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4th Nine-Weeks
Week # 07
Physics
CW 13: Electric Power – practice test
Name: ___________________________________ Date: _____________ Period: _________
You must show the formula selected and the work that leads to your answer to receive credit. The
answer must include the appropriate unit.
_________________ 1. An air conditioner draws 13 amps from a 220 V circuit. What is the
power?[2860 W]
_________________ 2. A 600 W electric coffee maker draws 4 amps of current. What is its
resistance? [37.5 Ώ]
_________________ 3. A 60 W light bulb operates on 120 V. What is its resistance? [240 Ώ]
_________________ 4. A flashlight has an 8 W light bulb and runs off a 9 V battery. How many
amps does it use? [0.889 A]
_________________ 5. A 100 W light bulb has a resistance of 145 Ώ . How many volts is it
designed for? [120.4 V]
_________________ 6. A hot water heater has a resistance of 17Ώ and draws 15 amps. What is
its power? [3825 W]
_________________ 7. A flashlight bulb draws 1.5 amps at 3 volts. What is the power? [4.5 W]
_________________ 8. An electric coffee pot has a power of 400 watts. The power line voltage
is 120 V. What resistance should the heating element have? [36 Ώ]
_________________ 9. How much current should a 3 kW clothes dryer draw from a 240 volt
circuit? [12.5 A]
_________________10. How many amps does a 50 watt automobile headlight draw in a 12 volt
circuit? [4.17 A]
_________________11. The resistance of a toaster’s heating element is 20 Ώ. It runs on
120 volts. What is the power? [720 W]
_________________ 12. A stereo provides 20 watts of power to an 8 Ώ speaker. How much
current flows? [1.58 A]
_________________ 13. A 2 ohm resistor is connected to a 6 V battery. It is immersed
in 0.250 kg of water at 30 °C for 15 minutes. What will be the
final temperature of the water? [45.5 °C]
Steps:
(a) calculate power in Watts
(b) convert to J/s
(c) convert time to Joules
(d) find final temperature
_________________ 14. A 5 ohm resistor is connected to a 12 V battery. It is immersed
in 0.700 kg of water at 20 °C. How many minutes does it take to
heat the water to 55 °C? [59.4 min]
_________________ 15. An electric toy car motor run’s off a 9 V battery. The force
needed to roll the car is 5 N. If the motor supplies 1.2 amps of
current, what is the top speed of the car? [2.16 m/s]
_________________ 16. An electric car motor run’s off a 12 V battery and requires 80 N
of force to push it across the ground. If the top speed of the
car is 27 m/s, how much current does the motor supply?
[180 A]
_________________ 17. A grain elevator motor runs on 240 volts and draws 15 amps of
current. How many minutes will it take to lift 6000 kg of grain
18 meters? [4.9 min]
Steps:
(a) Calculate power in Watts
(b) convert to J/s
(c) Calculate the amount of work done
(d) convert Joules to time
_________________ 18. An elevator motor runs on 240 volts and draws 25 amps of
current. How many seconds will it take to lift 140 kg to a height
of 370 meters? [84.6 s]
_________________ 19. A 12 volt battery has an internal resistance of 0.1 Ώ . What is the
maximum power this battery can deliver to a load? For maximum power
R = r. [360 W]
I
12 volts
E
R
P
load
R = 0.1 Ω
(load)
Internal
resistance
Circuit
r = 0.1 Ω
Internal
resistance
20. A 12 volt battery is connected to a light bulb by wires with a resistance of 2 Ώ. It is
connected to a 6 Ώ light bulb.
_________________ (a) What power is available to the light bulb? [13.5 W]
_________________(b) If percent efficiency is (useful power/total power) x 100, what is the
efficiency of the battery/light bulb system? [75%]
12 volts
r=2Ω
wires
R=6Ω
(light bulb)
I
E
R
P
Light bulb
Wires
Circuit
21. A 27 volt battery is connected to a light bulb by wires with a resistance of 2 Ώ. It is
connected to a 50 Ώ light bulb.
_________________ (a) What power is available to the light bulb? [13.52 W]
_________________(b) If percent efficiency is (useful power/total power) x 100, what is the
efficiency of the battery/light bulb system? [96%]
I
E
R
P
R = 50 Ω
(light bulb)
27 volts
Light bulb
Wires
r=2Ω
wires
Circuit
22. A heater requires 4 V and draws 0.25 amps.
(A) Complete the chart:
dropping resistor
12 V
heater
I, (A)
E, (V)
R, (Ω)
Dropping resistor
Heater
Circuit
__________________ (B) What value of dropping resistor is need so the heater will run
from a 12 volt car battery?
23. A CD player requires 9 V and draws 0.15 amps.
(A) Complete the chart:
dropping resistor
12 V
CD player
I, (A)
E, (V)
R, (Ω)
Dropping resistor
CD player
Circuit
__________________ (B) What value of dropping resistor is need so the CD player will run
from a 12 volt car battery?