P7 ENERGY

Question 1

what is the law of conservation of energy?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

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

Answer 3

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

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

what’s the equation associated with thermal energy?

Answer 7

△ e =mc△t

Question 8

what’s the equation associated with kinetic energy?

Answer 8

e=1/2mv^2

Question 9

what’s the equation associated with gravitational energy?

Answer 9

e=mgh

Question 10

what’s the equation associated with elastic energy?

Answer 10

e=1/2kx^2

Question 11

what’s the equation associated with nuclear energy?

Answer 11

e=△mc^2

Question 12

what’s the equation for efficiency?

Answer 12

efficiency = useful output energy transfer/input energy transfer

Question 13

when is energy stored in less useful ways?

Answer 13

when it is dissipated

Question 14

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

Answer 14

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

Question 15

describe ways to reduce unwanted energy transfer

Answer 15

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

Question 16

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

Answer 16

thickness and thermal conductivity of walls

Question 17

what is thermal conductivity?

Answer 17

how quickly energy is transferred through a wall

Question 18

what does a higher thermal conductivity mean?

Answer 18

a higher cooling rate

Question 19

how does a thicker wall reduce rate of cooling?

Answer 19

energy must travel further before it is transferred to the outside

Question 20

describe some ways to reduce unwanted energy transfer in homes

Answer 20

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)