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4th Nine- Weeks
Week # 07
Physics
Unit 19 Electric Power
DATE/DAY
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A
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TOPIC
ASSIGNMENT
Notes – Electric Power
CW 01
Notes completed
CW 02
Review for Test
CW 03
Ώ
R
Review for Test
TEST 04: Electric Power
LAB: Electric Power
P = IV
P = I2 R
P = V2/R
____________________ 1. A hair dryer draws 10 amps from a 120 volt electrical outlet.
What is the power?
____________________ 2. A toaster’s heating element has a resistance of 20 Ω. What is the
power? It runs on 120 V.
____________________ 3. A flashlight’s bulb has a resistance of 2 Ω and 1.5 amps of current go
through the bulb. What is the power of the bulb?
Energy
P = ___________ =
Time
Power = (force) (velocity)
Joules
____________
seconds
heat energy (Q) = m c Δt
work = energy = f x d [force x distance]
cwater = 4186 J / kg °C
4. How many minutes will it take this device to heat 2 kg of water from 20 °C to 100 °C? The
device runs on 120 V and has a resistance of 2 Ω.
The following are the steps you should take to answer the question:
_____________ (A) Calculate the power of the device.
_____________ (B) How many Joules/ seconds is this?
_____________ (C) Calculate the heat energy required to heat up the water.
_____________ (D) Calculate the time [convert Joules to minutes]. This is the final
answer to the question.
5. An electric motor draws 10 amps from a 120 V source. The motor is used to lift a 50 kg load to
a height of 20 m. How many seconds will it take?
The following are the steps you should take to answer the question:
_____________ (A) Calculate the power of the motor.
_____________ (B) How many Joules/ seconds is this?
_____________ (C) How much work was done?
_____________ (D) Calculate the time [convert Joules to seconds]. This is the final
answer to the question.
____________________ 6. An electric golf cart draws 5 amps from a 12 V battery. The force
needed to roll the golf cart is 40 N. What is the speed of the golf
cart?
7. Internal Resistance of Batteries: Batteries have resistance called internal resistance. This
resistance is mostly from used up chemicals. Fresh batteries have very low internal resistance
while batteries that are nearly “dead” have high internal resistance.
R = the load (light bulb, motor, CD player, cell phone, calculator, etc.)
r = internal resistance
12 V
R=3Ω
r = 0.2 Ω
A. Complete the chart
V
I
R, (Ω)
Load (R)
Internal
Resistance (r)
Circuit
B.
The useful power is _________________ watts.
The wasted power is ________________ watts.
C. Calculate the efficiency.
E=
Useful power
-------------------------------- x 100 =
Total power
___________________ %
Matching power source to power load:
For maximum power: r = R. This condition has an efficiency of 50%
For maximum efficiency:
R>>>r
(R much greater than r)
P
8. A 12 volt car battery has an internal resistance of 1Ω. It is used to play a car radio that draws
0.5 A of current.
A. Complete this chart:
V
I
R, (Ω)
P
Load (R)
Internal
Resistance (r)
Circuit
__________________B. How many volts are available to the load?
__________________C. How many volts are used up overcoming internal resistance?
__________________ D. What is the percent efficiency of this circuit?
9. Same car battery is used to try to start the car. The starter motor has a resistance of 0.05 Ω.
A. Complete the chart:
V
I
R, (Ω)
Load (R)
Internal
Resistance (r)
Circuit
__________________B. How many volts are available to the load?
__________________C. How many volts are used up overcoming internal resistance?
__________________ D. What is the percent efficiency of this circuit?
P
Dropping Resistors
A crude way of supplying low voltage from a high voltage source is to use a dropping resistor
in series. Energy is wasted as heat in the dropping resistor.
_________________10. A 12 volt battery is used to supply power to a 3 volt GPS system. The
GPS has a resistance of 500 Ω. What value dropping resistor is
needed?
V
I
R (Ω)
Circuit
_________________11. A CD player runs on 4.5 V and draws 0.2 amperes of current. What value
resistor should be placed in series with the CD player so it can be powered by
a 12 V battery?
V
I
R (Ω)
Circuit
_________________12. A cell phone requires 1.5 V and draws 0.05 amps of current. What value
of dropping resistor is needed so it will run from a 12 volt car battery?
V
I
R (Ω)
Circuit