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AS Physics - Module 3 > 3.3 - Work, Energy and Power > Flashcards

3.3 - Work, Energy and Power Flashcards

1
Q

What is work done?

A

(Or energy transferred) The product of the force and the distance moved by the force in the direction of the movement (can be thought of as the amount of force transferred from one form to another)

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2
Q

What is the equation for work?

A

Work done = force x distance moved in the direction of the force

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3
Q

Is work a scalar or a vector quantity and why?

A

Scalar - has a magnitude but no direction

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4
Q

What is the SI unit of energy?

A

Joule (J)

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5
Q

What is another measurement that is the same as the joule?

A

Newton metre (Nm) - one joule of work is transferred when a force of 1 newton causes an object to move a distance of one metre in the direction of the force

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6
Q

When using the W=Fd equation for vertical forces, what is the force?

A

Weight of the mass in the gravitational field strength: F=mg, therefore W=Fd=mgh

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7
Q

What is the typical work done in moving an apple to your mouth?

A

1N x 0.5m = 0.5J

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8
Q

What is the typical work done in walking upstairs?

A

750N x 3m = 2250J

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9
Q

What is the typical work done in pushing a shopping trolley along a supermarket aisle?

A

150N x 40m = 6000J

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10
Q

What is the typical work done by a crane lifting 40 tonnes by 40m?

A

40000kg x 9.81 x 30m = 11772000J = 1.10x10^7J

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11
Q

Is force a scalar or vector quantity and why?

A

Vector - it has a magnitude and a direction

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12
Q

If the force is in the direction of travel, how could you work out work done (in the direction of travel)?

A

W= F x cosθ x d

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13
Q

What is the equation for kinetic energy?

A

E = 0.5mv^2 (half times mass times velocity squared)

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14
Q

What is the equation for gravitational potential energy?

A

E = mgΔh (mass time gravitational potential energy time height)

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15
Q

What is the equation for thermal energy?

A

E = mcΔT (mass times the specific heat capacity of the material times the temperature difference)

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16
Q

What is the equation for elastic potential?

A

E =0.5Fe (half times force times the extension of the material)

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17
Q

What is the equation for nuclear energy?

A

E = mc^2 (mass times the speed of light squared)

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18
Q

What is the principle of conservation of energy?

A

The total energy of a closed system remains constant. Energy can neither be created nor destroyed, it can only be transferred from one form to another.

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19
Q

What is a closed system?

A

Any system in which all the energy transfers are accounted for. Energy or matter cannot enter or leave a closed system.

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20
Q

What does it mean if an energy transfer is not 100% efficient?

A

Not all the input energy was transferred into useful energy

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21
Q

What does efficiency of an energy transfer measure?

A

The percentage of the input energy that was transferred into useful energy

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22
Q

How do you work out efficiency?

A

[Useful energy out / energy in x 100 ] %

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23
Q

What is gravitational potential energy?

A

Energy an object has because of its position

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24
Q

What factors affect GPE?

A
  • Vertical height above the ground, Δh, measured in m
  • Mass, m, in kg
  • Gravitational field strength, g, in Nkg^-1
25
Q

How would you double the gravitational potential energy of an object?

A

Double on of its factors (height, mass or gravitational field strength)

26
Q

What is the relationship between gravitational potential energy gained and work done?

A

They are the same - E~p = work done = Fd

Force = mg therefore E~p = mgΔh

27
Q

What factors affect the kinetic energy of an object?

A

The object’s mass and velocity

28
Q

What is the relationship between kinetic energy gained and work done?

A

E~k = work done = F x Δs (F is the force that produces acceleration and Δs is change in displacement caused by the force)

29
Q

How do we use the equation for work done to calculate the the equation for kinetic energy?

