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IGCSE · 2 · multiple choice

Thermal physics · Question 5

Thermal physics · Original GioPhysics question with a detailed, mark-by-mark answer guide.

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Marks
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Answer
Complete
An electric heating element fitted at the bottom of a tank of watertank of waterelectric heating elementto the supply
Fig. 5.1A rectangular open-topped tank holds water to about four-fifths of its depth. A flat electric heating element lies horizontally inside the tank, well below the water surface and a short distance above the tank floor, spanning about half the width of the tank. Two leads run from the left-hand end of the element out through the side wall of the tank to the electricity supply.
multiple choice1 mark

An electric heating element is fitted at the bottom of a tank of water. Why is this the best position?

  1. A

    Water conducts thermal energy well downwards but not upwards.

  2. B

    Warm water is less dense, so it rises and a convection current circulates through the tank.

  3. C

    Thermal radiation travels upwards more easily than downwards.

  4. D

    The pressure is greatest at the bottom, so energy transfers faster there.

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Answer overviewCorrect answer: B — Warm water is less dense, so it rises and a convection current circulates through the tank.
01

Answer and reasoning

1 mark
How to approach it

Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.

  1. 1

    The correct choice is B: Warm water is less dense, so it rises and a convection current circulates through the tank.

  2. 2

    A is wrong twice over — water is a poor conductor and conduction has no preferred direction. D confuses pressure with energy transfer.

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