Quantity of Heat · 8 min read
Heat capacity and specific heat capacity
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Introduction to heat energy
Have you noticed that if you put a small cup of water and a big basin of water on the same gas cooker, the small cup gets hot faster? This is because different objects need different amounts of heat to increase their temperature. In Physics, we call this the study of heat capacity.
Quantity of heat
Heat is a form of energy that flows from a hot body to a cold body. When we talk about how much heat a body has absorbed, we use the letter Q. It is measured in Joules (J).
Heat capacity explained simply
Think of heat capacity as the 'stomach' of an object for heat. Some objects have a big stomach and need a lot of heat to get hot, like a large drum of water. Others have a small stomach and get hot quickly, like a small metal spoon.
In simple terms, it is the total heat needed to make a whole object hotter by 1 K.
Specific heat capacity explained simply
This is similar to heat capacity, but here we focus on just 1 kilogram of the material. For example, the heat needed for 1 kg of iron is different from the heat needed for 1 kg of water. Water has a very high specific heat capacity, which is why it stays warm for a long time in a flask.
Essentially, it tells us how much heat 1 kg of a particular material can hold for every degree rise in temperature.
The formula for heat calculations
We calculate the heat absorbed or lost using the formula: Q = m c (theta2 - theta1), where m is mass in kg, c is specific heat capacity, and theta is the change in temperature.
The units of specific heat capacity (c) are J kg-1 K-1 or J kg-1 C-1. The units of heat capacity (C) are J K-1 or J C-1.
Worked example
Calculate the heat required to raise the temperature of a 2 kg copper block from 30 C to 80 C. (Specific heat capacity of copper = 400 J kg-1 K-1).
1. Identify given values: m = 2 kg, c = 400 J kg-1 K-1, initial temperature = 30 C, final temperature = 80 C.
2. Calculate temperature change: 80 - 30 = 50 C.
3. Apply formula: Q = m c (change in temperature).
4. Substitute: Q = 2 x 400 x 50 = 40,000 J or 40 kJ.
5. The final answer is 40,000 Joules of heat energy.
Key points
- •Heat capacity (C) applies to the whole body, while specific heat capacity (c) applies to 1 kg.
- •The formula relating both is C = mc.
- •Water has a high specific heat capacity (4200 J kg-1 K-1) compared to metals.
- •Heat energy (Q) is measured in Joules (J).