Definitions
Internal Energy: The sum of the kinetic energy and potential energy of the molecules in a body.
Temperature: The average kinetic energy of all particles.
Heat: The transfer of internal energy
Specific heat capacity (c): The heat required to raise 1 kg of a substance by one degree (K).
Specific latent heat (L): The amount of heat required to change the state of 1kg of a substance without change in temperature. There are two types: fusion and vaporization
Laws
Boyle's law
The pressure of a fixed mass of a gas at constant temperature in inversely proportional to its value.
V↑=P↓
Pressure law
The pressure of a fixed mass of a gas with constant volume is directly proportional to its temperature.
Heat transfer
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Name
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What it is
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What medium
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Examples
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Conduction
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When the molecules at one end of a solid object are given energy, they vibrate more. This disturbs the neighbouring molecules and passing the energy along.
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Conduction transfers energy through solid or liquid materials. Metals are usually the best conductor and plastics the best insulators.
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Heat will travel slowly through a pot on a stove. |
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Convection
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The transfer of heat energy via liquid or gas.When heated a fluid expands marking it less dense, causing it to rise in the surrounding denser cooler fluid.
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Convection transfers energy through fluids such as gases and liquids. This is seen msot often in air or water. |
Radiator heats room by heating up the air. |
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Radiation
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Direct transfer from one body to another via infrared radiation.
Bodies of darker color both radiate and absorb the best.
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Radiation does not require any medium, unlike confuction and convection. Due to this, it is the only way to transfer heat in a vaccuum such as space. |
The sun emits radiation which heats up our planet. |
Specific heat capacity
Acrynym: SHC
Defintion: Amount of energy (joules) required to raise 1kg of a substance by one degree (Kelvin or Celcius)
Symbol: c
Si Unit: J/kg°K or J/kg°C
Caclulating thermal energy changes
Formula:
The change in thermal energy is equal to mass times specific heat capacity times by change in temperature.
Example question and answer
Jacob has 1500g of water which is at 17°C . How much heat must he add in kj, for the water to begin to boil. The specific heat capacity of water is 4.2 kj/kg°C.
Answer:
- First pull out the values needed from the question:
- Change temperature = 100-17= 83°C
- Mass = 1500g = 1.5kg
- SHC = 4.2 kj/g°C. (The question asks for kj, so no need to convert to j)
- Then put into formula:
What does the SHC tell us about the material?
| Specific heat capacity |
Meaning |
Insulator/conductor |
Examples |
| <800 (Low) |
It requires a small amount of energy to raise the temperature of the substance. |
Used as conductors as they don't waste alot of energy while transfering it to another substance. |
- Iron - 449 j/kg°K
- Copper - 385 j/kg°K
- Gold - 129 j/kg°K
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| >1200 (High) |
It requires alot of energy to raise the temperature of the substance. |
Used as insulators because it takes a long time to heat up and transfer the heat. |
- Rubber - 1880 j/kg°K
- Cotton - 1500 j/kg°K
- Water - 4182 k/kg°K
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Specific latent heat
Definition: Energy required to change the state of 1kg of a substance
Symbol: L
Unit: j/kg
When you change the state of a substance energy stops being used to change the temperature but instead to break the bonds. Because of this during a period of state change the temperature will remain constant.
The amount of energy required to change the state of a substance is called the latent heat. Latent is a word which means that something exists yet is hidden, so this is the "hidden temperature" change.
Fusion and vaporisation
Since there are two state changes there are also two types of specific latent heat
Latent heat of fusion: Energy (kj) to change a solid into a liquid
Latent heat of vaporisation: Energy (kj) to change a liquid into a gas.
Caculcating Latent heat changes
Formula:
The energy to change the state is equal to mass times specific latent heat for that state.
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