Heat Transfer: Conduction, Convection, and Radiation
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PAPER 2 - STRUCTURED QUESTIONS
1. (a) Describe an experiment to demonstrate convection in a fluid. You may use a diagram as well as words if you wish.
(b) When a fluid is heated it expands. Explain how this expansion leads to a convection current in a fluid.
(c) State an everyday example of convection
2. (a) A student holds a piece of copper wire AB just above the flame of a candle. After a little while, the copper between the fingers of the students becomes hot.
(i) Which of the following energy conversions occurs in the flame?
[ ] heat energy to chemical energy
[ ] chemical energy to heat energy
[ ] heat energy to mechanical energy
[ ] mechanical energy to heat energy
(ii) Name a process by which heat from the flame reaches A.
(iii) Name a process by which the end B becomes hot.
(b) What difference would it make to the results if the copper wire were replaced by a thin glass rod? Explain your answer.
3. This question is about transfer of the thermal energy.
(a) A plastic mat and a cork mat, each $2$ mm thick, were placed on a polished table. Two identical cups of hot drink at the same temperature were placed one on each mat. It was observed that the polish under the plastic mat was marked by the heat, but the polish under the cork mat was not. Explain this observation.
(b) An exterior door was painted, partly with black paint and partly with white paint. On a bright sunny day, the black part was hotter than the white part. What does this tell you about the black and the white paint?
(c) In cold countries, the air in a heated room is often warmer near the ceiling than near the floor. What does this tell you about the main process of heat transfer in the room?
This question includes visual content: A diagram is provided for question 2(a). It shows a lit candle with a flame. Above the tip of the flame is one end (marked 'A') of a horizontal straight wire labeled 'AB'. The other end (marked 'B') is being held by a human hand using two fingers. The point 'A' is positioned directly in the rising path of the flame's heat, while 'B' is at a distance on the same horizontal line.
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Hi Raziq, let's solve these physics questions regarding thermal energy transfer together. This page covers convection, conduction, and radiation.
Thermal Energy Transfer - Paper 2
Starting with question one. We need to describe an experiment for convection. A classic setup uses a beaker of water and a crystal of potassium manganate seven. When you heat the bottom, the purple dye rises and then sinks in a circular motion, demonstrating the flow of fluid.
1. (a) Convection Experiment
- Place a crystal of potassium manganate(VII) at the bottom of a beaker of water.
- Heat the beaker gently from the bottom.
- The purple dye rises with the warm water and sinks as it cools, showing a convection current.
In part b, we explain the mechanics of convection. When a fluid is heated, its particles move faster and spread apart, causing expansion. This decrease in density makes the warm fluid rise, while denser, cooler fluid sinks to take its place.
1. (b) Explanation
1. Heating causes the fluid to expand.
2. Expansion leads to a decrease in density.
3. The less dense warm fluid rises.
4. Cooler, denser fluid sinks to replace it, creating a current.
Common everyday examples of convection include a room heater warming up the air or water boiling in a kettle on the stove.
1. (c) Everyday Example
- Sea breezes and land breezes.
- Air currents around a room heater.
Move to question two about the candle and copper wire. First, let's look at the energy conversion in the flame itself.
2. Heat Transfer in a Wire
In a candle flame, the chemical energy stored in the wax is converted into heat and light energy. So, the correct option is chemical energy to heat energy.
(a)(i) Energy Conversion
- Chemical energy to heat energy
Heat reaches point A above the flame primarily through radiation and convection. Since A is directly above, warm air rising—which is convection—is the dominant mode.
(a)(ii) Process to Reach A
- Convection (and Radiation)
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