Energy Transfers

Question 1

What is the equation for gravity potential energy? Give SI units for all quantities involved.

Answer 1

mgh
Mass x Gravitational Field Strength x Height

Question 2

What is the equation for kinetic energy? Give SI units for all quantities involved.

Answer 2

1/2 m v^2
1/2 x Mass x (Velocity)^2
Energy (J), Mass(kg), Velocity(m/s)

Question 3

What is meant by the conservation of energy?

Answer 3

• The total amount of energy in a closed system remains constant
• Energy cannot be made or destroyed but can change from one form to another

Question 4

Describe the energy changes involves when a ball is thrown upwards and then returns to its starting position. Ignore air resistance

Answer 4

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

Question 5

State any changes in the total energy of a ball that is kicked, assuming that no external forces act

Answer 5

The total energy of the system remains constant due to the conservation of energy

Question 6

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

Answer 6

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

Question 7

Describe the energy transfers for a bungee jumper.

Answer 7

-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 lowest point, the jumpers initial GPE equals the EPE stored in the cord

Question 8

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

Answer 8

• Kinetic energy decreases since it is converted to elastic potential energy
• Since KE is proportional to (velocity)^2, as KE decreases, so does velocity.

Question 9

What is waste energy?

Answer 9

The energy that is not used by the device for its desired purpose, or not converted into the desired form.

Question 10

State two equations to calculate efficiency.

Answer 10

= Useful Output Energy / Total Input Energy
= Useful Power Output/Total Power Output

Question 11

State the consequence for energy transfer of a material with a high thermal conductivity.

Answer 11

The rate of energy transfer through the material is higher than for a material with a lower thermal conductivity.

Question 12

Do double-glazed windows have a higher or lower thermal conductivity that single-glazed windows?

Answer 12

Lower, meaning less energy transfers through them

Question 13

State three methods of reducing heat loss in a building.

Answer 13

1. Double glazing
2. Loft and wall insulation
3. Thicker walls

Question 14

How can you reduce unwanted energy transfer in a mechanical system?

Answer 14

Reduce friction by lubricating moving parts