All substances around us are made up of atoms and molecules, which are always in vibratory motion. Due to this motion, substances have an energy known as heat energy. Heat energy flows from hot substances to cold substances, or from the hot region to the cold region of a substance. When heat is supplied, the atoms and molecules vibrate and make neighbouring atoms and molecules vibrate, so heat is transferred from one part of the substance to another. This chapter covers the effects of heat, the transfer of heat, calorimetry, and three applications: the calorimeter, the thermostat and the thermos flask.
1. Effects of Heat
When heat energy is supplied to a substance it brings about many changes. The three important changes seen in daily life are:
- Expansion
- Increase in temperature
- Change in state
Expansion
- A heated metal ball does not pass through a ring it passed through when cold. When the ball is heated, its atoms gain heat energy, start vibrating and force each other apart – so expansion takes place.
- As the ball loses heat to the surroundings, it comes back to its original size.
- Expansion takes place in solids, liquids and gases; it is maximum in gases.
- Electric wires used for long-distance transmission of electricity expand during day time and contract at night. So they are not set very tightly; if set very tightly, they would break when they cool at night.