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. ELECTRIC POWER AND HEAT LOSS
The rate at which heat is dissipated in an electric circuit is referred to as the power loss.
Units: watts (W)
P = V I =I2 R =
P = V I
P = I2 R

5. 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

6. 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

7. 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

8. 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.

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

10. 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.

11. 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

12. (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.

13. Calculate the power output of the lamp.
Circuit Solving
Show R = 2 W.
Calculate the power output of the lamp.
12 V R
1 W
6 A
2 A
1. Show R = 2 W
find the potential drop (V1W ) across the 1 W resistor then the resistance.
Calculate the power output of the lamp.

14. A CD player’s specifications are 4. 0 V : 3. 5 W
A CD player’s specifications are 4.0 V : 3.5 W . Show the CD player has a resistance of 4.6 Ω.
The CD player is connected in parallel with the 2.0 Ω resistor. Calculate the new total resistance of the circuit.
1. Finding the current (I) when the player is operating at full power: P = VI 2.

15. A CD player’s specifications are labeled 4.0 V : 3.5 W .
The total resistance of the CD circuit is 5.4 Ω.
The equivalent circuit is drawn as shown.
Show the current in the equivalent circuit is 2.2 A.
Use the current to calculate the voltage across the 4 W resistor and state why the CD player will not work.
The two resistors in the voltage divider are replaced with a 12 Ω rheostat.
Re-draw the circuit with the rheostat and show how the rheostat could be used to make the CD work.
12 V
R = 5.4 W
CD circuit
equivalent circuit

16. CD supply voltage it too low
A CD player’s specifications are labeled 4.0 V : 3.5 W .
Show the current in the equivalent circuit is 2.2 A.
Re-draw the circuit with the rheostat and state why the CD could now work.
Calculate the voltage across the 4 W resistor and show why the CD player will not work.
Adjust the slide on the rheostat until the voltmeter reads 4 volts
Calculating the voltage across R1
VR1 = IR = 2.2 x 4
VR1 = 8.8 V
Voltage across CD player
12 V V = 3.2 V
CD supply voltage it too low
12 V
R = 5.4 W
equivalent circuit