Energy & Power Formulas
This lesson covers:
- The equation E = Pt
- The equation E = VIt
- The equation E = QV
- The equation P = IV
- The equation P = I2R
Do I have to remember all of these formulas?
It depends on your exam board and which exam you're doing. You usually have to memorise one or two and are given the rest - but it's important to check your specification or ask your teacher.
There are 3 formulas for energy |
Formula 1: Energy (E) = Power (P) x Time (t) E = Pt |
Formula 2: Energy (E) = Voltage (V) x Current (I) x Time (t) E = VIt |
Formula 3: Energy (E) = Charge (Q) x Voltage (V) E = QV |
Let's try using each of these formulas one by one.
An 800 W hairdryer is used for 30 seconds.
How much energy is transferred in total? Give your answer in Joules.
J
|
A circuit with a 12 V battery, and a current of 6 A is left on for 20 s.
How much energy is transferred in total? Give your answer in Joules.
J
|
The potential difference between the two terminals on a battery is 6 volts.
How much work (energy) is required to transfer 12 coulombs of charge across the terminals?
J
|
There are 2 different formulas for power: |
Formula 1: Power (P) = Current (I) x Voltage (V) P = IV |
Formula 2: Power = Current2 x Resistance P = I2 × R |
Again, let's try these formulas one by one.
A circuit has a potential difference of 12 V, and a current of 15 amps.
Calculate the power of the circuit.
W
|
A circuit with has a current of 3 A, and a total resistance of 30 Ω.
What is the power of the circuit?
W
|
Now let's try some harder questions, where you might have to rearrange the formulas.
A handheld fan draws a current of 6 A. Over 20 seconds it transfers a 1200 J of energy.
What is the voltage of the battery that the fan uses?
V
|
A circuit has a power of 20 W and a current of 4 A.
What is the resistance of the circuit? Give your answer to 3 s.f.
Ω
|
A component in a circuit has a potential difference of 5 V across it, and receives 30 J of energy.
How much charge passes through the component?
C
|
A circuit has a potential difference of 12 V, and produces 36 W of power.
Calculate the current of the circuit.
A
|
A lamp with a power rating of 60 W transfers 2520 J of energy.
How long was the lamp on for?
s
|