Thermal Physics

Thermal Physics

By Karen Lewis

​What is thermal energy?

Thermal energy (heat) transfer happens when there is a difference in temperature. The energy moves from the higher temperature area to the lower temperature area. Conduction.

Subject | Subject

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• Conduction is the main method of thermal energy transfer in solids

• Metals are extremely good at conducting heat

• Non-metals are poor at conducting heat whilst liquids and gases are extremely poor

• Poor conductors are called insulators

• Materials containing small pockets of trapped air are especially good at insulating, as air is a gas and hence a poor conductor

 

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• When a substance is heated, the atoms start to move around (vibrate) more

• As they do so they bump into each other, transferring energy from atom to atom

• Metals are especially good at conducting heat as the delocalised electrons can collide with the atoms, helping to transfer the vibrations through the material

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• Convection is the main way that heat travels through liquids and gases

  (Convection cannot happen in solids)

• When a liquid (or gas) is heated:

  • The molecules push each other apart, making the liquid/gas expand

  • This makes the hot liquid/gas less dense than the surroundings

  • The hot liquid/gas rises, and the cooler (surrounding) liquid/gas moves in to take its place

  • Eventually the hot liquid/gas cools, contracts and sinks back down again

  • The resulting motion is called a convection current

​If a question on heat mentions liquids or gases the answer will probably be about convection. Heat does not rise (only hot gases or liquids rise). 

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Radiation: Basics

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• All hot objects give off thermal radiation: the hotter they are, the more they emit

• Thermal radiation is part of the electromagnetic spectrum – infrared

• Thermal radiation is the only way in which heat can travel through a vacuum

  It is the way in which heat reaches us from the Sun

• The colour of an object affects how good it is at emitting and absorbing thermal radiation:

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• The amount of thermal radiation emitted by an object depends on a number of factors:

  • The temperature of the object (hotter = more radiation)

  • The colour of the object (black = more radiation)

  • The surface area of the object (greater surface area = more area for radiation to be emitted from)

Multiple Choice

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Multiple Choice

• Cold air rises as it is less dense than the warmer air in the room

• Cold air rises as it is more dense than the warmer air in the room

• Cold air falls as it is less dense than the warmer air in the room

• Cold air falls as it is more dense than the warmer air in the room

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Multiple Choice

• RADIATION

• CONDUCTION AND RADIATION

• CONVECTION AND RADIATION

• CONDUCTION AND CONVECTION

Multiple Choice

• Both on the top

• One on the top and one on the bottom

• One on each side

• Both on the bottom

Multiple Choice

• Conduction

• Radiation

• Evaporation

• Convection

Open Ended

Explain the answer to the last question

​Consequences of energy transfer

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CONDUCTION​

• The main means of thermal energy transfer in solids

• When heated, atoms vibrate more, knocking into each other and transferring energy from atom to atom as a result

• Metals are excellent conductors; Non-metals are poor; Liquids and gases are very poor

• If a question mentions metals, the answer will probably have something to do with conduction

• Trapped air is a very good insulator of heat. Air is a gas and so is a poor conductor. Trapping it prevents it from circulating and forming a convection current

​Consequences of energy transfer

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Convection​

• The means of thermal energy transfer in liquids and gases

• When heated, a gas will expand and become less dense. This causes it to rise (a convection current). Cooler (denser) gas falls, replacing the hot gas

• If a question refers to a liquid or gas (that isn’t trapped) then convection currents will probably form

• Heat sources placed at the bottom of things will generally create convection currents. Likewise, cooling units placed high up will cool any rising air, causing it to sink again

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​Consequences of Energy Transfer

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Thermal Radiation​

• The only way in which heat can travel through a vacuum

• Thermal radiation is heat transfer by infrared (part of the electromagnetic spectrum)

• Black objects are good at emitting and absorbing thermal radiation; shiny objects are poor at emitting and absorbing it

• If a question refers to the colour of something (black, white or shiny) then the answer will probably have something to do with thermal radiation

• If a piece of apparatus contains a vacuum then radiation will be the only way heat can travel through that part of the apparatus

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A common mistake made by candidates when explaining how an insulator keeps something warm is to state something along the lines of “The object warms up the insulator which then warms the object up”.

Avoid giving this kind of answer!

The real explanation is:

• The insulator contains trapped air, which is a poor conductor of heat

• Trapping the air also prevents it from transferring heat by convection

• This reduces the rate of heat loss from the object, meaning that it will stay warmer for longer

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Other things to watch out for:

• Heat does not rise (only hot gases or liquids rise)

• Shiny things do not reflect heat (they reflect thermal radiation)

• Black things do not absorb heat (they absorb thermal radiation)

And remember,  a good answer will often include references to more than one method of thermal energy transfer.

 

Open Ended

Which method of energy transfer is used in hot air balloons? Explain how this works

Thermal Energy

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• The molecules within a substance possess two forms of energy:

  • Kinetic energy (due to their random motion / vibration)

  • Potential energy (due to their position relative to each other)

• Together, these two form the total energy that makes up the internal energy of the system

• Internal energy is defined as:

The total energy stored inside a system by the particles that make up the system due to their motion and positions

​Heating and Temperature Change

• Heating a system changes a substance's internal energy by increasing the kinetic energy of its particles

  • The temperature of the material, therefore, is related to the average kinetic energy of the molecules

• The higher the temperature, the higher the kinetic energy of the molecules and vice versa

  • This means they move around faster

• This increase in kinetic energy (and therefore internal energy) can:

  • Cause the temperature of the system to increase

  • Or, produce a change of state (solid to liquid or liquid to gas)

Thermal expansion

• When (most) materials are heated, they expand

• This expansion happens because the molecules start to move around (or vibrate) faster, which causes them to knock into each other and push each other

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• The thermal expansion of materials can have some useful applications, but also has some undesirable consequences

 

• Applications:

  • The expansion of a liquid in a thermometer can be used to measure temperature

  • A bimetallic strip, consisting of two metals that expand at different rates, can be made to bend at a given temperature, forming a temperature-activated switch

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Multiple Choice

• gas is cooling

• liquid is freezing

• gas is condensing

• liquid is cooling

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• Heating the barrel

• cooling the iron ring

• soaking the barrel in water

• heating the iron ring