2.4.2a Energy transfers in everyday appliances

Appliances are designed to bring about energy transfers

1. Everyday appliances are built to produce a particular energy transfer; a kettle raises the thermal energy store of water, while a fan raises the kinetic energy store of its motor and blades.
2. The electrical supply can be a battery or the alternating potential difference of the ac mains.
3. In a battery-powered appliance the chemical energy store of the battery decreases, and energy is transferred electrically to the appliance.
4. In a mains appliance energy is transferred electrically from the ac mains to the appliance.
5. An appliance never creates energy; it only transfers it from one store to another.

Energy transferred grows with power and with time

1. A power of $1W$ means $1J$ of energy is transferred every second, so a more powerful appliance transfers more energy each second.
2. The energy an appliance transfers depends on its power and on how long it is switched on, linked by $E=Pt$, where $E$ is energy transferred in joules ($J$), $P$ is power in watts ($W$), and $t$ is time in seconds ($s$).
3. For the same time, a higher-power appliance transfers more energy.
4. For the same power, leaving the appliance on for longer transfers more energy.

Motors gain kinetic stores, heaters gain thermal stores

1. Different appliances are designed to raise different useful energy stores.
2. In appliances with an electric motor, energy is transferred electrically to the kinetic energy store of the moving parts; examples include an electric toothbrush, a washing machine, a fan and a vacuum cleaner.
3. In heating appliances, energy is transferred electrically to thermal energy stores; examples include a kettle, a toaster, an electric oven, an electric heater and a hairdryer.
4. A full description names the supply and the store that changes, for example: a battery-powered fan transfers energy electrically from the chemical energy store of the battery to the kinetic energy store of the motor and blades.

Power rating and the change in stored energy

1. Power ratings are shown in watts or kilowatts, and a high-power appliance causes a greater change in stored energy each second than a low-power one.
2. A $3000W$ kettle transfers $3000J$ each second, mostly to the thermal energy store of the water and kettle, while a $10W$ lamp transfers only $10J$ each second.
3. Run for the same time, the kettle transfers far more energy because its power rating is greater.
4. Total energy also depends on time, so a low-power appliance left on for a long while can transfer more energy overall than a high-power appliance used briefly.

2.4.2b Work done by an electric current: E = P t and E = Q V

Work is done when charge flows through a component

1. When a circuit is complete, charge flows through the components, and as it passes through a component energy is transferred.
2. In a lamp, electrical work transfers energy from the battery to the lamp, which then transfers energy to the surroundings by light and by heating.
3. The energy transferred depends on how much charge flows and on the potential difference across the component.

1. A larger potential difference means more energy is transferred for each coulomb that passes through the component.
2. This is why a potential difference of $1V$ means $1J$ is transferred for every $1C$ of charge.

Energy from power and time

1. The energy transferred by electrical work can be found from the power and the time: $E=Pt$, where $E$ is energy transferred in joules ($J$), $P$ is power in watts ($W$), and $t$ is time in seconds ($s$).
2. Power is the rate of energy transfer, so a device with greater power transfers more energy each second; a $2000W$ kettle transfers $2000J$ every second.

Energy from charge and potential difference

1. The energy transferred can also be found from the charge that flows and the potential difference: $E=QV$, where $E$ is energy transferred in joules ($J$), $Q$ is charge flow in coulombs ($C$), and $V$ is potential difference in volts ($V$).
2. The energy transferred increases if more charge flows.
3. It also increases if the potential difference is larger, because each coulomb then carries more energy; raising the potential difference across a motor means every coulomb transfers more energy to it.

Power links potential difference and current

1. Power depends on the potential difference across a device and the current through it: $P=IV$, where $P$ is power in watts ($W$), $I$ is current in amperes ($A$), and $V$ is potential difference in volts ($V$).
2. A higher potential difference means each coulomb transfers more energy, and a higher current means more charge flows each second, so raising either one raises the power.
3. Power is also the energy transferred over a given time, $P=\frac{E}{t}$, which is the same relationship as $E=Pt$ rearranged and shows that power measures how quickly energy is transferred.