Download Chapter 19 Review Current and Resistance

Chapter 19
Review
Current and
Resistance
1. A current of
2 amps flows for
30 seconds. How
much charge is
transferred?
I = q/t
It = q
2 x 30 = q
60 C = q
2. A charge of
10 coulombs is
transmitted over a time
of 20 seconds. What is
the current over this
period?
I = q/t
I = 10 C/20 s
I = 0.5 Amps
3. When you flip a
switch to turn on a
light, how long does
it take the electric
field in the wire to
develop?
Close to the
speed of light.
4. What is drift
speed? Why is it so
much slower than the
speed of the electric
field?
Drift speed is the actual
speed the electrons are
moving in the conductor.
It is much slower because
the electrons must follow
a zig-zag path.
5. What is the
difference in
AC and DC?
AC is alternating current.
In AC the direction of the
current changes rapidly.
DC is direct current. In DC
the current moves in the
same direction all the time.
6. If the current in a
wire is 10 A and the
resistance is 12 Ω,
what is the potential
difference across the
circuit?
V = IR
V = 10 x 12
V = 120 V
7. A motor of resistance
5 Ω carries a current of
12 A. What is the
potential difference
provided by the voltage
source?
V = IR
V = 12 x 5
V = 60 V
8. A light bulb with a
resistance of 144 Ω
is in a 120 V circuit.
What is the current in
the circuit?
V =IR
120 = I(144)
0.833 A = I
9. A 12 V battery is
connected to a heating
element. If the current
is 0.5 A, what is the
resistance of the
heating element?
V =IR
12 = 0.5 R
24 Ω = R
10. What four
factors affect the
resistance of a
conducting wire
and what effect do
they have?
1. Length - the longer the wire, the
higher the resistance
2. Cross section – the “fatter” the
wire, the lower the resistance
3. Material – some materials are
better conductors than others
4. Temperature – the higher the
temperature, the higher the
resistance
11. What is a
superconductor?
What is critical
temperature?
A superconductor is a material
that will conduct electricity with
zero resistance.
The critical temperature is the
temperature below which a
superconductor has zero
resistance.
(These critical temperatures
tend to be VERY low.)
12. If the resistance in a
circuit is doubled while the
voltage is held constant,
how does this change the
power output of the
resistor?
By V =IR, doubling the R
while V is held constant
would cut I in half.
By P = IV, if I is cut in half
while V is held constant,
the power output P is
cut in half.
13. If the voltage in a
circuit is doubled while
the resistance is held
constant, how does this
change the power
output of the resistor?
By V = IR, doubling V
while R is held constant
would cause I to double.
By P = IV, if I is doubled
and V is doubled, power
output P would be
quadrupled!
14. If the current in a
circuit is doubled while
the resistance is held
constant, how does this
change the power
output of the resistor?
By V = IR, doubling I while
R is held constant would
cause V to double.
By P = IV, if I is doubled
and V is doubled, power
output P would be
quadrupled!
15. If a circuit with a
current of 15 A has a
potential difference of
120 V, what is the
power output of the
circuit?
P = IV
P = 15 x 120
P = 1800 W
16. If a resistance of
240 Ω is in a 120 V
circuit, what is the
power output of the
resistance?
V = IR
120 = I(240)
I = 0.5 A
P = IV
P = 0.5 x 120
P = 60 W
17. A 1500 W hair dryer
is plugged into a 110 V
outlet. What is the
current of the circuit?
What is the resistance
of the hair dryer?
P = IV
1500 = I(110)
I = 13.6 A
V = IR
110 = 13.6 R
8.1 Ω = R
18. A device uses 4 A
when plugged into a
120 V outlet. What is the
total cost of running this
device for 5 hours if the
cost of energy is $0.12
per kilowatt·hour?
P = IV P = 4 x 120
P = 480 W
480 W / 1000 = 0.480 kW
0.480 kW x 5 hours =
2.4 kWh
2.4 kWh x 12 cents =
28.8 = 29 cents
19. A device uses 6 A
when plugged into a
120 V outlet. What is the
total cost of running this
device for 2 hours if the
cost of energy is $0.11
per kilowatt·hour?
P = IV P = 6 x 120
P = 720 W
720 W / 1000 = 0.720 kW
0.720 kW x 2 hours =
1.44 kWh
1.44 kWh x 11 cents =
15.84 = 16 cents