Temperature
The particle theory of solids, liquids and gases explains many of the macroscopic properties of matter.
The temperature of an object is given by the average kinetic energy of its particles.
Two objects in thermal equilibrium (at the same temperature) have particles with the same average kinetic energy.
When two objects at different temperatures are in contact, energy is transferred from the higher temperature object to the lower temperature object.
The internal energy of an object is the total kinetic and potential energies of the particles.
Phase changes between different states of matter cause changes to the potential energy of the particles of a substance.
Absolute zero is the temperature at which the particles of a substance have no kinetic energy. This occurs at -273oC.
Energy and Power
Energy can exist in different forms including internal (also referred to as heat or thermal energy), radiant (sound, light and other electromagnetic waves), kinetic, potential (chemical, electrical, gravitational, elastic). The unit of energy is the Joule.
Energy can be transferred (from one object to another) or transformed (to a different type)
Power is the rate of energy transfer and is calculated using Power = Energy / time. The unit of power is the Watt.
Electrical power is the rate at which electrical energy is transformed and can be calculated by Electrical power = Voltage x Current
The transfer of mechanical energy is called work and is calculated using W = F x d Work = Force ´ distance. The unit of work is the Joule.
Heat Transfer
When heat is transferred to an object, its internal energy can change and/or it can do work on its surroundings.
Heat energy is transferred by conduction, convection and radiation.
Conduction involves heat transfer between two objects in contact.
Convection involves heat transfer by convection currents in fluids.
Radiation involves heat transfer by electromagnetic waves. The frequency of radiation increases with increasing temperature.
Pressure in Gases
The pressure of a gas is due to the collisions of gas particles with the walls of a container.
Gas at a higher temperature has particles with more kinetic energy, so there are more collisions at higher speed.
The pressure of a gas is proportional to its temperature (Charles’ Law). The standard unit of pressure is the Pascal (Pa), which is the same as 1 Newton per square metre (N/m2).
Specific Heat Capacity
The specific heat capacity of a material is the energy needed to raise its temperature by 1Kelvin (or 1oC).
The energy and temperature rise are related by the equation E = mcDT
Energy = mass x specific heat capacity x temperature change
The units of specific heat capacity are Joules per kilogram per Kelvin (J/kg/K)
Specific heat capacity can be measured experimentally using ‘the electrical method’ or ‘the method of mixtures’.
Latent Heat
Latent heat is the energy added to a substance without a temperature change. This is the energy change during phase changes between solid, liquid and gas and depends on the substance.
The latent heat of fusion is the energy needed to melt a solid without a temperature rise.
The latent heat of vaporization is the energy needed to boil a liquid without a temperature rise.
The energy needed is given by Energy = mass x Latent Heat (E = mL)
Measuring Temperature
The centigrade scale of temperature uses two fixed points– the melting point of ice and the boiling point of water.
The Kelvin scale of temperature is defined by absolute zero and is designed so that 1 Kelvin = 1 oC. This gives absolute zero (0K) as -273.15 oC.
Temperature is measured using a thermodynamic property such as the thermal expansion of a solid, liquid or gas, electrical resistance or the frequency of electromagnetic radiation emitted.
A liquid in glass thermometer uses the thermal expansion of a liquid such as mercury or alcohol.
Many electrical thermometers use the change in resistance of a thermistor.
A constant volume gas thermometer uses the pressure of a gas.
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