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Electric Currents
Chapter 34
What is an electric current?
What makes current flow?
An Electric Current is simply electrons
traveling through a conductor
like currents in a river
Why does the water in a river flow?
Gravity is pulling it down, but a general rule of thumb is
that an object has a tendency to move towards positions
of lower energy.
DPE
Water will stop flowing when there is no PE
difference
This is also when the pressures are equal on
both sides
Water will stop flowing when there is no PE
difference
In order to keep water flowing we would need to maintain a
difference in height
The water would have to be continuously
pumped to keep it flowing
An electrical current is made up of little
charged particles (electrons).
What tells you the Potential Energy of an
electron. (Hint J/C)
Electric Potential (Volts)
Electrons travel from high to low voltages
2.0
1.5
1.0VV
0.5
0.0
-
+
+
+
+
As electrons flow the potential difference is
reduced.
No current flow when there is no potential
difference.
0V
In order to keep current flowing we need an
electron pump.
e1.5 V
A battery keeps current flowing by maintaining
a voltage difference (until it “dies”)
Chemical reactions in a battery pump electrons around a loop
electrons used up at the cathode
electrons are produced when the zinc “corrodes” at the anode
How would you measure the flowrate of
water?
Volume
gallon
time
minute
The rate at which electrons “flow” is measured in current
Q
I= t
the amount of
charge that flows
through a wire (C)
per time (s)
Electrical Current: Amperes or Amps for short
1 Coulomb
1 ampere =
1 second
What does as 15 amp circuit mean
One light bulb draws 1/3 amp
Another draws 2/3 amp.
How many more electrons pass through the
second as compared to the first in one second?
The greater the potential difference
(pressure) the faster the flow of water.
The bigger the electric potential difference,
the faster the flow of electrons.
As Voltage
Current
Voltage can be thought of as electrical pressure
How does the size of the hole
affect flowrate?
A small hole provides more RESISTANCE to water flow.
Electrical RESISTANCE of something
(like a wire) is how much it resists
electrons flowing through it.
It is measure in OHMS
W
The Electrical Resistance of an object
depends on 4 things
1.) THE MATERIAL
Copper wire
Glass rod
Which will have the higher resistance?
The Electrical Resistance of an object
depends on 4 things
2.) The Length
Short
Long
Which will have the higher resistance?
The Electrical Resistance of an object
depends on 4 things
3.) The thickness
THICK
THIN
Which will have the higher resistance?
The Electrical Resistance of an object
depends on 4 things
4.) The Temperature
The hotter an object the more it resists current.
When some materials are cooled WAY DOWN, they actually
lose all resistance and become SUPERCONDUCTORS
How would the resistance be affected if the
wire was stretched out.
As Resistance
Resistance is measure in
Current
Ohms
W
Tube
resistor
valve
switch
-
+
pump
battery
pressure drop across
voltage drop across
Tube
resistor
valve
switch
-
water flow
pump
e- flow
+
battery
Ohm’s Law
V
I= R
Current (amps)
Electrical Potential
(Volts)
Resistance
(Ohms)
Ohm’s Law
V
I= R
As Voltage
Current
As Resistance
Current
A 9-volt battery is place on your wet
tongue which has a resistance of
about 3000 ohms. How many amps
run through your tongue?
Amperage Chart
.001 A
.005 A
.015 A
.100 A
mild shock
hurts
loss of muscle control
probable death
If you touch a 120 V circuit
WET skin has a resistance
of about 3,000 W
high current
Dry skin has a resistance of
about 500,000 W
low current
Why is the bird safely perched on a power line
A technician installs a bird diverter to keep condors away from
points which may be grounded.
What resistance would you want, in order
to flow 15 Amps through a 120 V circuit?
How much current flows across a light bulb
with a resistance of 25 Ohms connected to
a 12 V batter?
Voltage from a 12 V battery (Direct Current or DC)
Current
T
AC or alternating current
Current 0
Time
The US use AC current at 60 Hz,
what does that mean
What happens to the current that
flow through the wire?
Current 0
Time
At times the current is moving:
left or right or not at all
Are there any electrons in a copper
wire which is not connected to a
battery?
e-
In a neutral wire, an electron moves randomly but doesn’t
really go anywhere
Path of an electron in an electric field
-
+
drift speed
An electron has a total speed of about 1,500,000 m/s
but usually has a drift speed of about .0001 m/s
Path of an electron in an electric field
-
+
drift speed
With a drift speed of about .0001 m/s
Why does it not take several minutes
for the lights to turn on, when you hit
the switch?
The wires are already full of electrons,
when one enters one exits.
The electron bumps into atoms in its path
+
-
This slows its drift speed, & is also why a wire gets hot
when current passes through it.
The hotter the wire, the faster the random motion of
the electron is.
The more frequently bumps into an atom, and loses
drift speed.
(the greater the resistance)
Temp
W
Electron motion applet
Path of an electron in an electric field
-
+
Which way would field lines point?
The electric company does not sell electrons
all they do is jiggle the electrons in your
wires which requires ENERGY
When you get electric shock, its your
own electrons that are shocking you!
In order for current to flow, a circuit must
be made
Which circuits will have current?
Which light bulb(s) will light?
Which light bulb(s) will light?
