Power
What unit is Power measured in?
Watts (W) or Joules per second (J/s)
What unit is Energy measured in?
Joules (J)
If an appliance is described as energy efficient, what does that mean?
The appliance uses less energy to do it's job.
State the law of conservation of energy.
What does the prefix “kilo-” mean?
1000
A light bulb uses 60 J of energy in 3 seconds.
What is its power?
20W
A stretched rubber band is ready to snap back.
What form of energy does it store?
Elastic
A device takes in 100 J of energy and produces 40 J of useful energy.
What is its efficiency (as a percentage)?
40%
What are the three ways that energy can be transferred?
Conduction, convection and radiation
Convert 3 kJ into metres.
3000J
A heater has a power rating of 200 W and runs for 5 seconds.
How much energy does it use?
1000J
A toaster heats up bread using electricity.
What is the main energy transformation that takes place?
Electrical to Heat
A machine uses 500 J of energy and outputs 350 J of useful energy.
Calculate its efficiency.
70%
A ball is dropped from a height. What happens to its gravitational potential energy as it falls?
It transforms into kinetic energy.
Convert 5000 mW into W.
5W
A device transfers 500 J of energy at a constant power of 50 W.
How long does it take?
10s
A battery-powered torch is switched on.
List the energy changes that occur from the battery to the light produced.
Chemical potential energy to Electrical energy to Light and Heat energy
A light bulb is 25% efficient and takes in 200 J of energy.
How much useful energy does it produce?
50J
A toy car at the top of a ramp has 100 J of gravitational potential energy. At the bottom, it has 70 J of kinetic energy. Where has the missing 30 J of energy gone?
The 30J of energy has been transformed into heat energy.
Convert 2.5MW to W.
2,500,000W
A microwave oven operates at 800 W for 2 minutes.
How much energy does it transfer in that time? (Hint: watch your units)
96,000J
A rollercoaster starts at the top of a hill, goes down, and then climbs another hill.
Describe how the energy changes throughout the ride.
Gravitational potential energy decreases as the rollercoaster goes down, then increases as the rollercoaster moves back up.
Kinetic energy increases as the rollercoaster speeds up while it moves down, then decreases as the rollercoaster slows as it moves back up.
A motor uses 800 J of electrical energy and wastes 200 J as heat.
What is the efficiency of the motor?
75%
A pendulum is released from a height. At its lowest point, it has 50 J of kinetic energy.
On the next swing, it only reaches a height where it has 40 J of gravitational potential energy.
Explain why the energy has decreased and how this still follows the law of conservation of energy.
The energy decreased because the pendulum experienced air resistance. The air resistance transformed some of the energy into heat energy. This shows that the missing energy was not destroyed but transformed.
A rocket has 3,000,000,000,000J of energy. Express this value in terajoules (TJ).
3TJ