Energy in the home

 

Common energy stores

Energy is the potential for a system to do work. Energy can exist in several forms:

 

• Thermal (heat)
• Gravitational potential (energy due to height)
• Kinetic (energy due to movement)
• Electromagnetic including visible light
• Sound
• Electrical
• Elastic potential
• Nuclear
• Chemical (energy stored in chemical bonds)

 

 

Some examples of common energy stores

 

Energy transfers

Energy can be transferred in 4 ways between the energy stores:

 

• Electrically - through a flow of charge (electricity).
• By heating - when there is a change in temperatures.
• Mechanically - when a force is applied to move an object through a distance.
• By radiation - when energy travels as a wave, for example as light or sound.

 

Common energy stores and transfers in the home

 

Lamp Energy is transferred when an electrical current flows through an object.   Electric charges flow through the tungsten filament inside the bulb. Energy is transferred to the thermal energy store of the filament bulb and causes the bulb to get hot.   Energy transfers from the thermal energy store of the bulb by heating and light.

Kettle Energy is also transferred when an object is heated. A kettle takes in electrical energy and transfers it heat in the element. This in turn heats up the water.   Charges flow through the heating element of the kettle. Energy is transferred to the thermal energy store of the element which causes the element to get hot.   Energy transfers from the hot element to the water which causes the water to heat up.

 

Electrical devices

Power is the rate at which energy is transferred by an appliance over a period of time. We can represent this using the following equation:

 

 

Where:

 

• Power – Measured in watts (W)
• Energy – Measured in joules (J)
• Time – Measured in seconds (s)

 

 

Calculating energy transfers

If you look at the electrical appliances in your home, you might find a label that show the power rating of the appliance.

Power is the amount of energy transferred each second. Power is measured in watts (W).

 

We can modify the equation above to calculate the energy transferred:

 

 

For example,

A 1000 Watt microwave oven is switched on for 60 seconds. How much energy is transferred to the microwave oven?

Energy transferred (J) = Power (W) x Time (s)
E = P x T
E = 1000 (W) x 60 (s)
E = 60,000 (J)

So, the energy used by this appliance per minute will be 60,000 J. If we convert this to kJ, the energy used will be 60 kJ (60 kilojoules).

 

 

Watts to kilowatts

Sometimes the energy of an electrical appliance is shown in kW (kilowatts) rather than in W (watts). This usually happens when an appliance uses large amounts of energy per second.

1 kW = 1000 W

Below is a table showing the conversion between power in W (watts) and power in kW (kilowatts) for different appliances:

 

Electrical appliance	Power in W	Power in kW
Kettle	2000	2.0
Microwave	3200	3.2
Oven	16000	16

 

 

Energy bills

The amount of energy that our homes use is measured by meters that energy companies keep track of. The readings from these meters are used by the companies to make an electricity or gas bill. The usual unit for energy is Joules (J), but the companies use kilowatt-hour (kWh) instead. So, they charge us for every kWh of energy that we use.
1 kWh is the amount of energy transferred to a 1kW appliance in 1 hour.

 

To work out the amount of energy in kWh used by an electrical appliance, we use the equation:

Energy transferred (kWh) = power(kW) x time(h)

For example,

1 kWh = 1 kW x 1 h

 

 

Calculating how much to pay

Household bills contain information such as the cost per kilowatt-hour, the number of kilowatt-hours used, and the total cost of the energy used.
They calculate the energy bill using the following equation:

Total cost (£) = amount of energy used (kWh) x cost per kWh

 

Summary:

 

• Many systems in our homes have energy stored in them.
• Energy can move from one energy store to another.
• Some devices can transfer energy faster than others because they have more power.