P7.2

Question 1

Power equation

Answer 1

Power = Work done / time

Question 2

What unit do electricity companies measure your energy usage in

Answer 2

kWh, kilowatt-hour

Question 3

What is kWh a measure of

Answer 3

measure of energy NOT POWER

Question 4

Define 1kWh

Answer 4

1kWh is the amount of energy transferred by a1kW appliance, in 1hour

Question 5

Answer 5

P = E/t = 18kWh/2h = 9kW

Cost = (kWh’s used) x (cost/kWh) = 18kWh x 10p/kWh = £1.80.

Question 6

Suggest and explain whether using higher - power devices is cheaper, more expensive or costs the same as using lower - power devices = CLASSIC 6 MARKER

Answer 6

Higher power devices transfer electrical energy faster to other forms.

In the same time, they will use more energy.

So, they will cost more to run.

However, if the higher power of the device makes itmore efficient at doing its job (e.g. – higher power hair dryer dries hair faster)…

… it won’t need to be on for as long a period of time…

… so may actually end of using less energy and so costing less money to use.

Question 7

3 Advantages of electrical energy

Answer 7

• is very easily transferred into other forms.
• is very easily transferred over long distances.
• is portable.

Question 8

What is a wasted energy transfer when current flows

Answer 8

Whenever an electric current flows, some energywill always be transferred as heat to the surroundings. = THIS IS WASTED

Question 9

Actually try this one - took me some time but I understand it now

Answer 9

Mr Sara is answer -

Question 10

When does thermal energy transfer occur

Answer 10

Thermal energy transfer will always occur when there exists a temperature difference between two points/media.

Question 11

Definition of temperature

Answer 11

Temperature is a measure of the average kinetic energy of particles (in a substance).

Question 12

What is a system

Answer 12

A system is a group of bodies interacting.

Question 13

When is the only time the energy of a system stays constant

Answer 13

when the system is closed.

Question 14

Define a closed system

Answer 14

one which has no external forces actingon it (no forces from bodies outside the system).

Question 15

Answer 15

A car travelling at a constant speed will have a constant kinetic energy.Thus, kinetic energy is transferred to the thermal (store) energy of the surroundings at the same rate as the chemical potential (store) energy of the fuel is transferred to kinetic energy of the car.

Question 16

What are the three main ways heat energy is lost from a house

Answer 16

1. Walls
2. Roof
3. Windows

Question 17

3 main ways to minimise heat loss from your house

Answer 17

• cavity wall insulation
• loft insulation
• double glazing

Question 18

How does double - glazed windows work

Answer 18

Traps air and stops it moving

Question 19

How does cavity wall insulation work

Answer 19

Traps air and stops it moving

Question 20

How does aluminium foil between a radiator panel and the war minimise heat loss

Answer 20

Reflects radiation back from the wall

Question 21

What does the rate at which heat energy is transferred through a wall depend on

Answer 21

• Temperature difference between the two sides of the wall (temperature gradient)
• Area of walls
• Thickness of the walls
• Material of the walls(constant called Thermal Conductivity)

Question 22

How does temperature gradient effect the rate at which heat energy is transferred through a wall

Answer 22

The greater the temperature difference between the two sides of the wall… the greater the rate of heat energy transfer.

Question 23

How does area of a wall effect the rate at which heat energy is transferred through a wall

Answer 23

The greater the area of the walls…the greater the rate of heat energy transfer.

Question 24

How does wall thickness affect the rate at which heat energy is transferred through a wall

Answer 24

The thicker the wall,the slower the rate of heat energy transfer.

Question 25

How does thermal conductivity effect the rate at which heat energy is transferred through a wall

Answer 25

The lower the thermal conductivity of the wall, the lower the rate of heat energy transfer.

Question 26

Definition of thermal conductivity

Answer 26

The heat energy transferred, per second, through a 1m thick wall, of area 1m2, when a temperature difference of 1oC exists across it.

Question 27

Units of thermal conductivity

Answer 27

W/mK

K = Kelvin

Question 28

Answer 28

Don’t understand this but maybe you do

Question 29

Law of conservation of energy

Answer 29

Energy can never be created nor destroyed.

Energy can only ever be transferred from one form to another.

Question 30

What is useful energy

Answer 30

Energy in the place we want it and in the form we can use it.

Question 31

What is wasted energy

Answer 31

Energy that is not useful energy.

Question 32

What is efficiency

Answer 32

a measure of how much of the input energy of a device, is transferred to useful energy.

Question 33

Efficiency formula

Answer 33

Question 34

Why do mechanical devices ALWAYS waste energy in the form of heat

Answer 34

This is owing to the force of friction which alwaysacts between moving parts/surfaces.

Question 35

What is a Sankey diagram - draw example

Answer 35

Question 36

How to reduce friction

Answer 36

Lubricate moving parts

Question 37

How to reduce the heating up of a wire

Answer 37

Use wires with as little resistance as possible

Question 38

How to reduce air resistance

Answer 38

Streamline shapes

Question 39

How to reduce sound caused by machinery

Answer 39

Tighten loose parts to reduce vibration

Question 40

Suggest how to improve the efficiency of an oven

Answer 40

Insulate the oven to ensure minimise wasted energy transferred to the surroundings as heat.(heat energy is useful only when transferred to the oven itself)

Question 41

Explain why using LEDs might not reduce your electricity bill

Answer 41

LEDs are more efficient then incandescent bulbs…

… so waste less energy by transferring less to heat energy in the surroundings.

A room lit by LEDs will feel colder than a room lit by incandescent bulbs.

Thus, you may need to turn on the heating.

Any electricity you may have saved in powering LEDs (as opposed to less efficient incandescent bulbs) will now just be used instead in heating the house.