1. What we will do today
State relationship between power, energy and time and carry out examples on this.
State the different expressions for electrical power and carry out calculations on these.

2. Power, Energy & Time
The power of an electrical appliance is the rate of energy transformation per unit time:
P = E t
P – Power (W)
E – Energy (J)
t - time (s)

3. Example 1
An electric fire transforms 2000 joules of energy each second.
What is the power rating of the fire?
How much energy is transformed in 15 s?
Solution
a) 2000 W
b) E = Pt
= 2000 x 15
= J

4. Equations for Power

5. Additional equations for power
Power = current x voltage
P = IV
Power = current2 x resistance
P = I2R
Power = voltage2 / resistance
P = V2 R

6. Tackling Power Problems
List all the quantities and their values.
Decide which equations you need to obtain your answer from the following (all given in relationship sheet):
P = IV
P = I2R
V = IR
P = V2 R
P = E t

7. Example 1
P = ?
R = 10 Ω
I = 2 A

8. Example 2
P = ?
R = 53 Ω
V = 230 V

9. Example 3
P = 1500 W
R = 35.5 Ω
I = ?

10. Example 4
P = 24 W
R = 6 Ω
V = ?

11. Example 5
P = 23 W
I = 100 mA
V = ?

12. 2003 Qu: 9

13. 2006 Qu: 11

14. 2005 Qu: 14

15. 2003 Qu: 24

16. 2005 Qu: 25