Heat transfer

Question 1

What is the SI unit of temperature?

Answer 1

Kelvin (K)

Question 2

Two metal spheres P and Q of identical material are maintained at the same temperature. The mass of sphere P is twice the mass of sphere Q.

Assuming negligible effects from the environment, which of the following statements incorrectly describes what happens when the spheres are placed in contact with each other?

A Heat flows from P to Q and vice versa.
B The temperature of P remains the same.
C The temperature of Q increases.
D No net heat flow occurs.

Answer 2

C
At thermal equilibrium, particles are still in constant random motion and therefore collide and transfer kinetic energy, hence option A is correct. Option B is correct because there is no change in temperature overall although there is heat flow.
Option C is wrong because it should be the temperature of Q remains the same.
Option D is correct because there is no change in temperature overall.

Question 3

What is the definition of conduction?

Answer 3

The transfer of thermal energy through a medium without any flow of the medium.

Question 4

Why is metal a good conductor of heat?

Answer 4

This is because metals have the presence of free-moving delocalised electrons in their structure which cab move quickly due to a smaller mass and randomly between the bigger metal cations, to allow a faster net transfer of kinetic energy to particles in cooler regions of the metal.

Question 5

Which state has the highest thermal conductivity and which state has the lowest thermal conductivity?

Answer 5

Solid—> highest
Liquid between the thermal conductivity of solid and gas
Gas (lowest)

Question 6

State of matter also influences thermal conductivity. How and why?

Answer 6

Particles are packed closer in solids, hence the frequency of collisions is generally higher in solids, enabling easier and faster transfer of kinetic energy through collisions.

Question 7

What is the definition of convection?

Answer 7

Convection is the transfer of thermal energy through a fluid by the movement of the fluid (liquid or gas) itself.

Question 8

What is the definition of a convection current?

Answer 8

The movement of fluid caused by a difference in the densities of various parts of the fluid.

Question 9

What is the answering structure for convection current?

Answer 9

Water/air at bottom of the____ gains heat from the ____, expands, becomes less dense than the surrounding water/air and rises. Cooler, denser water/air at the top of the/ surrounding air sinks by the sides of the flask to replace the heated water/ air. (2marks)

A convection current is set up (1mark)

Question 10

What is a sea breeze?

Answer 10

During the day, sun heats up the land much faster than the sea. The land is warmer than the sea. The warmer air above the land rises. Cool air above the sea moves inland to take its place.

Question 11

What is a land breeze?

Answer 11

At night, sea temperature drops more slowly. The sea is warmer than the land. The warmer air above the sea rises. Cool air above the land moves out to take its place. This results in land breeze.

Question 12

What is the definition of radiation?

Answer 12

The transfer of thermal energy in the form of electromagnetic waves, such ar but not limited to infrared radiation, without the aid of a medium.

Question 13

All matter above 0 K (absolute zero) emits thermal radiation in all directions.
True or False?

Answer 13

True

Question 14

A person is sitting next to a dog. The dog is sweating after a run and is warmer than the person.

Which of the following statement(s) about radiation is false?

(A) Net heat flow from the dog to the person is more.
(B) Heat flows from the dog to the person and vice versa.
(C)No net heat flow occurs.

Answer 14

C

Question 15

A black metal spoon and a white plastic spoon are both at 50 degrees celcius.

What is the main reason the black metal spoon feels warmer to the touch?

A black surfaces absorb heat at a faster rate than white surfaces.
B black surfaces emit heat at a faster rate than white surfaces.
C metal is a better conductor of heat than plastic.
D metal is a better radiator of heat than plastic.

Answer 15

C. When the hand touches the spoon, both conduction and radiation occurs but conduction is more significant here.

Question 16

Aluminium foil has a shiny side and a dull side. A hot sandwich is to be tightly wrapped in aluminium foil with no gaps so that it remains hot longer on a cold winter day.

Which suggestion and explanation are correct?

A The dull side should face outward as dull surfaces are good absorbers of heat.
B The shiny side should face inward to reflect heat to the sandwich.
C The shiny side should face outward as shiny surfaces are poor emitters of heat.
D The shiny side should face outward as shiny surfaces reflect heat well.

Answer 16

Answer: C

Option A is wrong as the environment is winter and is colder than the hot sandwich. Therefore what is the point of absorbing heat?

B is reasonable but C is a better answer, because if the shiny side face inward, it is less significant to reduce heat loss.

D is also wrong cuz the environment is colder so “reflect heat” is wrong.

