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# Uses and cost of electricity

## Heating effect of electric current

A current flowing through a wire or any other conductor causes heating of the wire. Electrical energy is converted into thermal energy due to the resistance of the wire.

An electric kettle converts electrical energy into thermal energy to boil water.

A conductor with high resistance converts more electrical energy into thermal energy than a conductor with low resistance.

The heating elements in many home appliances (e.g. electric ovens, water heaters) are made of nichrome because it has a high resistance relative to other metals.

A large proportion of the electrical energy passing through it is converted into thermal energy.

This is a typical heating element. It has a high resistance.

## Kilowatt hours as a unit of energy

One kilowatt hour is the total energy consumed when an appliance with a power of $1\text{ kW}$ ($1000\text{ W}$) is used continuously for $1\text{ h}$.

A kilowatt hour is a unit of energy like the joule. The kilowatt hour is frequently used in everyday life (such as on energy bills), while the joule is the SI unit for energy and is used in a scientific context.

The kilowatt hour is the product of units of power (kilowatt, energy per time) and time ($1 \text{ kW}\times 1 \text{ h} = 1 \text{ kW h}$).

A washing machine with a power of $\Tred{500 \text{ W}}$ is used for $\Tblue{2 \text{ hours}}$. The energy it consumes is equal to $\Tred{0.5 \text{ kW}} \times \Tblue{2 \text{ hours}} = 1 \text{ kW h}.$

An electricity meter measures the energy consumption of a house in kilowatt hours.

## Cost of electricity

Electricity companies charge for the usage of electricity in terms of dollars (or other currencies) per kilowatt hour ($\/ \text{kW h}$).

The cost of using an electrical appliance is given by: \begin{align*}\Tgreen{\text{total cost}} =& \text{power in kilowatts } \times \text{time in hours} \\ & \times \text{cost per kilowatt hour}\end{align*}

A light bulb with a power of $100 \text{ W}$ is used for $10 \text{ h}.$ The cost of electricity is $\ \, 0.15 \, / \text{ kW h}$.

The cost of using the light bulb is: $$0.1 \text{ kW} \times 10 \text{ h} \times \ \, 0.15 \, / \text{ kW h} = \Tgreen{\ \, 0.15} = \Tgreen{15 \text{ c}}$$

An electricity meter measures the energy consumption of a house. Electricity companies charge the owners of the house based on the meter reading monthly.