**Our Objective**

To plot temperature–time graph for a hot object as it cools.

**The Theory**

The degree of hotness or coldness of a body is known as temperature.

If the hot coffee and cool juice are put open on the table what change will happen after some time?

We can observe that the hotness of coffee decreases and the coldness of cold juice decreases. If we measure the temperature of the coffee and juice, we can observe that the temperature of coffee and juice are equal.

The flow of energy from higher temperature position to lower temperature position is known as heat.

Heat transfer occurs until two objects have the same temperature. A hot body loses heat. A cold body gains heat.

**Transfer of heat occurs in three methods. **

**Conduction** - Transfer of heat through vibration of particles. Only energy transfer occurs through this method. It does not involve any material flow.

**Convection -** Heat is transferred through the flow of fluid. Fluid flow from higher temperature to lower temperature position.

**Radiation** - Transfer of heat through electromagnetic waves.

The rate of change of temperature is proportional to the rate of heat transfer.

Rate of loss of heat depends on the temperature between object and its surroundings.

As the difference between the temperatures of object and surroundings decreases the rate of heat loss decreases.

When we plot a graph between time on X –axis and temperature of a hot object on Y - axis we get a graph as shown in the fig.

The slope of the graph gives rate of temperature change. The temperature of a hot body changes at a faster rate at the beginning. As time goes on the difference between the temperature of the body and surroundings decreases, temperature of hot body changes at slower rate. As time goes on the slope of the graph becomes less steep.

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**Learning Outcomes**

- Students understand how the temperature of a hot body decreases.
- Students understand the relation between the rate of loss of heat and temperature difference between object and surroundings.
- Students understand the concept of thermal equilibrium.