1. Latent Heat Can water boil at temperatures other than 100 oC?
If you opened a bottle of water on the moon what would happen?
Why does a gas come out of deodorant cans but yet when you shake them you hear a liquid?
How do refrigerators work?
What are geothermal heat pumps and how do they work?
PhysicsSFC
1/15/2019

2. This heat is called latent heat.
A change of state occurs when a substance changes from one state to another.
Heat In
Heat In
Solid Liquid Gas
Heat Out
Heat Out
The easiest way for a change of state to happen is by adding or removing heat.
This heat is called latent heat.
PhysicsSFC
1/15/2019

3. Latent Heat But what is latent heat? Energy Supplied (J)
If a piece of ice is heated its temperature rises (as seen on the thermometer) until it reaches 0 oC.
Ice
Thermometer
100
Temperature (oC)
At 0 oC the heat causes the ice to melt. The temperature remains at 0 oC until all the ice has melted. This type of heat is called ‘hidden’ or latent heat as there is no change of reading on the thermometer while the ice is melting.
PhysicsSFC
1/15/2019

4. Latent Heat But what is latent heat? Energy Supplied (J)
When the ice melts fully its temperature starts to rise again as it is heated.
Water
Thermometer
100
Temperature (oC)
At 100 oC the heat needed to change water into steam is again called ‘hidden’ or latent heat as there is no change of reading on the thermometer while the water is evaporating.
PhysicsSFC
1/15/2019

5. Latent Heat
Latent heat of fusion is the heat needed to change a solid to a liquid without a change in temperature
Specific latent heat of fusion is the heat needed to change 1 Kg of a solid to a liquid without a change in temperature
Latent heat of vaporisation is the heat needed to change a liquid to a gas without a change in temperature
Specific latent heat of vaporisation is the heat needed to change 1 Kg of a liquid to a gas without a change in temperature
PhysicsSFC
1/15/2019

6. Latent Heat
A less obvious way to change the state of a substance is by changing pressure.
E.g. Water boils at 100 oC at standard pressure.
Water boils at around 80 oC on Mount Everest. Why?
There is less air pushing down on top of Mt. Everest than at sea level. This in turn results in lower atmospheric pressure.
But why does pressure affect the boiling point of a liquid?
PhysicsSFC
1/15/2019

7. Latent Heat Why does pressure affect the boiling point of a liquid?
In order to evaporate, the particles in liquids must have enough energy to break free of each other and form a gas.
Atmospheric pressure pushes down on liquids. When the particles have enough energy to overcome this pressure they can escape from one another as gas particles.
Atmospheric Pressure
So why does water boil at a lower temperature on Mt. Everest?
If you opened a bottle of water on the moon what would happen?
PhysicsSFC
1/15/2019

8. Latent Heat
Recall: A change of state occurs when a substance changes from one state to another.
When close to its boiling point a liquid can be changed into a gas by reducing the surrounding pressure on the liquid.
Heat (latent heat of vaporisation) is absorbed in in the process.
An increase in pressure changes the gas back into a liquid and the latent heat is released.
Decrease Pressure
Heat In
Liquid Gas
Heat Out
Increase Pressure
This is the basic principle on which a heat pump (refrigeration) works
PhysicsSFC
1/15/2019

9. Latent Heat Heat Pump (refrigeration) Expansion valve Heat Out Heat In
Compressor – pushes a gas through piping that gets narrower.
Heat Pump (refrigeration)
Expansion valve
As a result the pressure on the gas increases and it changes to a liquid.
Liquid
Liquid
Heat Out
Heat In
Latent heat of vaporisation given out.
Liquid
Liquid
Expansion valve releases the pressure on the liquid.
Outside ‘fridge
Inside ‘fridge
Gas
Gas
As a result the liquid changes to a gas.
Gas
Gas
Compressor
Latent heat of vaporisation taken in.
PhysicsSFC
1/15/2019

10. Latent Heat Can water boil at temperatures other than 100 oC?
If you opened a bottle of water on the moon what would happen?
Why does a gas come out of deodorant cans but yet when you shake them you hear a liquid?
How do refrigerators work?
What are geothermal heat pumps and how do they work?
PhysicsSFC
1/15/2019