BOOK Problems
Page 546 2-16, 26 - 32
Energy
No resistance =
No energy loss
9V
9V battery
0V
Electric POWER
Energy
Power =
time
If you were looking for the
power of a waterfall,
what 2 factors would
determine it
the height of the falls,
the change in PE of the water
the flowrate of the water
For electric power, it is very similar
the flowrate of electrons
the change in PE of the electron
P =IV
Power
J/s
(Watt, W)
Current
C/s
Voltage
J/C
I
P
Energy
9V battery
I
P
Energy
9V battery
The voltage is the same so each electron gives the same amount of
energy, but more electrons pass through each second
V P
Energy
9V battery
0V
V P
Energy
120 V battery
Even if the current is the same, the Power increases because
each electron drops off more energy!
What power is used by a 2 amp toaster on a 120 V
circuit?
A 60 watt bulb is made to flow .50 A, what
voltage is it intended to be plugged into?
If you were to connect 2 resistors
together (or anything else) there are
two ways to do it.
parallel
series
Which will be harder to
push water through
Twice as hard
When resistor are positioned in series the total
resistance just adds up
Total or EQUIVALENT resistance
4W
4W
4W
8W
8W
4W
8W
12 W
12 W
What is the total current running through each circuit
4W
2.0 A
8V
4W
4W
1.0 A
8V
Because the current can only go in 1 path, the
current through both resistors is the same
1.0 A
4W
1.0 A
1.0 A
8V
4W
1.0 A
Same flow rate everywhere
Pressure drops after resistance
50 Pa
100 Pa
100 Pa
0 Pa
Voltage drops after each resistor
8V
0V
4W
4W
1.0 A
8V
Voltage drop is given by Ohms Law
V = IR
4W
4W
4 V drop
4 V drop
1.0 A
8V
nd resistor?
What
the
voltage
drop
across
the
resistor?
What
Whatisisisthe
thevoltage
voltagedrop
between
across
thethe
resistors?
2first
Energy
4W
Energy
8W
24 V
A lamp with a resistance of 360 W and one
that is 144 W are connected in series to a
120 V circuit. Which burns brighter?
3 light bulbs each with a resistance of 2 W, are connected
to a 9V battery. What is the current and voltage at each
point along the circuit
Which bulb is the brightest?
9V
What happens if one bulb burns out?
9V
What happens if one bulb burns out?
9V
What happens to the brightness of a
bulb, if another is connected in series.
?
9V
9V
The bulbs are dimmer, the same
voltage with more resistance means…
less current
9V
9V
Which will be easier
to pump
water
through?
There
are Twice
the paths
open to the water
The resistance is only 1/2 , so twice
as much water will flow
Resistors can also be added in Parallel
Which circuit has the higher Resistance to
current flow?
4W
4W
4W
Equivalent Resistance
4W
4W
4W
4W
2W
4W
4W
4W
4/3 W
4W
4W
4W
4W
4W
4W
As more resistors are added
in PARALLEL,
the equivalent resistance
DROPS
R1
REQ
R2
=
R3
1
1
=
REQ
R1
+
1
R2
+
1
R3
+ etc…
Equivalent Resistance
4W
4W
4W
4W
1
1
1
=
+
2W 4W 4W
4W
4W
4W
1
1
1
1
=
+
+
4/3 W 4 W 4 W 4 W
What is the equivalent resistance of
a 10 ohm and 20 ohm resistor in series and parallel?
Page 559 3, 4, 5, 6, 9, 10, 11
due next class
What is the equivalent resistance
4W
REQ=2.7 W
8W
What is the total current
out of the battery?
I=3.3 A
9V
Is the current the same through each resistor?
Which resistor has the greatest voltage drop?
4W
8W
9V
9V
0V
Which resistor will have the greatest current?
4W
8W
9V
Will the flow rate be the same through both tubes?
Both tubes
are fed the
same
pressure
Which resistor will consume the greatest power?
4W
8W
9V
What would happen the the Voltage, Current, and Resistance if
another resistor was added in parallel?
24 W
4W
8W
9V
How does the brightness
change if another bulb is added
in series?
What changed?
½ Resistance
Twice current
battery drains
twice as fast
9V
9V
What if one burns
out?
9V
What if one burns
out?
9V
A light bulb with a thick
filament and one with a thin
filament are connected in
parallel to a battery.
Which will burn brighter?
9V
to other circuit breakers
Hot From
Power
company
Circuit Breaker
or fuse (15 A)
8 Amp
Microwave
7A
Fridge
unused outlet
to other circuit breakers
Hot From
Power
company
Circuit Breaker
or fuse (15 A)
8 Amp
Microwave
7 Amp
Fridge
1 Amp
light
to other circuit breakers
Hot From
Power
company
Circuit Breaker
or fuse (15 A)
8 Amp
Microwave
7 Amp
Fridge
1 Amp
light
What is the equivalent resistance of the
circuit below
4W
2W
6W
What is the equivalent resistance of the
circuit below
4W
4W
4W
4W
4W
4W
How much current would flow from a 100 V batttery?
Book problems
page 560: 16, 17, 27, 28, 29
What is the current out of the battery?
Which of these resistors will have
more current?
80 W
20 W
12 V
I1
40 W
4W
20 W
15 W
40 W
10 W
4W
24 V
What is the equivalent resistance of the
circuit below
4W
6W
20 W
10 W
2W
40 W
Book problems: p561 29-32
Energy
9V battery
Energy
9V battery
nd resistor?
What
the
voltage
drop
across
the
resistor?
What
Whatisisisthe
thevoltage
voltagedrop
between
across
thethe
resistors?
2first
Energy
4W
Energy
8W
24 V
8W
4W
9V
Energy
Energy
8W
Energy
4W
9V