Medium
Physics
Forces, Energy & Light
Motion, energy and light, described without the equations.
Sample question
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The speed of light in a vacuum is closest to which value?
Energy, Heat and Temperature
Thermodynamics has a reputation for being dry, which is unfair, because it explains kettles, coastlines and why no engine will ever be perfect. Ten questions on energy and the way it moves.
About this topic
Energy is never created or destroyed, but it does get steadily less useful, and those two statements are most of thermodynamics. Everything else is bookkeeping: how much energy a substance can hold, how fast it moves from one place to another, and how much of it you can turn into work before the rest escapes as waste heat. This quiz stays with the everyday consequences. Why the sea moderates a coastal climate, why boiling water refuses to get hotter than boiling, and why the Sun can warm your face across 150 million kilometres of nothing at all.
Questions & answers
Energy, Heat and Temperature: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1What does the temperature of a substance actually measure?
- A The number of particles packed into a given space
- B The average kinetic energy of its particles Correct
- C The total amount of heat energy it contains
- D How quickly it will transfer heat to something else
Answer: B. The average kinetic energy of its particles
Why: Temperature tracks the average energy of motion per particle, not the total energy present. A bathtub of warm water holds far more heat energy than a red-hot needle, despite being much cooler.
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2Which SI unit measures energy?
- A The joule Correct
- B The watt
- C The pascal
- D The kelvin
Answer: A. The joule
Why: One joule is roughly the energy needed to lift a small apple one metre. The watt is a rate rather than an amount, meaning one joule delivered every second.
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3Left to itself, which way does heat always flow between two objects in contact?
- A From the cooler object to the hotter one
- B From the larger object to the smaller one
- C From the denser object to the less dense one
- D From the hotter object to the cooler one Correct
Answer: D. From the hotter object to the cooler one
Why: This one-way tendency is the second law of thermodynamics in its plainest form. A fridge only reverses the direction by using electrical energy to pump the heat uphill.
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4Energy from the Sun crosses empty space to reach Earth. By which method?
- A Convection
- B Evaporation
- C Radiation Correct
- D Conduction
Answer: C. Radiation
Why: Radiation carries energy as electromagnetic waves and needs no material to travel through. Conduction and convection both require matter, which is why a vacuum flask blocks them so effectively.
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5A metal spoon and a wooden spoon have been sitting in the same drawer. Why does the metal one feel colder?
- A Metal reflects your body heat back at your hand
- B Metal conducts heat away from your skin much faster Correct
- C Metal is genuinely at a lower temperature than the wood
- D Wood generates a small amount of heat of its own
Answer: B. Metal conducts heat away from your skin much faster
Why: Both are at room temperature, but the metal drains warmth from your fingertips quickly and your nerves report the loss rather than the temperature. It is the same reason a tiled floor feels colder underfoot than a carpet.
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6Water is boiling steadily in an open pan. What happens to its temperature while it boils?
- A It stays at the boiling point until all the water has turned to steam Correct
- B It keeps rising slowly for as long as the heat is on
- C It drops as the hottest particles escape as steam
- D It fluctuates up and down with the bubbling
Answer: A. It stays at the boiling point until all the water has turned to steam
Why: The energy going in is spent breaking the bonds between liquid particles rather than raising the temperature, and it is called latent heat. Steam therefore carries far more energy than water at the same temperature, which is why steam burns are so severe.
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7Coastal towns have milder summers and winters than places inland at the same latitude. Which property of water explains this?
- A Its low density compared with soil and rock
- B Its transparency to sunlight
- C Its ability to dissolve most salts
- D Its unusually high specific heat capacity Correct
Answer: D. Its unusually high specific heat capacity
Why: Water absorbs and releases a great deal of energy for each degree it changes, so the sea warms and cools slowly and drags the local climate along with it. The same property is why water makes such a good coolant in engines and power stations.
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8If a car doubles its speed, what happens to its kinetic energy?
- A It becomes eight times as large
- B It stays the same, since the mass has not changed
- C It becomes four times as large Correct
- D It doubles
Answer: C. It becomes four times as large
Why: Kinetic energy depends on the square of the speed, so twice the speed means four times the energy. That relationship is why stopping distances grow so sharply as speed rises.
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9Why can no heat engine convert all of its fuel energy into useful work?
- A Energy is quietly destroyed during every conversion
- B Some energy must always be dumped to a cooler reservoir as waste heat Correct
- C Fuel always contains impurities that will not burn
- D Friction can in principle be removed but never has been
Answer: B. Some energy must always be dumped to a cooler reservoir as waste heat
Why: The second law sets a ceiling on efficiency that depends on the temperatures the engine runs between. A modern car petrol engine converts only around a third of its fuel energy into motion, with most of the rest leaving as heat.
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10A ball bouncing on a floor returns lower each time. Where has the missing energy gone?
- A Into heat and sound as the ball and floor deform Correct
- B It has been destroyed by the impact
- C Into extra mass gained by the ball
- D Into the gravitational field, which absorbs it
Answer: A. Into heat and sound as the ball and floor deform
Why: Energy is conserved but spreads out into forms that cannot easily be recovered, chiefly warmth in the ball and the surface. A well-inflated basketball returns about three-quarters of its drop height, and a squash ball far less until it is warmed up.
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