Chapter 8: Thermal Processes

Question 1

Define ‘conduction’

Answer 1

Conduction is a process of energy transfer by the transfer of increased vibrations at a molecular level from one molecule to another. It does not involve the flow of the medium

Question 2

Explain this process

A non-metal bar being heated by a candle, with particles closer to the candle vibrating more vigorously

Answer 2

• When one end of the bar is heated due to the flame of the candle, the molecules in direct contact with the flame vibrate with greater kinetic energy.
• These molecules move more vigorously, bumping into the neighbouring molecules and causing them to vibrate with greater kinetic energy as well, which increases the temperature of the bar.
• Eventually the vigorous vibrations are pased to the molecules at the other hand end of the bar, generally raising the temperature of the whole bar.
• The process of passing more vigorous vibrations from one end of a material to another end is called conduction
• Energy is transferred from a hotter region to a cooler region within the same material by the process of heating.

Question 3

Define ‘convention’

Answer 3

Convention is the process of energy transfer caused by convention currents in a fluid which occur due to density differences within the fluid

Question 3

Free electrons in a metal bar transfers energy to the positive ion

Answer 3

• When a flame is positioned on one end of the metal bar, the free electrons near the flame will gain kinetic energy and move with greater kinetic energy.
• These free electrons move quickly between the spaces of the positive ions, bumping and colliding with the positive ions, bumping and colliding woith the positive ions along the way, causing thye ions to vibrate with greater kinetic energy.
• The free electrons more quickly to the other end of the bar, bumping and colliding with the positive ions along the way, causing the ions to vibrate with greater kinetic energy and raising the temperature of the metal bar.

The temperature of metals increases much faster than non-metals as non-metals do not have free electrons in their molecular structure.

Question 4

Define ‘convention currents’

Answer 4

Convention currents involve the general movement of warmer fluid away from the hot flame and the movement of cooler fluids towards the flame to get heated in turn.

Question 5

Explain how this experiment works

Answer 5

• The hot Bunsen burner causes the water above it to increase in temperature. As the water increases in temperature, the molecules move around in greater velocity, pushing one another apart, causing the volume of the warmer water to increase. The warmer water has a lower density compared to the surrounding cooler water
• Water molecules of the cooler water move about with less velocity and are closer together, so the cooler water has a higher density. This density difference will cause the warmer water to move upwards away from the flame and the cooler water to move downwards towards the flame to take its place.
• The cooler water moves in to take the place of the warmer water, and in turn gets heated by the flame. When the temperature of the water increases to become higher than that of the surrounding fluid, its density will become lower and the water will then rise. The whole process repeats until the whole fluid is heated.
• This movement of fluids due to density differences in the process of heating is called convection current

Convention currents involve the general movement of warmer fluid away from the hot flame and the movement of cooler fluids towards the flame to get heated in turn.

Question 6

Explain how this experiment works

Answer 6

• The hot candle flame causes the air just above it to increase in temperature. As the air increases in temperature, the air particles move around with greater velocity, pushing one another apart, causing the volume of the warm air to increase. The warmer air has a lower density compared to the surrounding cooler air and will rise.
• Air particles of the cooler air move about with less velocity and are closer together, so cooler air has a higher density. The density difference will cause the warmer air to move upwards away from the flame and the surrounding cooler air to move towards the flame to take its place.
• When the cooler air moves in to take the place of the warmer air, it gets heated by the flame. When the temperature of the air increases to beocme higher than that of the surrounding fluid, its density will become lower, causing the air to rise. The whole process repeats until the whole fluid is heated. Smoke from the smouldering paper shows the general direction of air movement.

Question 7

Explain how this experiment works

Answer 7

• A coloured ice cube is added to a beaker of water. The cold ice gains internal energy and melts, causing the water around the ice cube to cool.
• The water around the ice cube and from the melted ice is cold. Particles at a lower temperature move about with less velocity and are closer together. The cold water has a higher density compared to the water around it and will sink
• The warmer water will move in to take the place of the cold water, becomes chilled by the ice cube. The process repeats until the whole fluid is cooled

Question 8

Define ‘radiation’

Answer 8

Radiation is the transfer of energy in the from of electromagentic waves like infared radiation without the aid of a medium, unlike convection and conduction which requires the movement of molecules (also known as the medium). Radiation can occur in vacuum.

Question 9

What are the factors affecting emisison and absorption of infared radiation?

Answer 9

• Surface and colour of object
• Surface temeprature compared to surroundings
• Surface area

Question 10

Explain how does the surface and colour of the object affect radiation?

Answer 10

A rough black object can be a good absorber (wehn the object’s temperature is lower) or a good emitter (when the object’s temperature is higher) of infrared radiation compared to another object with smooth silvery surface

Table of surface and colours of good and poor absorbers/emitters

Question 11

Explain how does the surface temperature compared to surroundings affect radiation?

Answer 11

The larger the temperature difference, the greater the rate of emission of infrared radiation.

• Initially the temperature difference between the object and the surrounding is the largest. The rate of infrared emission will be the highest causing the temperature decrease at a high rate.
• As the object cools, the temperature difference is smaller. The rate of infrared emission will decrease and the temperature decrease will be at a slower rate.
• Eventually the object cools to become the same temperature as the environment. The rate of infrared emission will decrease until it is the same as the infrared absorption from the environment. The temperature remains constant.

Temperature-time graph of the emission of infrared radiation

Question 12

Explain how does surface area affect radiation

Answer 12

Two objects with the same mass and made of the same material have different surface areas. The object with the larger surface area will emit or absorb infrared radiation at a higher rate.

For example, between a solid steel cylinder and a hollow steel cylinder of the same mass and temperature, the hollow cylinder will have a larger surface area compared to the solid cylinder and will emit or absorb more infrared radiation