Work, Power and Efficiency

Question 1

what is a closed system

Answer 1

a system where no energy can be transferred to or from the surroundings - the total energy stays the same

Question 2

what is work done

Answer 2

energy transferred when a force moves an object

Question 3

what is the unit for energy

Answer 3

joules (j)

Question 4

what is one joule of work

Answer 4

the work done when a force of 1N causes an object to move 1m in the direction of the force

Question 5

describe the energy transferred when a car slows down

Answer 5

energy is transferred mechanically from the kinetic store of the car to the thermal store of the brakes - some energy is dissipated to the thermal store of the surroundings

Question 6

describe the transfer of energy when an electric kettle is used to heat water

Answer 6

the electric current in a kettle transfers energy to the heating elements thermal store - energy is then transferred by heating from the heating elements thermal store to the store of the water

Question 7

describe the energy transferred when a battery powered toy car is used

Answer 7

energy is transferred electrically from the chemical store of the battery to the kinetic store of the car - some energy is dissipated to the thermal store of the surroundings

Question 8

describe the energy transferred when a falling apple hits the ground

Answer 8

energy is transferred from the kinetic store of the apple and dissipated to the thermal store of the surroundings by sound waves

Question 9

name the unit that represents one joule transferred per second

Answer 9

watt (W)

Question 10

a motor is 30% efficient what does that mean

Answer 10

30% of the energy is usefully transferred and 70% is dissipated

Question 11

How do we calculate work done?

Answer 11

Work done = force × distance
W = Fd

Question 12

What units do we use for the work done equation

Answer 12

Work (Joules, J), Force (Newtons, N), Distance (metres, m)

Question 13

What properties does a moving object have?

Answer 13

A moving object has mass and velocity (speed in a direction)

Question 14

What is the kinetic energy store of an object?

Answer 14

The kinetic store of an object relates to its motion. The faster an object moves, the more energy it has in its kinetic store.
A moving object also has mass and if a larger mass travels at the same speed it would have more energy in its kinetic store.

Question 15

How would we calculate the amount of energy in the kinetic store of an object?

Answer 15

Kinetic energy = 1/2 × mass × velocity2
Ek = 1/2mv2

Question 16

What units do we use for kinetic equation

Answer 16

Kinetic Energy (Joules, J), Mass (kilograms, kg), Velocity (metres per second, m/s)

Question 17

What is the elastic potential energy store of an object?

Answer 17

The elastic potential store of an object relates to how stretched it is. The more stretched (extended) it is, the more energy it has in its elastic store.

Question 18

How would we calculate the amount of energy in the elastic potential store of an object?

Answer 18

Elastic potential energy = 1/2 × spring constant × extension2
Ee = 1/2ke2

Question 19

what units do we use for elastic potential equation

Answer 19

Elastic Potential Energy (Joules, J), Spring Constant (Newtons per metre, N/m), Extension (metres, m)

Question 20

What is the gravitational potential energy store of an object?

Answer 20

The gravitational potential store of an object relates to how high above the Earth’s surface it is. The higher it is, the more energy it has in its gravitational potential store.

Question 21

How would we calculate the amount of energy in the gravitational potential store of an object?

Answer 21

Gravitational potential energy = mass × gravitational field strength × height
Ep = mgh

Question 22

Units for GPE equation

Answer 22

Gravitational Potential Energy (Joules, J), Mass (kilograms, kg), Gravitational field strength (Newtons per kilogram, N/kg), Height (metres, m)

Question 23

gravitational field strength on the Earth

Answer 23

9.8 N/kg

Question 24

gravitational field strength on the moon

Answer 24

1.6 N/kg

Question 25

What is power?

Answer 25

Power is the rate of transferring energy, also considered as the rate of doing work.

Question 26

How would we calculate the power of an energy transfer?

Answer 26

power = work done/time *or* energy transferred/time P = W/t *or* E/t

Question 27

Units for power equation

Answer 27

Power (Watts, W), Work done (Joules, J), Time (seconds, s)

Question 28

What does it mean if a kettle has a 3 kW rating?

Answer 28

This kettle transfers 3000 J of energy to the thermal store of the water in it per second. This kettle does 3000 J of work to the water in it every second.

Question 29

What is the law of conservation of energy?

Answer 29

Energy cannot be created or destroyed, only stored and transferred.

Question 30

How do we apply the law of conservation of energy

Answer 30

The total energy output from a device is equal to the total energy input.

Question 31

What do we mean for a device to be efficient?

Answer 31

If a device is efficient it transfers its energy usefully.

Question 32

If a device is inefficient what does it do?

Answer 32

If a device is inefficient it loses energy to the surroundings (usually by heating), we call this energy dissipated or wasted, as it is transferred to less useful stores

Question 33

How do we calculate the efficiency of an energy transfer?

Answer 33

efficiency = useful output transfer/total input for a percentage ×100 This equation can be used with both energies and powers.