1. Energy (power, conservation & dissipation of energy, thermal conductivity)

Question 1

What is power?

Answer 1

The rate at which energy is transferred or the rate at which work is done

Question 2

What is the equation for power?

Answer 2

Power = work done
———————–
time

Question 3

What are the units for power?

Answer 3

Watts

Question 4

What are the units for time?

Answer 4

Seconds

Question 5

What are the units for work done (W)?

Answer 5

Joules

Question 6

What are the units for energy transferred (E)?

Answer 6

Joules

Question 7

What is an energy transfer of 1 joule per second equal to?

Answer 7

A power of 1 watt

Question 8

What is the conservation of energy?

Answer 8

Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed

Question 9

What is an example that illustrates the definition of power?

Answer 9

• If you’re given two motors A and B, that both lift the same weight through the same height, the motor that can do the same work faster is more powerful - as the energy transferred is at a faster rate

Question 10

What are some ways that you can reduce unwanted energy transfers?

Answer 10

• Lubrication
• Thermal insulation

Question 11

What can useful energy be defined as?

Answer 11

The energy that is transferred from store to store and used for an intended purpose

Question 12

What can wasted (dissipated) energy be defined as?

Answer 12

The energy that is not useful for the intended purpose and is dissipated to the surroundings

Question 13

Give an example of lubrication?

Answer 13

• The oil in a motor reduces friction
• So less energy is lost (as heat) through friction

Question 14

Give an example of thermal insulation?

Answer 14

• Double glazing windows
• Means that less useful thermal energy is lost

Question 15

What is thermal conductivity?

Answer 15

Thermal conduction is the process where energy is transferred by vibrating particles in a substance

Question 16

What is the direction of energy transfer always from?

Answer 16

Hot to cold

Question 17

Why does a higher thermal conductivity mean a higher rate of energy transfer by conduction?

Answer 17

• The higher the thermal conductivity of a material
• The more heat is allowed to travel through the material more easily
• So there is a higher rate of energy transfer by conduction across the material

Question 18

Do materials with high thermal conductivity or low thermal conductivity heat up faster?

Answer 18

High thermal conductivity

Question 19

How does thermal conductivity work in a building?

Answer 19

The rate of cooling is low if the walls are thick and thermal conductivity of the walls are low

Question 20

If the walls of a building were thin metal sheets what would happen?

Answer 20

Heat would be lost very quickly

Question 21

What is conduction?

Answer 21

The process where vibrating particles transfer energy to neighbouring particles

Question 22

What is convection?

Answer 22

The process where energetic particles move away from hotter regions to cooler regions

Question 23

Where does conduction mainly occur in?

Answer 23

Solids

Question 24

Where does convection mainly occur in?

Answer 24

Only liquids and gases