energy

Question 1

How do you convert Kj into J?

Answer 1

To convert kilojoules (kJ) to joules (J), you need to multiply the value in kilojoules by 1000.

Question 2

Explain why a person should not just consider the energy inside food when making choices about what to eat.

Answer 2

Other nutrition ideas are important too, for example:

1. We need vitamins from vegetables and fruits.
2. High energy foods can be high in fats, salts and sugars which are unhealthy in large amounts.

Question 3

What unit of measurement describes the rate at which energy is used?

Answer 3

Watts (W)-
Something that uses one joule of energy per second has a power of one watt.

Question 4

How do you convert kW to W?

Answer 4

To convert kilowatts (kW) to watts (W), you need to multiply the value in kilowatts by 1000.

Question 5

What formula can be used to calculate the energy used in one minute?

Answer 5

Energy used in one minute= Power rating x 60

Example-
If a device has a power rating of 300W, it will use:
300 x 60 = 18,000 joules of energy in one minute.

Question 6

Calculate the energy used for the following devices:

1. A 40W lightbulb that is switched on for 60 seconds.
2. A 450W motor that is switched on for 60 seconds.
3. A 0.8kW mixer that is switched on for 60 seconds.
4. A 20kW car that is powered for 60 seconds.
5. A 1800W toaster that is powered for 30 seconds.

Answer 6

1. 40 x 60 = 2 400 J
2. 450 x 60 = 27 000 J
3. 0.8 x 60 = 48 kJ
   (or 800 x 60 = 48 000 J)
4. 20 x 60 = 1 200 kJ
5. 1800 x (60 / 2) = 54 000 J

Question 7

Name 6 energy resources.

Answer 7

• Biomass
• Hydroelectric
• Solar power
• Fossil Fuels
• Wind power
• Geothermal

Question 8

Define renewable and non-renewable resources.

Answer 8

Renewable resources can be easily replaced once they are used up.
Non-renewable resources cannot be easily replaced once they are used up.

Question 9

Put these sentences in the correct order from 1–5 to describe how oil is formed.

_ The oil can be extracted by drilling into the rock.

_ Rock puts pressure on the layers of remains, heating and
squashing them.

_ Animals and plants living in the sea die and build up on the sea
bed.

_ The high temperature and pressure slowly causes the remains to turn
into oil.

_ Mud settles on top slowly turns to rock, trapping the remains
below.

Answer 9

5 The oil can be extracted by drilling into the rock.

3 Rock puts pressure on the layers of remains, heating and
squashing them.

1 Animals and plants living in the sea die and build up on the sea
bed.

4 The high temperature and pressure slowly causes the remains to turn
into oil.

2 Mud settles on top slowly turns to rock, trapping the remains
below.

Question 10

Name 5 different energy stores.

Answer 10

• Kinetic
• Thermal
• Gravitational
• Chemical
• Elastic

Question 11

Define ‘kinetic energy store’.

Answer 11

Objects have energy in their kinetic store when they are moving.
Objects speed up or slow down.

Question 12

Define ‘thermal energy store’.

Answer 12

Also called heat energy store, objects have energy in their thermal store when they are hot.
Material warms up.

Question 13

Define ‘gravitational energy store’.

Answer 13

Objects have energy in their gravitational store whenever they are lifted up above the surface of the Earth (or other planets too!).
Objects move nearer to or further from the surface of the planet.

Question 14

Define ‘chemical energy store’.

Answer 14

Objects have energy in their chemical store if their atoms are combined together in complex ways. Foods, fuels and batteries have energy in their chemical stores.
Atoms combine.

Question 15

Define ‘elastic energy store’.

Answer 15

Also called strain energy store, objects have energy in their elastic store when they are stretched or squeezed.
Material is stretched, squeezed, or relaxed.

Question 16

What is the ‘conservation of energy’?

Answer 16

In any energy transfer, the total amount of energy has to stay the same.
This is a very important idea in Physics called the conservation of energy:
Energy cannot be created or destroyed, only transferred between stores

The type of energy stored can change, but the total amount in joules must always be the same.

Question 17

Example

A ball falling:
Gravitational store decreases → Kinetic store increases
This tells us that as the ball gets closer to the surface of the earth, it moves faster because its kinetic store is increasing.

Answer in the same format as the above:
1. A burning candle.
2. Wind-up toy car.
3. Wind-up electric torch.
4. Battery powered fan.
5. A motor lifting a mass.

Answer 17

1. A burning candle
   Chemical potential store decreases → Thermal store increases
2. Wind-up toy car
   Elastic potential store decreases → Kinetic store increases
3. Wind-up electric torch
   Elastic potential store decreases → Light waves increase
4. Battery powered fan
   Electric current decreases → Kinetic store increases
5. A motor lifting a mass
   Electric current decreases → Gravitational potential store increases

Question 18

What energy is useful and what energy is wasted?:

1. Filament lamp
2. Car motor
3. Toaster grill

Answer 18

1. Filament lamp
   Useful: Lights up the room (light energy)
   Wasted: Gets warm and heats up the surroundings (thermal store)
2. Car motor
   Useful: Makes the car move (kinetic store)
   Wasted: Makes lots of noise (sound energy)
3. Toaster grill
   Useful: Cooks food with heat (thermal store)
   Wasted: Glows red (light energy)

The more useful energy a device has compared to the wasted energy, the more efficient it is. Efficient devices do not waste much energy and are generally better to use, as they use less resources to run.

Question 19

How do you compare efficiency?

Answer 19

To compare efficiency we use a percentage. This tells us how much of the energy transferred is useful.
You do not need to calculate this yourself until 3rd Form.

Question 20

Example

A light bulb has an input energy of 500J. Of this, 50J is transferred to light energy.

Calculate the wasted energy of the light bulb.
500 – 50 = 450J

Describe what store the wasted energy has been transferred to.
The thermal store of the surroundings.

This bulb is only 10% efficient, meaning that only 10 out of every 100 joules used are going into useful light energy. The rest is wasted.

Work out the missing values in the same format as the above:

1. Computer screen
   Total: 500 J
   Useful: 400 J
   Wasted: ___ J
2. Elevator
   Total: ___ kJ
   Useful: 350 kJ
   Wasted: 120 kJ
3. Refrigerator
   Total: 6200 J
   Useful: ____ J
   Wasted: 3800 J
4. Radio set
   Total: 540 J
   Useful: 450 J
   Wasted: ___ J
5. Car engine
   Total: 9800 kJ
   Useful: ____ kJ
   Wasted: 3200 kJ
6. Phone speakers
   Total: ____ J
   Useful: 1820 J
   Wasted: 1330 J

Answer 20

1. Computer screen
   Wasted: 100 J
2. Elevator
   Total: 470 kJ
3. Refrigerator
   Useful: 2400 J
4. Radio set
   Wasted: 90 J
5. Car engine
   Useful: 6600 kJ
6. Phone speakers
   Total: 3150 J

Question 21

Explain the difference between energy and power.

Answer 21

Energy: total amount of work done.
Power: how quickly the work is done.