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# AQA A-level Physics Electric energy and power

This page covers the following topics:

Electric power is the rate at which energy is transferred. It is measured in Watts, where 1000 W = 1 kW. Power is calculated by the following formula: Power = Work Done/time, therefore each Watt is equal to 1 J of energy per second. The amount of money payed for electricity is calculated based on the amount of electrical energy transferred to an appliance of a specific power and the time during which it is switched on. The unit used to calculate the amount that must be paid for electricity is the kilowatt-hour (kWh), where 1 kWh is the energy converted by a 1 kW applicance used for an hour.

The relationship between power, voltage and current is given by: Power = Voltage Ć Current.

Ohm's Law can be used with the relationship of power, voltage and current to come up with a different relationship betweeen power, current and resistance, given by P = IĀ²R.

Energy is transferred to an object to do work on it and is calculated in Joules. Work is done on an object whenever a force moves it. The work done can be calculated in Joules using the following formula: Work Done = Force Ć distance moved in direction of force. The efficiency is the proportion of the energy or power transferred to an object which is used in useful ways. It is calculated using the following ratio: Efficiency = useful energy or power out/total energy or power in. Since efficiency is a ratio, it has no units.

The energy transferred to an appliance can be related its power rating and amount of time for which it is switched on through the following formula: Power = Energy transferred/time.

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Calculate the potential difference across an appliance with a power rating of 400 W and a current of 1.2 A through it.

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A wall socket supplies 240 V. Given that a laptop has a power rating of 50 W, calculate the current flowing through the laptop.

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A vacuum has a power rating of 1500 W. Given that there is a current of 12 A flowing through it, calculate the resistance flowing through it.

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Given that the power rating of an appliance is 950 W and its resistance is 750 Ī©, calculate the current flowing through it.

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An electric motor is used in a lift. Given that the power rating of the lift is 6500 W and the lift is carried up in 50 s, calculate the energy transferred in the motor.

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Given that the power rating of a laptop is 60 W, calculate the amount of energy it transfers in 20 minutes.

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A light bulb has a power rating of 90 W. Calculate the energy it transfers in 2.5 minutes.

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