Question 17

Which of the following action(s) will reduce a person’s carbon footprint?
1 switching on the air-conditioner the entire day
2 taking public transport instead of private transport
3 recycling plastic bottles

A 1 only
B 2 only
C 1 and 3 only
D 2 and 3 only

Answer 17

D

Question 18

What are the three factors influencing the rate of heat transfer by radiation?

Answer 18

Colour of surface, Temperature of surface and surface area.

Question 19

Darker, dull surfaces absorb thermal radiation at a slower rate.
True of false?

Answer 19

False. Darker, dull surfaces absorb thermal radiation at a higher rate.

Question 20

Brighter, shiny surfaces absorb and emit thermal radiation at a lower rate.

True or false?

Answer 20

True

Question 21

Explain how the temperature of surface relates to thermal radiation.

Answer 21

Matter at a higher temperature emit thermal radiation at a higher rate.

Matter at a higher temperature than its surroundings will emit thermal radiation to its surroundings at a higher rate than it absorbs from its surroundings, resulting a net loss of heat to its surroundings by radiation, causing the matter to start cooling down.

The rate of emission of thermal radiation decreases as the matter’s temperature gets closer to that of its surroundings, towards thermal equilibrium.

Question 22

How does surface area affect the rate of absorption and emission of thermal radiation?

Answer 22

The larger the surface area, the greater the rate of absorption and emission of thermal radiation.

Question 23

Explain how putting a lid over a pot of hot soup helps to keep it warm for a longer time.

Answer 23

The lid prevents the hot air above the hot soup from rising out of the pot. (1mark)

Convection currents cannot be formed between air inside and outside the pot, (1mark) maintaining a low(er) temperature difference between the soup and air above, reducing heat loss from the warmer soup to the cooler air above it via conduction (1mark).

The trapper air is also saturated with a pours of the liquid, reducing evaporation of the liquid and thus reducing cooling due to evaporation (1mark).

Question 24

Explain how tucking in one’s legs close to the body help keep one warm on a cold night.

Answer 24

Tucking of your legs reduce the body’s surface area exposed to the colder surrounding air and hence reduce heat loss by radiation (main form). This position allows more radiant heat emitted by the body to be reabsorbed by another part of the body.

Question 25

What are the stuff to take note when answering heat transfer questions?

Answer 25

1. Examine context: Which object/area is hotter?
   What is the objective/purpose?
2. State clearly the direction of heat transfer, like which is hotter/colder
   — from where/what — to where/what
3. Include words indicative of the heat transfer mechanism(s)
   Like conducts/emits/absorbs
   There can be more than 1 form of heat transfer at any point in time.
4. Analyse the heat transfer layer-by-layer/ object-to-object.

Question 26

A swimmer climbs out of the swimming pool on a warm, dry day. Almost immediately, he begins to feel cold.
Which statement explains the cooling effect?
A The water allows a convection current to remove heat from his skin.
B The water gains heat from his body as it evaporates.
C The water on his skin is a good conductor of heat.
D The water reflects infra-red radiation away from his body.

Answer 26

B

There is no convection current between liquid and gas. It can only occur between either a liquid or a gas.

Question 27

A matchstick only lights up when placed above a naked flame but does not light up if placed at the side of a naked flame. The matchstick does not touch the flame in both situations.

Three possible reasons are given as follows:
1 Radiation takes place in all directions, increasing the temperature at the top of the flame.
2 Hot air rises, therefore, the temperature at the sides of the flame is not as high as at the top of the flame
3 Both convection and radiation are happening at the same time, so their effects cancel out at the side of the flame.

Which explanations are correct?
A 2 only
B 3 only
C 1 and 2 only
D 1, 2 and 3 only

Answer 27

A
Only A is good reason that explains this

Question 28

For expansion and contraction, bimetallic strips are used. What is the material at the top of the bimetallic strips and the material at the bottom?

Answer 28

Brass is at the top
Steel is at the bottom

Question 29

Describe how the bimetallic strip works.

Answer 29

Hotter than the reference temperature, brass expands more and its greater length puts it on the outsides of the curve, so the bimetallic strip bends downwards.
When it is colder than the reference temperature, brass contracts more and its shorter length puts it on the inside of the curve, causing the bimetallic strip to bend upwards.
(The metal that is a better thermal conductor will expand more and contract more (darker colour metal strip) Over here, my explanation is according to an assessment book , it needs to be clearer

Question 30

A matter is at -273.14 degrees celcius. Does it emit thermal radiation?

Answer 30

Yes. -273.14+273.15=0.01K All matter above 0 K (absolute zero) emits thermal radiation in all directions.