1. Natural Sciences Grade 7
Term 3: Energy and Change
Energy transfer to surroundings

2. Topic 5 Energy transfer to surroundings
Useful and wasted energy [part 2]
Natural Sciences - Grade 7

3. Natural Sciences - Grade 7
Useful energy – Energy systems [e.g. mechanical devices and electrical appliances], are designed to provide useful energy outputs.
For example: We use a light bulb to light up a dark room,
so therefore the light energy can be regarded as the useful energy.
Most of the energy produced by the light bulb is in the
form of heat. Heat can be regarded as wasted energy.
Energy flow in systems
In order to improve the energy efficiency of machines and devices we need to determine the amount of:
Input energy: This is the amount of energy needed for the device to operate properly.
Useful output energy: This is the amount of useful energy produced by the device.
Wasted energy: This is the amount of energy that is not used for the required output. It is converted to another form of energy.
Natural Sciences - Grade 7

4. Example of wasted and useful energy
For example: A 100W light bulb needs 100J of electrical energy per second to function properly, yet it only produces approximately 5J of light energy per second. That means that 95J of heat energy per second are wasted!
The useful output energy in a system is always less than the input energy, because we seldom get an energy system that is 100% effective. Some of the input energy is transferred to the surroundings and is not useful.
The more input energy converted to useful energy, the greater the energy efficiency of the system. An energy efficient system produces less ‘wasted’ energy.
Natural Sciences - Grade 7

5. Energy inefficient power plants
Most electricity in South Africa comes from power plants.
Only about one third of the energy from the coal is converted to useful energy.
By the time the electrical energy reaches us from the power plant, it has already undergone several energy conversions.
Every time a conversion occurs, some energy is ‘wasted’. This means that it does not reach the point (a home or building).
Natural Sciences - Grade 7

6. Reducing ‘wasted’ energy
We need to reduce wasted energy to improve energy efficiency. This is important as fuel and electricity are becoming increasingly expensive.
Did you know?
- Energy efficiency of cars:
Cars waste about 65% of energy from the fuel in the form of
heat.
Natural Sciences - Grade 7

7. Natural Sciences - Grade 7
Key concepts
‘Wasted’ energy must be reduced in order to save our energy sources such as coal and fuel.
The more input energy converted to useful energy, the greater the energy efficiency.
In a solar collector, a black dull surface absorbs sunlight. Solar heat is absorbed by copper pipes. Insulating materials trap the heat.
The sun can be used to heat water and produce electricity. Energy produced by the sun is called solar power.
Heat moves from an area of higher heat to an area of lower heat.
An insulator is a material that does not easily transfer heat energy.
Natural Sciences - Grade 7