energy change in a system (topic 1)

Question 1

give one reason why the students results can only be used as an estimate of the mean power at her location

Answer 1

• energy used to heat metal pan
• energy transfer to the surroundings
• intensity of solar radiation may have varied during investigation

Question 2

and give reason for answer (2)

Answer 2

• b a c
• biggest temperature difference (80 °C)

(this is because, the higher the temperature difference the more energy will move)

Question 3

dark matt, dark shiny, light matt, light shiny

which can of water will reach room temperature the quickest and give a reason (2)

Answer 3

• (the can that is) dark matt
• because it is the best absorber (of infrared radiation)

Question 4

apart from material of the can, mass of water and starting temperature, suggest three control variables for the students investigation

Answer 4

• same shape of can
• surrounding temperature is the same for all cans
• same surface underneath cans
• same position in the room

Question 5

(3)

Answer 5

• fox A
• smaller ears
• thicker fur
• these minimise energy transfer

Question 6

what is meant by a device being very efficient (1)

Answer 6

most (input) energy is usefully transformed

Question 7

(2)

Answer 7

• the room is losing energy / heat
• at the same rate as the heater supplies it

Question 8

(6)

Answer 8

plastic cap:
- plastic is a poor conductor
accept insulator for poor conductor
- stops convection currents forming at the top of the flask so stopping energy transfer by convection
- molecules / particles evaporating from the (hot) liquid cannot move into the (surrounding) air so stops energy transfer by evaporation
- plastic cap reduces energy transfer by conduction, convection, evaporation

glass container:
- glass is a poor conductor so reducing energy transfer by conduction
- glass reduces energy transfer by conduction

vacuum:
- both conduction and convection require a medium, particles
- so stops energy transfer between the two walls by conduction and convection

silvered surfaces:
- silvered surfaces reflect infrared radiation
- silvered surfaces are poor emitters of infrared radiation
- infrared radiation (partly) reflected back (towards hot liquid)
- silvered surfaces reduce energy transfer by radiation

Question 9

(2)

Answer 9

• (the ears have a) small surface area
• so reducing energy transferred (from the fox)

Question 10

Which cup, A, B or C, has the greatest rate of cooling?
Using the graph, give a reason for your answer (2)

Answer 10

• c
• biggest drop in temperature during a given time

Question 11

explain, in terms of particles, how evaporation causes the cooling of water

Answer 11

• the fastest particles have enough energy
• to escape from the surface of the water
• therefore the mean energy of the remaining particles decreases
• the lower the mean energy of particles the lower the temperature (of the water)