2. Electric Power
Power is the energy transformed by a device per unit time:
The symbol for Power is P
and the unit is watt W,
note 1W = 1Js-1

4. Calculate the power supplied by the battery
The supply voltage is 12 V.
The potential drop across the lamp is 7.0 V
The current I = 2.0 A.
Calculate the power supplied by the battery
Calculate the power used as heat and light by the lamp.
Calculate the power used as heat by the resistor R.
Calculate the value of the resistance R

5. The potential drop across the lamp is 7.0 V The current I = 2.0 A.
The supply voltage is 12 V.
The potential drop across the lamp is 7.0 V
The current I = 2.0 A.
Calculate the power supplied by the battery
Power P = VI  P = 12 x 2  P = 24 watts.
Calculate the power transformed into heat and light by the lamp.
P = VI = 7 x 2 = 14 W
Calculate the power transformed into heat by the resistor R.
P = VI = = 10 W
Calculate the value of the resistance R

6. The two lamps are each rated 6 V : 24 W Each resistor is rated 10 Ω
The supply voltage is 12 V.
The two lamps are each rated 6 V : 24 W
Each resistor is rated 10 Ω
Calculate V1 and V2
Lamp(L1) in parallel with R1 = Lamp (L2) in parallel with R2
Therefore the voltage is split equal, i.e. V1 = V2 = 6.0 V
Calculate the rate of heat transfer in each of the 10 Ω resistors
Calculate the current supplied by the battery
Current through each 10 W resistor: from I = P/V; I = 3.6/6 = 0.6 A
Current through each lamp: from I = P/V; 24/6 = 4, I = 4 A
Current from battery I = = 4.6 A

7. 18 W
48 W
12 V
In this circuit the supply voltage is 12 V.
One lamp is rated 12 V:18 W and the other one is rated 12 V:48 W.
Which lamp will use the most energy?
Explain your answer.
The 48 W lamp
Examiners state that when you are given a question that has a 50:50 chance of success, then there shall be no opportunity to gain credits for that question.
In order to succeed you must be able explain your answer ie which lamp you chose.

8. Energy
18 W
48 W
12 V
The 48 W lamp
Merit

9. Excellence The lower wattage lamp has the greater resistance.
Energy (2)
48 W
18 W
12 V
In this circuit the supply voltage is 12 V.
One lamp is rated 12 V:18 W and the other one is rated 12 V:48 W.
Which lamp will use the most energy?
Explain your answer.
The 18 W lamp
The lower wattage lamp has the greater resistance.
Both lamps have the same current through them.
Energy = VIt (substituting IR = V into the equation)
Energy = I2Rt
Energy is directly proportional to R
So the 18 W uses the most energy.
You have to understand the phenomena and principles of physics to succeed.
To demonstrate your understanding of physics you have to answer questions with accurate explanations.
Excellence

10. Excellence The 18 W lamp Energy (2) Alternative answer 12 V 18 W 48 W
In this circuit the supply voltage is 12 V.
One lamp is rated 12 V - 18 W and the other one is rated 12 V - 48 W.
Which lamp will use the most energy?
Explain your answer
The 18 W lamp
Approximate working resistance of lamps
Excellence

11. (By the way, power strips are wired in parallel.)
Johnny “Danger” Powells uses one power strip to plug in his microwave, coffee pot, space heater, toaster, and guitar amplifier all into one outlet.
This is dangerous because…
(By the way, power strips are wired in parallel.)
25 A
Coffee Pot
Microwave
10 A
5 A
Toaster
P=VI gets too large - burns wire
1. The resistance of the kitchen circuit is too high.
2. The voltage across the kitchen circuit is too high.
3. The current in the kitchen circuit is too high.