Transfer of Energy

Transfer of Thermal Energy

30 MCQs, Answer and some explanations

Multiple Choice

A black plastic bag is filled with cold water and hung up in a sunny place.

What causes the water temperature to rise?

• Convection inside the black bag.

• The black bag absorbing radiation.

• The black bag acting as an insulator.

• The black bag emitting radiation.

Multiple Choice

Which of the following would not produce convection currents in a container filled with water at room temperature?

• Floating an ice cube on the water

• Placing a lighted bunsen burner below the container

• Placing a piece of hot metal in the water

• Placing a very hot lighted bulb just above the water surface

Forming of Convection Current

Convection can only be formed when:

fluid is heated at the bottom, or fluid is cooled at the top

This is due to:

warmer fluid being less dense and rises, or cooler fluid being denser and sinks

Multiple Choice

Two identical metal plates are painted, one white and the other black. They are placed at equal distances from a radiant heater as shown in the figure below. The heater is turned on for five minutes and then turned off.

Which metal plate absorbs less energy (when heater is turned on) and which plate emits less energy (when heater is turned off)?

• Absorb less - black

  Emit less - black

• Absorb less - black

  Emit less - white

• Absorb less - white

  Emit less - black

• Absorb less - white

  Emit less - white

Multiple Choice

Bread can be baked by placing it below, but not touching, a heating element.

Which is(are) the main process(es) that transfers thermal energy from the heating element to the bread?

• Conduction only

• Radiation only

• Conduction and convection only

• Convection and radiation only

Baking

Convection current cannot be formed to bake the bread in this oven (in the diagram). Air around the heating element is heated and stays at the top of the oven.

Heat can only reach the bread via infrared radiation.

Multiple Choice

A boy wrapped a sweet corn in aluminium foil for a barbeque. He placed the corn beside the fire.

Which side should be on the outside and why?

• The shiny side, as it is a better absorber of thermal radiation.

• The shiny side, as it is a better emitter of thermal radiation.

• The dull side, as it is a better absorber of thermal radiation.

• The dull side, as it is a better emitter of thermal radiation.

Multiple Choice

The diagram shows a brick wall between a man and a camp fire.

How does thermal energy from the campfire reach the man?

• Radiation,

  Conduction,

  Convection

• Radiation,

  Conduction,

  Radiation

• Radiation,

  Convection,

  Convection

• Radiation,

  Convection,

  Radiation

Heat Transfer

Convection cannot happen in brick wall (solid). When the brick wall is warm, it causes the air near it to be heated and then the air rises. This warmer air does not reach the man who is standing beside the brick wall.

(Thermal energy is radiated from the fire to the brick wall and from the brick wall to the man. Heat is transferred across the brick wall via conduction.)

Multiple Choice

Four metal cans are identical except for the colour and the texture of their outer surfaces.

100 cm³ of water at 70°C is poured into each can.

Which cools the most rapidly?

• A

• B

• C

• D

Multiple Choice

The diagram shows a thick copper plate that is very hot. One side is black, the other is shiny. A student places her hands the same distance from each side as shown.

Her left hand feels warmer than her right hand. Which statement is the correct conclusion from the experiment?

• The black side is hotter than the shiny side.

• The black side radiates more heat.

• The shiny side absorbs less heat.

• The shiny side is cooling down faster than the black side.

Multiple Choice

The figure shows a piece of paper wrapped around a copper rod and placed above a candle flame. It was observed that the paper did not catch fire even when it was placed above the flame for some time.

Which statement best explains why this happened?

• Hot air rises.

• The paper is a poor absorber of heat.

• The copper rod conducts thermal energy away.

• The copper rod is not burnt by the flame.

Multiple Choice

A solar heater uses heat energy from the sun to heat up water. The solar panels are painted black because

• dark surfaces absorb radiation well.

• dark surfaces conduct heat well.

• dark surfaces emit radiation well.

• dark surfaces reflect radiation well.

Multiple Choice

A cook makes the pudding “baked Alaska”.

The pudding is placed in a very hot oven until the top of the egg whites turns brown. It is then removed from the oven.Why does the ice cream stay cold?

• Air is a good conductor of heat and conducts heat away from the ice cream.

• Air is a poor conductor of heat and stops the heat from reaching the ice cream.

• The metal dish is a good conductor of heat and conducts heat away from the ice cream.

• The metal dish is a poor conductor of heat and stops the heat from reaching the ice cream.

Multiple Choice

Which of the following statements best explains why drops of liquid form on the outside of a glass of ice water on a hot day?

• Water molecules in the ice water gain energy from the hot air.

• Water molecules in the air transfer thermal energy to the glass.

• Water molecules in the air are attracted to the surface of the glass.

• Water molecules in the ice water conduct heat through the glass to the outside of the glass.

Multiple Select

Heat transfer by conduction can occur in (select all that apply)

• Solid

• Liquid

• Gas

• Vacuum

Multiple Select

Heat transfer by convection can occur in (select all that apply)

• Solid

• Liquid

• Gas

• Vacuum

Multiple Select

Heat transfer by radiation can occur in (select all that apply)

• Solid

• Liquid

• Gas

• Vacuum

Multiple Choice

Three glasses of water of the same temperature but different volume are placed side-by-side touching each other as shown below.

Which of the following statements is correct?

• Heat flows from glass X to Y and Z.

