Energy and Efficiency

Appliances transfer energy but not all the energy is transferred in useful ways. The choise of appliance is influenced by their efficiency and cost effectiveness.

(Most Kids Hate Learning GCSE Energy Names)

• Magnetic - Energy in magnets and electromagnets 
• Kinetic (Movement energy) - The energy in moving objects 
• Heat (Thermal energy) 
• Light - Also called radiant energy
• Gravitational Potential - Stored in energy in raised objects
• Chemical - Stored in fuel, foods and batteries 
• Sound - Energy released by vibrating objects 
• Electrical - Energy in moving changes or static electric changes
• Elastic Potenetial - Stored in stretched or squashed objects 
• Nuclear - Stored in the Nuclei of atoms

Different types of energy can be transferred from one type to another. Energy transfer diagrams show each type of energy, whether it is stored or not and the processes taking place as the energy is transferred.

The energy transfer diagram shows the useful energy transfer in a car.The car engine transfers chemcial energy, which is stored in the fuel, into kinetic energy in the enging and wheels.

This diagram shows the energy transfer diagram for the useful energy transger in an electric lamp. You can see that the electric lamp transfers or converts electrial energy into light energy.

Notice that these energy transfer diagrams only show the useful energy transfers. Car engines are also noisy and hot, and electric lamps also give out heat energy 

Sankey diagrams summaries all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved. 

Energy can be transferred usefully, stored or dissipated. It cannot be created or destroyed. Notice that 100 J of electrical energy is supplied to the lamp. Of this, 10 J is transferred to the surroundings as light energy. The remainder, 90 J (100J - 10 J) is transferred to the surroundings as heat energy.

• The energy transfer to light energy is the useful transfer
• The rest is 'wasted': it is eventually transferred to the surroundings, making them warmer. 
• This 'wasted' energy eventually becomes so spread out that it becomes less useful.

The efficiency of a device is the proportion of the energy supplied that is transferred in useful ways. You should be able to calculate the efficiency of a device as a decimal or as a percentage.

Electric Lamps

Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. Hower, most of the electrical energy is transferred as heat energy instead of light energy.

Modern energy-saving lamps and LEDS work in a different way: they transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energy-saving lamp. (next page)

From the diagram you can see that much less electrical energy is transferred, or 'wasted' as heat energy from the energy-saving lamp. Its more efficent than the filament lamp.

The efficent of a device such as a lamp can be calculated:

efficiency = useful energy out ÷ total energy in (for a decimal efficiency)

efficiency = (useful energy out ÷ total energy in) × 100 (for a percentage efficiency).    

The efficency of a filament lamp is 10 ÷ 100 = 0.10 (or 10 percent).

This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy.)

We calculate the amount of electrical energy transferred by an appliance and how much it costs tp run. This is useful for comparing the advantages and disadvantades of using different electrical appliances for a particular purpose.

Electrical enery calculations 

The amount of electrical energy trasferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh 

E= P x t 

• E is the energy transferred in kilowatt-hours, kWh 
• P is the power in killowatts, kW
• T is for the time in hours, h

Power is measured in kilowatts here instead of the more usual watts. To convert from W to K you must divide by 1,000

Time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600

Example calculation:

A 2 kW electrical fire is switched on for 3 hours. It uses 2 x 3 = 6 kWh of electrical energy.

Joules, watts and seconds 

You also use the equations E = P x t 

• E is the energy transferred in joules, J
• P is the power in watts, W 
• T is the time in seconds,s 

Electricity meters measure the number of units of electricity used in a home or other building. The more units used, the greater the cost. The cose of the electricity used is calculated using this equation:

total cost = number of units x cost per unit 

e.g. If 5 units of electricity are used at a cost of 8p per unit the total cost will be 5 x 8 = 40p

units (kWh) = power (kW) x time (h) so....

total cost = power (kW) x time (h) x cost per unit