Power and Conservation
Physics: AQA A Level, topic 5.1.9
Power in circuits
When current flows through a component with resistance, power is dissipated.
Power
- Power is the rate of transfer of energy.
- As current flows through a component with resistance, work is done. This work is a power output.
- Power could be heating something up, turning a motor, a buzzer making a noise or a bulb lighting up.
Calculation
- The equation for the power output in a circuit is:
- Power = current x voltage
- We can combine this with Ohm's law:
- To rearrange the power equation to give:
Energy
- Power is the flow of energy per unit time. We can find energy by multiplying power by time.
- The equation for energy is:
- Energy = power x time
Conservation Laws
In any circuit both charge and energy must be conserved. This means it cannot be created or destroyed.
Kirchhoff's first law
- The total current entering a junction is equal to the total current leaving a junction.
- This means that the same amount of current that leaves the cell must arrive back at the cell.
Kirchhoff's second law
- The total electromotive force in a series circuit is equal to the sum of potential differences across each component.
- Energy is put into the circuit by the voltage at the cell.
- When current flows through a component there is a voltage drop.
- The total of all the voltage drops across every component in the circuit must be equal to the amount of voltage put in by the cell.
Test yourself on Power and Conservation
What is the equation for power in a circuit?