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Heat Transfer: Conduction, Convection and Radiation — Answers

Answer sheet with model answers and success criteria for the 8th Grade energy, light, sound & electricity worksheet.

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Success TutoringWorksheetsAnswer sheet8th Grade · Science · Energy, Light, Sound & Electricity

Heat Transfer: Conduction, Convection and Radiation — Answers

Part A: Three ways heat moves

For each example, choose the main way thermal energy is transferred.

Thermal energy always moves from a hotter object or region to a colder one.

Conduction: particles in a solid vibrate faster when heated and pass energy to neighboring particles by collisions. Metals are the best conductors because they also have free electrons that carry energy quickly.

Convection: in a liquid or gas, heated particles spread out, so the warm fluid becomes less dense and rises. Cooler, denser fluid sinks to replace it, forming a convection current.

Radiation: all objects give off infrared radiation. It travels as waves, needs no particles and can cross a vacuum. Dark, dull surfaces absorb and emit radiation well; light, shiny surfaces reflect it.

1.A metal spoon is left in a cup of hot soup and its handle becomes hot.
  1. AConduction
  2. BConvection
  3. CRadiation

Answer: Conduction

Energy passes from particle to particle along the solid metal.

2.Warm air rises above a heater and cooler air sinks to take its place, warming the whole room.
  1. AConduction
  2. BConvection
  3. CRadiation

Answer: Convection

Warm air is less dense and rises, setting up a convection current.

3.You feel the warmth of a campfire on your face from a few meters away.
  1. AConduction
  2. BConvection
  3. CRadiation

Answer: Radiation

Infrared radiation travels through the air to your skin as waves.

4.Energy from the Sun reaches Earth through the vacuum of space.
  1. AConduction
  2. BConvection
  3. CRadiation

Answer: Radiation

Only radiation can travel through a vacuum, because it does not need particles.

5.Water at the bottom of a saucepan is heated and rises, while cooler water sinks.
  1. AConduction
  2. BConvection
  3. CRadiation

Answer: Convection

Convection happens in liquids and gases (fluids).

6.Convection can happen in solids.

Answer: False

In a solid, the particles cannot move from place to place, so convection currents cannot form. Convection needs a fluid (liquid or gas).

Part B: Conductors and insulators

7.Match each material to how well it conducts heat.
  • Copper
  • Aluminum
  • Glass
  • Wool
  • Poor conductor
  • Very good conductor
  • Good insulator (traps air)
  • Good conductor

Answer: Copper → Very good conductor; Aluminum → Good conductor; Glass → Poor conductor; Wool → Good insulator (traps air)

8.Explain why saucepans are often made of metal but have plastic or wooden handles.

Answer: Metal is a good conductor, so it transfers heat quickly from the stove to the food. Plastic and wood are insulators, so the handle stays cool enough to hold.

9.Many insulating materials, such as wool, foam and feathers, contain lots of trapped air. Why does trapped air make a good insulator?

Answer: Air is a gas with widely spaced particles, so it is a poor conductor. Because the air is trapped in small pockets, it cannot move to form convection currents either.

10.Which color of shirt would keep you coolest standing in strong sunlight?
  1. ABlack
  2. BDark blue
  3. CWhite
  4. DDark green

Answer: White

Light, shiny surfaces reflect more radiation, so a white shirt absorbs the least energy from the Sun.

Part C: Investigating insulation

A student filled three identical cups with 200 mL of water at 80 °C. One cup had no cover, one was wrapped in foam and one in cloth. After 10 minutes the temperatures were: no cover 45 °C, foam 71 °C, cloth 62 °C.

11.Calculate the temperature drop for each cup after 10 minutes.

Answer: No cover: 35 °C; foam: 9 °C; cloth: 18 °C

  1. No cover: 80 − 45 = 35 °C
  2. Foam: 80 − 71 = 9 °C
  3. Cloth: 80 − 62 = 18 °C
12.Calculate the average rate of cooling of the cup wrapped in cloth, in °C per minute.

Answer: 1.8 °C per minute

  1. Temperature drop = 18 °C over 10 minutes
  2. 18 ÷ 10 = 1.8 °C per minute
13.Which material was the best insulator? Use the data to justify your answer.

Answer: Foam, because its cup had the smallest temperature drop (only 9 °C), so it lost thermal energy most slowly.

14.State two variables that should be kept the same to make this a fair test.

Answer: Any two of: the starting temperature of the water, the volume of water, the type and size of cup, the thickness of the wrapping, the room temperature, the time between readings, whether a lid was used.

Part D: Explain a design

15.A vacuum flask keeps hot drinks hot. It has a double glass wall with a vacuum between the layers, shiny silvered surfaces facing the vacuum, and a plastic stopper. Explain how each feature reduces heat transfer by conduction, convection or radiation.[4 marks]

Model answer: The vacuum between the glass walls contains no particles, so heat cannot be transferred across it by conduction or convection. The shiny silvered surfaces reflect infrared radiation back towards the drink, reducing heat loss by radiation. The plastic stopper is an insulator, which reduces conduction, and it seals the top so warm air cannot rise and escape, reducing convection and evaporation.

Success criteria:

  • Vacuum stops conduction because there are no particles
  • Vacuum stops convection because there is no fluid to move
  • Shiny surfaces reflect infrared radiation
  • Stopper is an insulator and prevents convection currents carrying heat out of the top
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