A
  • Work = F x Δs (change in displacement)
  • From F=ma, E~K = ma(Δs)
  • From equation of motion v^2 = u^2 + 2as, can write as = 0.5(v^2 - u^2)
  • Times both sides by m gives E~K = 0.5m(v^2 - u^2)
  • Considering an acceleration from rest means E~K = 0.5mv^2
30
Q

What is the relationship between loss in gravitational potential energy and gain in kinetic energy?

A

Loss in potential energy = gain in kinetic energy

mgΔh = 0.5mv^2

31
Q

What is the equation, from mgΔh = 0.5mv^2, to work out the velocity of an object that has fallen from rest through a height?

A

v = √2gΔh

32
Q

How will doubling the mass of an object affect the kinetic energy of an object?

A

Double the kinetic energy

33
Q

How does doubling the velocity affect the kinetic energy?

A

Increases it by a factor of 4 - (2v)^2 = 4v^2

34
Q

When calculating speed from gravitational potential energy, why is it always more than the speed the object actually got to?

A

The calculated speed assumes there were no other forces acting on her and that none of the energy put in was wasted, when in reality, there are other forces e.g. air resistance and some energy is wasted through heat or sound

35
Q

What are the equations for power?

A 
Power = work done / time taken
Power = force x velocity
36
Q

What is power?

A

The rate of doing work

37
Q

Is power a scalar or vector quantity?

A

Scalar - it only has a magnitude, no direction

38
Q

What is power measured in?

A

Watts (W) or Joules per second (Js^-1) - rate of energy being transferred from one form to the other

39
Q

How do you convert between Watts and kilowatts?

A

1000W = 1kW

40
Q

How do you convert between megawatts, kilowatts and watts?

A

1MW = 1000kW = 1000000W

41
Q

What is the equation linking time, energy and power?

A 
Energy = Power x time 
E = Pt
42
Q

What factors affect the total amount of energy transferred by a device?

A
  • Power rating

- Length of time which it is used for

43
Q

How are the two equations for calculating power linked?

A

Power = work done / time taken
Power = (force x distance) / time taken
As velocity = distance / time, power = force x velocity

44
Q

What is equation for efficiency?

A

Efficiency = [useful output energy / total input energy x 100 ] %

45
Q

Why is the input energy always different (and more than) the useful output energy?

A

Always less than because energy can’t be created (or destroyed), only transferred from one form to another. Less because some energy is wasted

46
Q

What is a waste energy in a reaction?

A

One that can not be used

47
Q

For an electric motor, what is the energy transfer and what is the typical efficiency?

A

Electrical –> kinetic/potential

85%

48
Q

For a solar cell, what is the energy transfer and what is the typical efficiency?

A

Light –> electrical

10%

49
Q

For a rechargeable battery, what is the energy transfer and what is the typical efficiency?

A

electrical –> electrical

30%

50
Q

For an electric radiator, what is the energy transfer and what is the typical efficiency?

A

Electrical –> thermal

99%

51
Q

For a power station, what is the energy transfer and what is the typical efficiency?

A

Nuclear –> electrical

40%

52
Q

For a car (diesel or petrol), what is the energy transfer and what is the typical efficiency?

A

Chemical –> kinetic/potential

55%

53
Q

For a steam engine, what is the energy transfer and what is the typical efficiency?

A

Chemical –> kinetic/potential

8%

54
Q

How much energy is being wasted from the sun and why?

A

90% because solar technologies are only about 20% efficient

55
Q

How do we get electrical energy from our home?

A

Power stations: water heated, steam turns a turbine which powers a generator. The electricity produced is then distributed via the National Grid

56
Q

What is the efficiency range of a coal power station for generating electrical energy?

A

39-47%

57
Q

What is the efficiency range of an oil power station for generating electrical energy?

A

38-44%

58
Q

What is the efficiency range of a natural gas power station for generating electrical energy?

A

30-39%

59
Q

What is the efficiency range of a nuclear power station for generating electrical energy?

A

33-36%

AS Physics - Module 3 (1 decks)

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