Thermal Energy & Heat Transfer — Practice · Grade 8 Science

Chapter: Energy & Heat Transfer

What this practice covers

By the end of Thermal Energy & Heat Transfer you should be able to:

10 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: thermal energy, heat, conduction, convection, radiation, thermal equilibrium, model, evidence, system boundary, assumption.

Problem 1

A 25°C metal spoon is placed in 80°C soup. They reach thermal equilibrium at 65°C. By how many degrees did the spoon's temperature change?
Answer and explanation

40 °C

Temperature change = 65 − 25 = 40°C.

Problem 2

Which method of heat transfer does NOT require any matter or medium?
  1. conduction
  2. convection
  3. radiation
  4. friction
Answer and explanation

radiation

Radiation travels as electromagnetic waves and can cross empty space with no medium needed.

Problem 3

Stirring a pot of soup and watching it circulate as it heats is an example of…
  1. conduction
  2. convection
  3. radiation
  4. sublimation
Answer and explanation

convection

Convection is heat transfer through the movement of a fluid, like circulating soup.

Problem 4

Explain why a metal spoon in hot soup feels hotter to the touch than a wooden spoon in the same soup.
Answer and explanation

Metal conducts heat much faster than wood, so thermal energy transfers quickly from the soup through the metal spoon to your hand, while wood is a poor conductor and transfers heat much more slowly.

Problem 5

Two objects are placed in contact: one at 90°C, one at 30°C. They reach thermal equilibrium at 60°C. By how many degrees did the cooler object's temperature rise?
Answer and explanation

30 °C

The cooler object rose from 30°C to 60°C, a change of 60 − 30 = 30°C.

Problem 6

Why can a vacuum flask reduce heat transfer even though it cannot stop all radiation? Explain the reasoning in your own words.
Answer and explanation

The gap limits conduction and convection; radiation remains possible.

A vacuum largely removes matter needed for conduction and convection across the gap.

Radiation can still cross the gap, so reflective surfaces and other design features help.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Why does trapped air in insulation reduce some thermal transfer? Show the reasoning, not just the conclusion.
Hints

More than one process can occur in the same scene.

Answer and explanation

Air conducts poorly compared with many solids, and small trapped pockets limit bulk convection. Radiation may still contribute.

Problem 8

A warm mug cools on a table. Name two possible energy-transfer paths.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Conduction to the table, convection to surrounding air, and radiation are possible; select any two and identify the receiving surroundings.

Problem 9

Net heat transfer follows a temperature differenceNet heat transfer follows a temperature differenceWarmer object80 °CCooler object20 °CNet energy transferFor equal masses of the same substance in an ideal isolated comparison:equilibrium is 50 °C, not 100 °C; energy transfers are not temperature addition.
The model compares equal masses of the same substance initially at 80 and 20 °C, with no phase changes or net exchange with other objects. Net energy transfer is from warmer to cooler, reaching 50 °C under the stated ideal assumptions. The red arrow shows energy transfer, not flow of a substance called heat or cold. Different masses or substances would require a different final-temperature account.
For this unit’s illustrated case, would simply adding the initial temperatures give the equilibrium temperature? Explain what is held comparable here.
Answer and explanation

No. For this ideal equal-mass, same-substance case the result is the midpoint 50 °C, not 100 °C. Amount, substance, phase and outside transfers affect the prediction.

Heat names energy transferred because of a temperature difference. Thermal energy names an energy contribution within an account. When temperatures match, the simple model has no net heat transfer between these two objects, though microscopic exchanges can still occur.

No. For this ideal equal-mass, same-substance case the result is the midpoint 50 °C, not 100 °C. Amount, substance, phase and outside transfers affect the prediction.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 10

An object receives energy across a vacuum gap from a warmer source. Which of conduction, convection or radiation can explain the gap transfer? What would a full net-exchange account include?
Answer and explanation

Radiation can cross the vacuum. Convection needs moving material and conduction needs an interacting material pathway. A full net account includes both objects’ emission/absorption and their temperatures.

Use the presence or absence of matter to distinguish mechanisms.

Do not mistake a selected one-way arrow for every transfer in the full exchange.

Radiation can cross the vacuum. Convection needs moving material and conduction needs an interacting material pathway. A full net account includes both objects’ emission/absorption and their temperatures.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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