MEASUREMENTS OF HEAT

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Measurements of heat
Measurements of heat

At the end of this chapter, you should be able to:

  1. Demonstrate the difference between temperature and heat energy.
  2. Describe the terms of heat capacity and specific heat capacity.
  3. Identify the SI units of specific heat capacity.
  4. Demonstrate how to measure specific heat capacity of solids and liquids.
  5. Describe the terms latent heat, specific latent heat of fusion and of vaporization.

Demonstrate the solving of numerical problems on heat measurements

  1. 1.1. HEAT AND TEMPERATURE

When you hold a small block of ice in your hand it feels cold because the heat from your palm is drawn into the block of ice. The block of ice is at a lower temperature than your hand. Similarly, when you touch a warm piece of metal; it fells warm because heat travels from the metal to your hand the metal is at higher temperature than your hand.

  1. Heat

Heat energy is referred to as ‘thermal energy’ and it is defined as the energy which flows from a body at higher temperature to one which is at lower temperature. When there is a flow of heat from one body to another the internal (molecular) energy of the body receiving the heat is increased, thereby increasing the kinetic energy of the molecules.

When two objects at the same temperature come into contact with each other there is no flow of heat energy. This implies that two objects are in ‘thermal equilibrium (i.e.: they are at the same temperature). If two objects at different temperature come into contact, heat energy flows until they reach a state of thermal equilibrium.

Like other forms of energy, heat is measured in ‘Joule (J)’. When discussing nutrition, the energy value of different foods is measured in ‘Calories’ (cal)

  • Temperature

Temperature is the measure of how hot or cold an object is, expressed against a chosen scale. Temperature is thus the property of an object which determines the direction in which heat will flow when the object is placed in contact with another object. The instrument used to measure temperature is called ‘a thermometer’ and the scale used is either Kelvin scale (K) or commonly, the Celsius scale (˚C).

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What Will You Learn?

  • Demonstrate the difference between temperature and heat energy.
  • Describe the terms of heat capacity and specific heat capacity.
  • Identify the SI units of specific heat capacity.
  • Demonstrate how to measure specific heat capacity of solids and liquids.
  • Describe the terms latent heat, specific latent heat of fusion and of vaporization.
  • Demonstrate the solving of numerical problems on heat measurements

Course Content

HEAT CAPACITY
if you Heat the same bodies to different temperatures and drop it into identical calorimeters containing the same amount of water, you will observe that they attain different final temperatures. The heat capacity, C, of an object is the quantity of heat which must be transferred to it to give it one-degree rise in temperature. Bodies of different masses, of the same material and at the same temperature have different amount of heat. The specific heat capacity of a substance is defined as the amount of heat energy required to raise the temperature of a unit mass of a substance by 1 °C or 1K. It is used to describe the heat capacity of a substance per unit mass. The SI unit for the specific heat capacity ‘c’ is J∙kg^(-1)∙℃^(-1) or J∙kg^(-1)∙K^(-1) (or simply J/kg∙°C and J/kg∙K). It is given by the relation: Q=mc∆T. The specific heat capacity can be found experimentally in the laboratory using two different methods, namely, the electrical method and the method of mixture.

LATENT HEAT
Latent heat is that heat that does not show its presence in terms of a change in temperature. It is the amount of heat that is absorbed by an object in order to undergo a change of state. Whenever energy is transferred between a substance and its surroundings, the substance undergoes a change in temperature. The latent heat of fusion of a substance is the amount of heat the substance requires (absorbs) to change its state from solid to liquid at a constant temperature. The specific latent heat of fusion of a substance is the amount of heat energy required to change unit mass (1kg) of the substance from solid to liquid without change of temperature. The symbol for specific latent heat of fusion is Lf and it SI unit is joule per kilogram (J/kg). Specific latent heat=(heat energy)/mass Then the heat energy Q or E_L supplied to cause a change of state of a solid; mass m is given by Q = mLf Latent heat of vaporisation of a substance is the amount of heat energy required (absorbed) by the substance to change its state from liquid to gas at a constant temperature. The specific latent heat of vaporisation of a substance is the heat energy required to change a unit mass of a substance from liquid to gas without change in temperature. The symbol for specific latent heat of vaporisation is LV and its SI unit is J/kg. The heat energy Q supplied to cause change of state of mass m of a liquid to gas is given by Q = mLv Note: the specific latent heat of vaporisation of steam is the quantity of heat energy required to change 1 kg of water at 100°C to 1kg of steam at100°C.

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