Electrical energy
Since:
\({Power} =\frac{\text{energy~transferred}}{\text{time~taken}}\)
Then:
Energy = power x time
Hence:
Electric energy = electric power x time (t)
Electric energy = VI x t
Electric energy = VI t
This is usually remembered in the form:
Electric energy = ItV
Where:
Electric energy is in joules, J
I = current in amperes, A
t = time in seconds, s
V = voltage in volts, V
Key point
- Electric power P = VI
- Electrical energy = ItV
Example
What is the power of a small electric motor if a current of 2 A flows when connected to a 12 V power supply?
P = VI
I = 2 A
V = 12 V
P = 2 A x 12 V
P = 24 W
The power of the electric motor is 24 W.
Question
A light bulb is rated 60 W, 240 V. What does that mean?
When the voltage across the bulb is 240 V the bulb has a power of 60 W.
It converts 60 J of electrical energy into light and heat energy every second.
Question
A kettle has a power of 2.2 kW and is connected to mains voltage of 240 V.
- What current flows when the kettle is operating normally?
- What is the resistance of the kettle’s heating element?
1.
\({I} =\frac{\text{P}}{\text{V}}\)
P = 2.2 kW = 2200 W
V = 240 V
\({I} =\frac{\text{2200 W}}{\text{240 V}}\)
I = 9.17 A
2.
\({R} =\frac{\text{V}}{\text{I}}\)
V = 240 V
I = 9.17 A
\({R} =\frac{\text{240 W}}{\text{9.17 A}}\)
R = 26.2 \(\Omega\)
The resistance of the kettle’s heating element is 26.2 \(\Omega\)