• Heat flows from glass Y to X and Z.

• Heat flows from glass Z to X and Y.

• There is not net flow of heat among the three.

Multiple Choice

A student uses a double-walled glass vessel to contain a hot liquid.

Which of the following reduces heat loss by radiation?

• Painting the surface Q black

• Painting the surface R black

• Painting the surface S silver

• Presence of vacuum in between the walls

Multiple Choice

Density changes are responsible for which methods of thermal energy transfer?

• conduction only

• convection only

• conduction and convection only

• radiation only

Multiple Choice

Which of the following about conduction is true?

• Black is good conductor of heat.

• Conduction can only transfer thermal energy in solids.

• Conduction transfers thermal energy faster in denser medium.

• Objects with white shiny surfaces are poor conductors of thermal energy.

Multiple Choice

The diagram shows a crystal being heated in a beaker of water. The crystal releases a dye that shows how the water circulates around the beaker.

What causes the water above the crystal to rise?

• The water contracts and its density decreases.

• The water contracts and its density increases.

• The water expands and its density decreases.

• The water expands and its density increases.

Multiple Choice

The outer layer of a firefighter's coat is usually silver coloured.

This is because silver surfaces are

• bad absorbers of heat.

• bad conductors of heat.

• bad emitters of heat.

• good conductors of heat.

Multiple Choice

A metal container consists of three compartments X, Y, and Z, each of which contains water at a different temperature. Energy flows from X to Y and from Z to Y.

What are the possible temperatures of X, Y, and Z?

• 30°C, 40°C, 50°C

• 30°C, 50°C, 40°C

• 40°C, 30°C, 50°C

• 40°C, 50°C, 30°C

Multiple Choice

Double glazing in windows consists of 2 glass panes separated by a vacuum or air filled space to reduce heat transfer. It is commonly found in buildings in temperate climates and in aeroplanes.

Which mode(s) of heat transfer is reduced by double glazing?

• conduction only

• convection only

• conduction and convection only

• conduction and radiation only

Multiple Choice

On a cold day, which colour of sweater should you wear to feel warmer?

• Black, because a black surface is a good absorber of radiant heat.

• Black, because a black surface is a poor emitter of radiant heat.

• White, because a white surface is a good absorber of radiant heat.

• White, because a white surface is a poor emitter of radiant heat.

Multiple Choice

The diagram below shows two copper cans filled with boiling water. One can is insulated with wool.

The temperature of the water in each can is taken every minute.

Which of the following graphs shows the results obtained?

Multiple Choice

A student filled an upright ring-shaped container completely with warm water. He then attempted to set up a convection current by placing a pack of dry ice at a corner of the container as shown.

Which one of the following is a correct illustration of the convection current that was set up?

Multiple Choice

Two plastic cups are placed one inside the other. Hot water is poured into the inner cup and a lid is put on top as shown.

Which statement is correct?

• Heat loss by convection is prevented by the small air gap.

• Heat loss by radiation is prevented by the small air gap.

• The bench is heated by convection from the bottom of the outer cup.

• The lid is used to reduce heat loss by convection.

To keep water hot:

Lid - prevent warm air above the water to be escaping (prevent convection of air to happen)

Small spacer - so that there is air gap between the two cups

Air gap - minimise heat lost via conduction and convection (air is poor conductor; convection current cannot be formed)

Plastic (material of the cup) - minimise heat transfer to air or bench via conduction (plastic is a poor conductor of heat)

Multiple Choice

The diagram shows part of a water-heating system which works by convection.

Which diagram shows the most likely flow of water in the system?

• A

• B

• C

• D

Movement of Water using Convection

Warm water rises and cold water sinks. At the boiler, the water is heated. The warmer (relatively) water will rise and and flow into the hot water tank from the top pipe. The colder (relatively) water sinks and flow into the boiler at the bottom pipe, and gets heated.

Multiple Choice

Four similar containers was filled with the same amount of water of the same temperature. They are then placed on four different hot metal plates. The lower surfaces of the metal plates are heated to the same temperature. The metal plates have the same size but are made from four different metals. The time taken for the temperature of the water to rise in the four containers is noted below.

Which metal is the best conductor?

• 20°C rise in 180 s

• 20°C rise in 200 s
  

• 10°C rise in 150 s

• 10°C rise in 100 s

Good conductor

A good conductor allows heat to pass through it fast. Hence, if the metal plate made of best conductor, the temperature of the water will rise fast.

Multiple Choice

The diagram shows four metal rods immersed in a basin of hot water and the temperatures near the exposed end are measured.

Which of the combinations is rod 1?

(Aluminium is a poorer conductor compare to copper.)

• Aluminum, black

• Aluminum, white

• Copper, black

• Copper, white

Copper is a better conductor than aluminium.

Black is good emtter of radiation.

Open Ended

In an igloo,

1) the walls are made from compressed snow, which contain tiny pockets of air

2) the ventilation holes are found near the top and

3) the the living & resting area is above a large hole dug below the igloo.

Explain why 1 to 3 are made this way.

Igloo

1) The tiny pockets of air in the compressed snow are poor conductors and minimise heat loss to the surroundings.

2) The ventilation hole is near the top because warmer air (less dense) rises out of this hole; fresh air can enter from the bottom of the igloo to replace it. (see diagram)

3) The living & resting area is located away from where the air would flow.

for finishing the quiz!