Definition:
It is the transfer of thermal energy through a medium, without medium moving.
- Main mode of heat transfer in solids, also occur in liquids and gases.
- Conductors: Metals (eg. Silver, gold, copper, brass)
- Insulators (poor conductors): Glass, rubber, plastic, wood, wool, cotton, water, air
- Different materials conduct heat at different rates (depends on whether material is conductor or insulator)
- Molecular vibration
- Free electron diffusion
- slow to transfer thermal energy
- no transfer of particles
- occurs in both conductors and insulators
1) Heat supplies thermal energy to particles
2) Thermal energy is transferred from one particle to another by vibration of particles
3) Thermal energy is converted to kinetic energy
4) Kinetic energy of vibrating particles at hot end is transferred to neighbouring particles
Free electron diffusion
- increase speed of transfer of thermal energy
- involves transfer of particles
- occurs in conductors only
1) Only conductors contain free electrons
2) Thermal energy in heat converted to kinetic energy
3) Free electrons gain kinetic energy and move faster
4) Electrons diffuse into cool end
5) Electrons collide with particles and transfer kinetic energy into them
Conduction in liquids and gases
Liquids and gases are POOR conductors of heat.
As shown, water at the top of test tube boils, but ice remains. The rate of heat transfer from top to bottom is extremely slow as water is a poor conductor. Hence, ice melts extremely slowly.
Bunsen burner does not light up match when it is placed a distance away from Bunsen burner, hence air is a poor conductor. The rate of heat transfer from flame to the match is extremely slow. Heat is trasnferred by conduction and also radiation.
Which is a poorer conductor? Air or Water?
Air.
In air, particles are spaced further apart than those in solids. Collisions between molecules are less frequent in liquids, lesser in gases. Transfer of kinetic energy from fast-moving particles to neighbouring particles is slower.
Applications of conduction
- Conductors
- Insulators
1) Cooking utensils made of metals (eg. aluminium, stainless steel) are able to conduct heat and cook food faster.
2) Soldering iron rods are made of iron with tip made of copper. Copper is a much better conductor of heat than iron
Insulators
1) Wool is used as winter clothings. Wool, being an insulator, cause heat to be trapped in wool, unable to get out of wool, keeps body warm.
2) Handles of cooking utensils made of insulators so that hand will not get scalded while using it.
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