Topic 8: Energy- Forces Doing Work

Question 1

What’s the law of energy conservation?

Answer 1

Energy cannot be created or destroyed -it can only be transferred into different forms

Question 2

What are the energy changes involved when a ball is thrown upwards and then returns to its starting position?

Answer 2

• Upwards: KE is converted to GPE
• Peak: Maximum GPE, zero KE
• Downwards: GPE is converted to KE

Question 3

State any changes in the total energy of a ball that is kicked

Answer 3

Total energy of the system remains constant due to the conservation of energy

Question 4

Describe the energy changes that occur in a filament light-bulb

Answer 4

• Electrical energy is transferred into light and heat energy
• Light is a useful energy form, heat is waste energy

Question 5

Describe the energy transfers for a bungee jumper?

Answer 5

• When falling the GPE is converted to KE of jumper
• As the cord tightens, KE is converted and stored as Elastic Potential Energy (EPE)
• At the lowest point, jumper’s initial GPE equals the EPE stored in the cord

Question 6

Explain why a bungee jumper slows down once the cord begins to stretch?

Answer 6

• KE decreases since it’s converted to EPE
• Since KE is proportional to (velocity)^2 as KE decreases, so does velocity

Question 7

Describe the energy changes in a power station?

Answer 7

• Heat energy released in the reaction heats water to make steam
• Steam moves the turbine, converting KE
• KE turns the generator
• Electrical energy as the generator generates electricity

Question 8

In what ways can the energy of a system be changed?

Answer 8

• Through work done by forces
• Through electrical input
• Through heating

Question 9

How can energy be changed through work done forces?

Answer 9

A force which moves an object (KE) causes work to be done over a distance

Question 10

Why do wasteful mechanical processes cause a rise in the temperature of the surroundings?

Answer 10

They cause energy to be transferred by heating the surroundings

Question 11

What’s the definition of power?

Answer 11

The rate at which energy is transferred (or rate at which work is done)

Question 12

What’s the unit of power?

Answer 12

Watt (W)

Question 13

What is one watt equivalent to?

Answer 13

1 joule per second (J/s)

Question 14

What does a power rating of 10W mean for an electrical device?

Answer 14

It converts 10 Joules of energy every second

Question 15

What is efficiency?

Answer 15

The proportion of energy that is converted usually, rather than wasted/ dissipated