P7 ENERGY Flashcards

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1
Q

what is the law of conservation of energy?

A

when there are energy transfers in a system, there’s no net change to the total energy of a closed system

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2
Q

what are the changes in the way energy is stored when an object is projected up a slope?

A

energy is transferred mechanically from the kinetic energy store of the ball to its gravitational potential energy store

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3
Q

what are the changes in the way energy is stored when a bat hits a ball?

A

some energy in the kinetic energy store of the bat is transferred mechanically to the thermal energy store of the bat, ball and their surroundings. some energy is transferred mechanically from the kinetic store of the bat to the kinetic store of the ball. the rest of the energy is carried away by sound

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4
Q

what are the changes in the way energy is stored when a car slows down (without braking)?

A

energy in the kinetic store of the car is transferred mechanically (due to friction) and then by heating to the thermal energy stores of the car and road by heating

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5
Q

what are the changes in the way energy is stored when a kettle is boiling water?

A

energy is transferred electrically from the mains to the thermal energy store of the kettle’s heating element. it is then transferred by heating to the thermal energy store of the water

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6
Q

what are the changes in the way energy is stored when an object is accelerated by a constant force?

A

work is done mechanically on an object and is converted to the object’s kinetic store.

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7
Q

what’s the equation associated with thermal energy?

A

△ e =mc△t

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8
Q

what’s the equation associated with kinetic energy?

A

e=1/2mv^2

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9
Q

what’s the equation associated with gravitational energy?

A

e=mgh

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10
Q

what’s the equation associated with elastic energy?

A

e=1/2kx^2

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11
Q

what’s the equation associated with nuclear energy?

A

e=△mc^2

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12
Q

what’s the equation for efficiency?

A

efficiency = useful output energy transfer/input energy transfer

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13
Q

when is energy stored in less useful ways?

A

when it is dissipated

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14
Q

describe how energy is transferred from batteries or from ac from the mains in different domestic devices

A

in a battery and mains supply powered motor, the kinetic energy of the electrons moving in the wire is transferred electrically into the kinetic energy store of the moving motor. and the kinetic energy of the moving electrons is transferred electrically into thermal energy from the collisions of electrons and positive ions

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15
Q

describe ways to reduce unwanted energy transfer

A

lubrication (since energy can be lost due to friction) and thermal insulation

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16
Q

what is the rate of cooling of a building affected by?

A

thickness and thermal conductivity of walls

17
Q

what is thermal conductivity?

A

how quickly energy is transferred through a wall

18
Q

what does a higher thermal conductivity mean?

A

a higher cooling rate

19
Q

how does a thicker wall reduce rate of cooling?

A

energy must travel further before it is transferred to the outside

20
Q

describe some ways to reduce unwanted energy transfer in homes

A

double glazing (because air has a lower thermal conductivity than brick), hot water tank jacket (reduces conduction, keeps water hot) and thick curtains (reduce heat loss by conduction and convection through windows)