Week 17 · Theme 3, Unit 3 Energy and Fuels, Concept 3.1 Devices and Energy (B) · 3 lessons
2
Energy in, energy out
Discovery Education Science P4: Term 2, Unit 3, pp. 11–25
GoalName the energy going into and coming out of everyday devices, and tell the useful output from the wasted output.
New words
energy input · the energy that goes into a deviceenergy output · the energy that comes out of a devicewasted energy · energy that comes out in a form we do not want, usually heat
Explanation
Every device has an energy input and an energy output. The input of a table lamp is electrical energy. The outputs are light and heat. The light is the useful output because it is the job of the lamp. The heat is not wanted: we call it wasted energy, but it is not lost, it spreads into the air. A hair dryer’s input is electrical energy. Its outputs are heat, the motion of the fan and moving air, and sound. The noise is not part of its job. In a chain, at every link, some energy escapes as other forms, mostly heat. Engineers try to make devices that waste less energy. These are called efficient devices. An efficient lamp turns more of its input into light and gives out less heat.
Device
Energy in
Useful energy out
Other energy out
table lamp
electrical
light
heat
hair dryer
electrical
hot moving air
sound
radio
electrical
sound
heat
fan
electrical
moving air
sound, heat
Worked examples
A kettle has electrical energy in and heat out, and that heat is useful because it warms the water. A bulb in a lamp has electrical energy in; light is useful and heat is wasted.
Mariam compares two lamps. Each takes in 10 units of electrical energy. Lamp A gives 2 units of light and 8 units of heat. Lamp B gives 7 units of light and 3 units of heat. Both add up to 10. Lamp B is more efficient: it wastes only 3 units.
A washing machine takes in electrical energy. It gives out the motion of the drum (useful), and also some heat and a humming sound (not the job of the machine).
Class activity
Input and output. With a grown-up, choose four devices at home that are switched on safely: a lamp, a fan, a radio, a mobile phone. Record in a table: Device, Energy in, Useful energy out, Other energy out. Ask the grown-up which devices feel warm after being on for a while (you never touch hot parts). Which device wastes the most as heat?
Practice
1
What is the energy input of a radio? What is the useful output?
Input: electrical energy. Useful output: sound.
2
A lamp takes in 20 units of energy and gives 5 units of light. How many units come out as heat?
20 − 5 = 15 units.
3
Which lamp is more efficient? Say the letter.
Lamp
Energy in
Light out
Heat out
A
10 units
3 units
7 units
B
10 units
8 units
2 units
B: more of its energy becomes light.
4
Is the heat from a lamp “lost”? Explain.
No. It is wasted for the lamp’s job, but the energy has moved into the air.
