Year 9 Science · Unit 3 · Lesson 3
Foundation Worksheet
Learning Goals
Fill the gap
Choose the correct word or number from the word bank. Each one is used once. Two will not be used.
Energy = useful energy output + waste energy output. Efficiency (%) = useful energy output ÷ energy input × . The SI unit of energy is the , and 1 kJ = J. The energy that does the job we want is called energy.
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
If a device’s useful output is 800 J and its input is 1,000 J, its efficiency is 80%.
Correct it:
Efficiency can be more than 100% if a device is designed well enough.
Correct it:
Waste energy disappears, so it cannot be measured or accounted for.
Correct it:
If a device is 40% efficient and its input is 1,000 J, the waste energy is 600 J.
Correct it:
The equation energy input = useful output + waste output is another way of stating the law of conservation of energy.
Correct it:
Sort it: input, useful or waste?
The values come from three devices: a camp stove, an LED globe and an electric drill. Write each value in the correct box.
Energy input
Useful output
Waste output
1. A lamp takes in 400 J of electrical energy and gives out 60 J of light. Calculate (a) the waste energy and (b) the efficiency of the lamp. Show your working.
(a) Waste energy J
(b) Efficiency %
2. A 2 kg pot plant sits on a shelf 1.5 m above the floor. (a) Use GPE = mgh, with g = 10 m/s², to calculate its gravitational potential energy. (b) The pot plant falls off the shelf. Ignoring air resistance, how much kinetic energy does it have just before it hits the floor?
(a) GPE J
(b) Kinetic energy J
Investigation: which phone charger wastes the least energy?
A class charged the same phone from flat to full with three different chargers. A plug-in energy meter measured the electrical energy each charger took from the wall. Each time, the phone’s battery stored 48 kJ of energy.
| Charger | Energy from the wall (kJ) | Waste energy (kJ) | Efficiency (%) |
|---|---|---|---|
| A | 60 | 12 | 80 |
| B | 64 | ||
| C | 55 |
Source: sample class results.
3. Complete Table 1. Round each efficiency to the nearest whole number. Charger A is done for you.
4. Which charger should the class recommend? How much less energy does it waste than charger B for each full charge?
5. A student says: “My new charger is 95% efficient, so hardly any energy is wasted. It’s basically perfect.” While charging a tablet, the charger takes 60 kJ from the wall. Do you agree with the student? Explain using the definition of efficiency.
Waste energy kJ