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physics > p1 - conservation and dissipation of energy > Flashcards

p1 - conservation and dissipation of energy Flashcards

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1
Q

law of conservation

A

energy cannot be created, nor destroyed. it can also be transferred from store to store

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2
Q

explain the energy changes in a torch

A

1) chemical energy store in the battery
2) energy transferred by electric current in the wires
3) energy transferred to the surroundings; either by light waves or via an increase in the thermal energy store of the surroundings

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3
Q

energy transfers in a closed system

A

no net change of the total energy but transfers take place within the system due to the conservation of energy

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4
Q

dissipation

A

energy transferred to the surroundings

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5
Q

useful energy

A

energy being transferred to where it is required

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6
Q

wasted energy

A

energy that is not usefully transferred

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7
Q

define power

A

the rate at which energy is transferred/ rate at which work is done

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8
Q

power equation (linking energy and time)

A

power (watts, W) = energy transferred (joules, J) / time (s)

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9
Q

power equation (linking work done and time)

A

power (watts, W) = work done (joules) / time (s)

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10
Q

energy efficiency equations

A

efficiency = useful output (J) / total input (J)
efficiency = useful output energy transfer/ total input energy transfer

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11
Q

percentage energy efficiency

A

efficiency = useful output/ total input x 100

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12
Q

what is a sankey diagram?

A

summarises all the energy transfers taking place in a process

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13
Q

describe energy changes in a filament light bulb

A

1) transfer of electrical energy through wires
2) energy transferred through light waves (useful)
3) increase in the thermal store of the surroundings (wasted)

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14
Q

how can the efficiency of a system be increased?

A
  • reducing waste output via thermal insulation
  • recycling waste output
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15
Q

what is kinetic energy?

A

energy in a moving object

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16
Q

kinetic energy equation

A

KE (J) = 1/2 x mass (kg) x velocity^2 (m/s)
1/2 m v^2

17
Q

elastic potential equation

A

elastic potential energy (J) = 1/2 x spring constant (N/m) x extension^2 (m)
1/2 k x^2

18
Q

gravitational potential equation

A

GPE (J) = mass (kg) x gfs (N/kg) x height (m)
mgh

19
Q

two motors lift the same mass through the same height. motor A does it in half the time. which dissipates the most power?

A

motor A dissipates more power. the energy transfer is less but the time taken is less

20
Q

describe the energy change when a ball is thrown upwards and then returns to its starting position

A
  • upwards: the KE is converted into GPE
  • at its peak: maximum GPE, no KE because the ball is no longer moving
  • downwards: GPE is converted back into KE
21
Q

describe the energy transfers in a bungee jump

A
  • when the person falls: the GPE is converted into KE
  • when the cord tightens: that KE is converted and stored as EPE
  • when the lowest point is reached: the EPE stored in the cord = the jumper’s initial GPE
22
Q

why does a bungee jumper slow down once the cord begins to stretch?

A
  • KE decreases as it is converted into EPE
  • KE is proportional to velocity^2 so as one decreases so does the other
23
Q

examples of chemical energy stores

A

food, batteries, fuel (wood, coal, petrol)

24
Q

examples of energy stores

A

chemical, gravitational potential, kinetic, elastic potential, magnetic, electrostatic

25
Q

what is the spring constant?

A

the measure of a spring’s stiffness

26
Q

the greater the spring constant…

A

the greater the force required to stretch or compress

27
Q

watt conversions: 1kW (kilo) and 1mW (mega)

A

1kW = 1000 watts
1mW = 1,000,000

28
Q

watt = joule conversion

A

1 watt = 1 joule per second

29
Q

why can’t an object reach max velocity?

A
  • not all GPE is converted into KE, much is wasted to the thermal energy store of the surroundings
30
Q

what is elastic potential energy?

A

a force acting on an object which may cause it to change shape

31
Q

how is EPE?

A
  • the stored until the force is removed and the deformed object returns back into its original shape
32
Q

what is GPE?

A

the energy stored in an object that is raised above the ground
the object has the potential to fall due to gravity

33
Q

what is work done?

A
  • when an object is moved by a force work is done
  • energy is transferred to the object
  • energy transferred to object = the work done on the object
34
Q

work done equation

A

work done (J) = force applied (N) x distance moved (m)

35
Q

what is friction?

A

when a force is applied, friction is what resists movement

36
Q

examples of ways in which energy can be transferred

A

mechanically, electrically, via light and sound waves, heating, infrared radiation

37
Q

weight equation

A

weight = mass (kg) x gfs (N/kg)

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