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Heat

Change of state

Take the same amount of water in two similar beakers. Put a thermometer in each beaker, and heat the beakers. Heat one beaker for 5 minutes and the other for 10 minutes. What do you observe?

You will observe that the water in the beaker that is heated for the longer time becomes hotter than the other beaker. The thermometer in this beaker shows a higher temperature. Why does this happen?

The answer is very simple—the longer the time we heat water, the hotter it becomes. So, we can say the rise in the temperature of a body is directly proportional to the heat energy contained in the body.

Let us do an experiment to explore more properties of heat.

As shown in the figure, take two beakers: one with 100 mL of water and the other with 200 mL of water. Now, start heating the beakers, and heat them till the temperatures in both the beakers become the same. Do they require the same time?

You will observe that the time taken to raise the temperature of water by 30°C in the 200-mL beaker is double the time taken to raise the temperature of water (by the same measure) in the 100-mL beaker. Perform this experiment a few more times by raising the temperature to different levels.

Every time you will get the same result.

Hence, we can conclude that the amount of heat absorbed by a body to raise its temperature is directly proportional to the mass of the body. The more the mass of a body, the more is the heat required by it to raise its temperature.

Again, we take the same two beakers. Fill the beaker containing 200 mL of water with 100 mL of oil instead.

Heat the two beakers for the same amount of time. You will observe that the temperature of oil increases more than that of water for the same amount of heat energy supplied. Thus, it can be concluded that water needs more heat energy than oil to be raised to the same degree of temperature.

Hence, it can be concluded that heat energy contained in a body is directly related to the chemical composition of the body.

Therefore, from all the experiments we can conclude that

• When the mass (m) is constant, the rise in the temperature (Δθr ) of a body…

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