.
3 ohms.
d.
0.75 ohms.
e.
36 ohms.
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7.
When a 12 volt battery is connected to a circuit of resistance 2 ohms how much current will flow
through the circuit?
I = ___________
8.
As more lamps are put into a series circuit, the overall current in the circuit
a.
increases.
b.
decreases.
c.
stays the same.
9.
As more lamps are put into a parallel circuit, the overall current in the circuit
a.
increases.
b.
decreases.
c.
stays the same.
10.
Two lamps, one with a thick filament and one with a thin filament, are connected in parallel to a
battery. The current is greatest in the
a.
thick filament.
b.
thin filament.
c.
the same in each lamp.
11.
Two lamps, one with a thick filament and one with a thin filament, are connected in parallel to a
battery. The voltage is greatest across the
a.
thick filament.
b.
thin filament.
c.
the same in each lamp.
12.
Two lamps, one with a thick filament and one with a thin filament, are connected in series to a
battery. The current is greatest in the
a.
thick filament.
b.
thin filament.
c.
the same in each lamp.
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13.
Two lamps, one with a thick filament and one with a thin filament, are connected in series to a
battery. The voltage is greatest across the
a.
thick filament.
b.
thin filament.
c.
the same in each lamp.
14.
If you plug in a lamp that is rated for 110 V into a 220 V outlet, the current in the lamp will be
a.
half what it should be.
b.
the same as if it were plugged into 110 V.
c.
more than twice what it should be.
d.
twice what it should be.
e.
none of these.
15.
If B goes out in the following DC circuit what happens to A?
a.
A
b.
c.
d.
A gets brighter
A stays the same.
A gets dimmer.
A goes out.
B
V
16.
If B goes out in the following DC circuit what happens to A and C?
a.
A gets brighter
B
b.
A stays the same.
c.
A gets dimmer.
A
d.
A goes out.
a.
C gets brighter
b.
C stays the same.
c.
C gets dimmer.
d.
C goes out.
C
V
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17.
If A goes out in the following DC circuit what happens to B and C?
a.
B gets brighter
B
b.
B stays the same.
c.
B gets dimmer.
A
d.
B goes out.
a.
C gets brighter
b.
C stays the same.
c.
C gets dimmer.
d.
C goes out.
C
V
18.
Answer the following questions true or false about the following diagram.
a.
(T/F) Bulb A is not as bright as bulb
B
B.
b.
(T/F) Bulbs B and C are the same
A
brightness.
c.
(T/F) If bulb B is unscrewed bulb C
will become brighter
d.
(T/F) If bulb B is shorted out (i.e. a
wire connected between B and C) bulb B will
become dimmer.
C
e.
(T/F) If bulb B is shorted bulb C gets
brighter.
f.
(T/F) If Bulbs B and C are
unscrewed bulb 1 gets brighter.
g.
(T/F) If a wire is connected between
V
points A and C all bulbs go out.
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