# Physics - Rate of Heat Transfer (AQA Syllabus A)

One of the topics studied in GCSE Science is the transfer of heat energy This is one of eight quizzes on that particular subject and it looks specifically at how we can increase or decrease the rate of heat transfer via convection, conduction and radiation.

Heat is thermal energy and can be transferred from one location to another by conduction through solids, convection in fluids and as heat rays (thermal radiation). Conduction and convection involve the movement of particles but thermal radiation involves the electromagnetic waves known as infrared. These travel at the same speed as other electromagnetic radiation - the speed of light. This makes radiation the fastest way for heat to move from one place to another. It is so fast that it only takes just over 8 minutes for heat to reach the Earth from the Sun, about 150,000,000 km away!

Conduction is how thermal energy is transferred through solids and the speed at which this happens varies. It is fastest in metals, which are good conductors of heat, and we can use them to increase the rate of heat transfer. Materials that do not allow heat conduction to happen quickly are called insulators. These are things like plastics, wood and materials that contain trapped air or other gases. We can use insulators to decrease the rate of heat transfer.

Convection occurs in fluids and is the reason that hot air rises. A fluid is a liquid or gas. An area of a fluid that is warmer than surrounding areas is less dense and it rises. Cooler fluid then moves in to take its place. If the heating continues, the cooler fluid soon warms up and rises away from the heat source setting up a convection current. When you boil something in a pan, convection is the process that spreads the heat through the liquid.

Finding ways to increase and decrease the rate of thermal energy transfer is important in many applications. A scientist working in the Antarctic or a mountaineer climbing Everest both need clothing that slows down how fast that heat is transferred away from their bodies. Designers of LED lighting systems and computers on the other hand, need to find ways of increasing the rate of heat transfer to cool down the two devices.

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1. A group of students carried out an investigation into the rate at which different foods cool down. They measured the temperature of each food at 30 second intervals until the food had cooled from 80oC to room temperature. They then drew graphs of the results. In each case, the graph was the same shape. Which of the following describes the shape of their graphs?
The bigger the temperature difference between an object and its surroundings, the faster heat is transferred, so the food cooled down faster (steep negative gradient) at hotter temperatures
2. If 18,000 J of energy was transferred from an electric heater over the course of 2 minutes, what is the rate that heat was transferred?
This actually gives you the power rating of the heater - 1 joule per second is 1 watt so a 150 watt heater was used
3. 18,000 J of energy was transferred from an electric heater to a 2kg block of steel. The temperature increased by 18oC. Based on these figures, what is the specific heat capacity of the steel?
You need to rearrange the equation to isolate the specific heat capacity. It is unlikely that you would be asked to give the correct units in an exam but it is worthwhile knowing them, just in case you are.
4. A cool box is used to keep food cooler than the surroundings. They are often used to carry chilled or frozen food home from a supermarket or to keep food fresh at a picnic. Which of the following materials would be suitable to make the walls of a cool box?
It's light and has a low U-value
5. The smallest fox in the world, the fennec fox, lives in the Sahara desert. It has very large ears. How do its ears help it to keep cool?
This is testing your knowledge of how surface area and volume affect the rate of heat transfer. Since the ears have a large surface area compared to their volume, they will lose thermal energy quickly
6. Which of the following would be the worst conductor of heat?
Materials not made from metals are poor heat conductors
7. Which of the following features of a vacuum flask is designed to reduce the rate of heat transfer by radiation?
Smooth shiny surfaces reflect infrared (thermal) radiation the best
8. Which way does thermal energy travel?
Thermal energy always spreads out into the cooler parts of the environment around the hotter area
9. Which of the features of a vacuum flask is designed to slow down the rate of heat transfer by reducing both conduction and convection?
The cork base will reduce heat loss by conduction whilst the lid reduces heat loss by convection but the vacuum minimises both at the same time
10. LEDs are low energy alternatives to conventional light bulbs but they still generate a lot of heat when they are switched on. They need to be attached to a heat sink to keep them cool. Which of the following would make a good heat sink for an LED?
A metal plate with fins is a heat conductor which has a large surface area with a low volume. This makes it efficient at transferring heat from the LED into its surroundings - the air

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