Edexcel A2 Physics Notes Unit 5, Topic: Thermal Energy. 423m6f

109: Investigate, Recognise and use the expression ^E = mc^0. -

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The effect that transferred heat energy has on the temperature of an object depends on three things: o The amount of heat energy transferred. o The mass of the object. o The specific heat capacity of the material from which the object is made. How much the temperature rises is dependent upon the nature of the material and this is given by a property called specific heat capacity ‘c’. Specific heat capacity is defined as the amount of energy needed to raise the temperature of 1kg of a particular substance by 1K. Different materials have different specific heat capacities because their molecular structures are different and so their molecules will be affected to different degrees by additional heat energy. The expression E = mc^___ for a certain amount of energy ‘E’ transferred to a material, the change in temperature ___, is related to mass of material ‘m’ and the specific heat capacity, ‘c’. Specific heat capacity has units in Jkg-1K-1. You can use change in temperature ___(theta), in both Kelvin or Celsius as the intervals are the same on both scales.

110: Explain the concept of internal energy as the random distribution of potential and kinetic energy amongst molecules. -

Molecules vibrate or move faster when their kinetic energy increases. Their potential energy also changes as they move about faster. It is the kinetic energy that determines the temperature of a material. If the average kinetic energy of the molecules of a substance increases, then it is at a higher temperature. This is Internal Energy. Internal Kinetic Energy on the scale of the particles that make up an object is separate from the idea of overall movement of the object. A cricket ball moves due to its kinetic energy but its molecules are vibrating within it and it is this internal kinetic energy that determines its temperature.

111. Explain the concept of absolute zero and how the average kinetic energy of molecules is related to the absolute temperature. - If you take energy away from a substance, its temperature becomes lower. Once all of this internal energy is taken away, the molecules will not have any internal kinetic energy left. At this point, the temperature of the substance is said to be absolute zero. - The gaps in the Kelvin Scale are similar to the Fahrenheit and Celsius. Kelvin scale starts at absolute zero at which there is no kinetic energy in molecules. So the Kelvin scale of temperature is sometimes called the absolute temperature. - As the units begin from a definite zero and go up in proportion to the amount of energy being added to the molecules of the substance being measured, it is

clear that as the average kinetic energy of the molecules increases so does the absolute temperature. 112. Recognize and use the expression 1/2mc2 = 3/2kT. - The expression for the average kinetic energy of the molecules is = ½ mc 2. ‘c’ is the speed of the molecule and ‘m’ is its mass. - The root mean square speed is sqrt(c2) from the expression above. - The average kinetic energy of any molecule in a gaseous sample is proportional to the absolute temperature of the gas. - This is expressed as 1/2mc2 = 3/2kT where ‘k’ is the Boltzmann constant; 1.38 x 10-23 JK-1. Temperature ‘T’ must be kelvin (K). 113. Use the expression pV = NkT as the equation of state for an ideal gas. - Assuming a gas is an ideal gas, we can put together an equation relating pressure, volume, temperature and amount of gas. - pV = NkT - where N is the number of molecules of the gas and k is the Boltzmann constant. The temperature must be absolute temperature in Kelvin. - The above equation is the equation of state for an ideal gas, expressed in of the number of molecules present.