Worksheet: Power and Ohm’
s Law
KEY
Name ___________________
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State Ohm's Law: _V=IR__
Give the units for: V _volt (v)__ ; I __amp (a)__ ; R __ohm ( )_______
Restate these units, using J, s, and/or c: V _J/C__ ; I _C/s_
State the electric power formula: _____P = IV_________
Give the unit: P _Watt (W)___ ; Restate, using J, s, and/or c: P _J/s_____
Another formula for calculating power is P = Work or Energy/time
Rearrange the formula to solve for energy: E = _Pt___
7. The kilowatt hour (The kw●hi
sauni
tformeas
uri
ng_________________.
)
is a unit for __energy___________ which Power Companies sell to their
customers. Why doesn't the power company use the MKS unit, watt
second instead? __The Ws is too small a unit to use conveniently. ______
For these problems, show the formula used:
8. What is the resistance of an electric frying pan that draws 11 amps when
connected to a 110 v circuit? V = IR R = V/I
R = 110 v / 11a R = 10 ohm
What is the power of the frying pan? P = IV = (11 a) ( 110 v) = 1200 W
9. If a 120 v line to a socket is limited to 15 a by a fuse, will it operate a
1200 w dryer without blowing the fuse? Calculate current and see if it exceeds 15 a or
calculate power and see if it exceeds 1200 W.
P = IV
I = P/V
I = 1200 W / 120 v = 10 a
10 a is less than 15 a so it will work!
If you calculate power, you get 1800 W as the maximum it can handle. 1200 W is less than that, so again it
will work!
10. If the power company charges us 8 cents/ kw●hfor electricity, what
does it cost to operate the 1200 W hair dryer for 15 minutes?
$ = E (in kWh) x Cost Rate (in $/kWh) OR $ = Pt x Cost Rate
Convert power to kw and time to hour first.
$ = 1.2 kW x .25 h x $0.08/kWh = $0.02 or 2 cents
11. If the power company sells electrical energy at 11 cents/ kw●h, how much
does it cost to run a 100w radio for 3.0 hours?
$ = E (in kWh) x Cost Rate (in $/kWh) OR $ = Pt x Cost Rate
Convert power to kw first.
$ = 0.1 kW x 3.0 h x $0.11/kWh = $0.03 or 3 cents
12. What is the resistance of a 150 W light bulb running on a 120 V circuit?
R=?
P = IV
V = IR
V = IR (you need to calculate I first)
150 W = (I) x (120 v)
I = 1.25 a
R = V/I
R = 120 v / 1.25 a
R = 96 ohm
Do At Home: Find the power rating on 5 different electrical appliances in your
home. List these below. Look on the back of household appliance to find these answers. Examples –
Hairdryer = 1250 W, Alarm Clock = 5 W, Clothes Dryer = 5,000 W, etc.
PHYSICSFundamentals
© 2004, GPB
9-07a