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Everything, whether it’s a solid, liquid or gas, has energy. The only difference between these states of matter is the amount of energy they have. This energy comes in two forms within the substance, kinetic energy and potential energy. The kinetic energy is from the movement of the particles within the mass. You can visualise the potential energy as if there were springs between the particles in a solid, which forces them to vibrate about their fixed positions. The sum of the randomly distributed potential energy and kinetic energy of the particles in the system is called the internal energy.
Heat always moves from a region of higher temperature to a region of lower temperature. This will continue to flow until both bodies have the same temperature. At this point it is said to be in thermal equilibrium. There are three ways in which heat is transferred. These are conduction, convection and radiation. Conduction is the transfer of heat by neighboring atoms vibrating against each other and is the most common method of heat transfer between solids. Convection is the transfer of heat from one place to another by the movement of a liquid or a gas. As the particles gain heat they spread out, become less dense and rise above the colder air causing the colder air to sink, this is how a radiator (despite the name) heats up your room. Radiation is the transfer of heat energy by electromagnetic waves in the infrared part of the spectrum and this sort of energy can be transferred through a vacuum. It is important to note that heat or thermal energy is not the same as temperature; heat is measured in joules whereas temperature is usually measured in Kelvin.
We all know that some things heat up more easily than others but why is this the case? The reason is due to a property of the substance called the specific heat capacity. It is defined as the energy required to raise the temperature of a unit mass of a substance by one kelvin. The formula which relates the change in the internal energy of a body and the change in temperature of the body is ΔE=mcΔθ. Where delta E is the change in internal energy, m is the mass of the substance being heated, c is the specific heat capacity and delta theta is the change in temperature of the substance which can be in kelvin or Celsius since the change would be the same.
You can use this formula to measure the specific heat capacity of a liquid for example with this simple experiment. Pour a known mass of liquid into a calorimeter, which will reduce heat losses to ensure most of the energy put in, is used to heat the liquid. The calorimeter will have a heating element inside it connected to a power supply with an ammeter and voltmeter in the circuit. A thermometer will also be placed in the calorimeter to measure the change in temperature. Finally a stopwatch should be used to measure the time it takes to go from it’s initial temperature to the it’s final temperature. Then using the equation E=IVt and ΔE=mcΔθ we can find the specific heat capacity as c=IVt/mΔθ.
When you heat a solid it raises the kinetic energy of the molecules, which as a result raises the internal energy of the substance. However when you continue to heat the substance it will reach a point where all of the energy supplied will go into meting the substance whilst keeping it at a constant temperature. So the potential energy of the particles will increase whilst the kinetic energy remains constant. The energy needed to change the state of a unit mass of a substance from solid to liquid or from liquid to solid at constant temperature is called the specific latent heat of fusion. Whereas the energy required to change the state of a unit mass of a substance from liquid to gas or from gas to liquid at constant temperature is called the specific latent heat of vaporisation. If you were to sketch a graph of temperature against time when heating a substance you can see that when it changes state, there is no change in temperature until it all the substance reaches this state.
1. CGP AS & A2 Physics for OCR A, ISBN: 9781847624192....