5.1.9

# Power and Conservation

Test yourself

## 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
• $P=IV$
• We can combine this with Ohm's law:
• $V=IR$
• To rearrange the power equation to give:
• $P=I^2R=\frac{V^2}{R}$

### 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
• $E=Pt$

## 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.