Ways of transferring thermal energy.pptx

Year 9 IGCSE Physics

1. FORCES AND ENERGY WAYS OF TRANSFERRING THERMAL ENERGY

2. HEAT AND PARTICLE MOVEMENT When a substance is heated, its particles start to move faster. In solids the particles vibrate around fixed positions. This vibration gets more vigorous when the substance is heated. The particles, therefore, take up more space. This means that solids expand(increase in size) when heated. In liquids, particles vibrate and move around but the particles are not in fixed positions. When heated, liquid particles vibrate and move around faster taking up more space. This means the liquid expands when heated. In gases, particles are far apart and move in straight lines until they collide with other particles or walls of the container. When heated, particles move faster and collide with more force and more frequently. The particles will take up more space in a heated gas, hence gases expand when heated.

3. CONDUCTION Conduction is the process by which heat energy is transferred through collisions between particles. It works best in solids where particles are close together and can only move by vibrating. Metals are best conductors of heat because of the way particles are arranged and existence of free electrons . Wood, plastics, cotton and wool are poor conductors. Materials that do not conduct heat are called insulators. Cooking pan is made from metal because it is a good conductor while its handle is made from plastic because it is a poor conductor. Conduction does not work well in liquids and gases. Conduction cannot happen in a vacuum.

4. CONVECTION When a gas or liquid is heated at the bottom, becomes less dense and start to float up through the colder, more dense parts. This upward movement of warmer liquid or gas is called convection . The colder, more dense particles then sink down to occupy the space. This movement around the heated liquid or gas is called convection current . The convection current causes all of the liquid or gas to eventually become heated. Convection can happen in liquids and gases because the particles are free to move. Convection cannot happen in solids because the particles are not free to move. Convection cannot happen in a vacuum as there are no particles to move.

5. RADIATION Radiation is a method of thermal energy transfer that uses waves and does not depend on particles. It occurs through a vacuum, through gases and through transparent solids, liquids and gases. All objects give out thermal energy through radiation. The hotter the object, the more radiation it will give out ( emit) . Hot objects emit thermal energy though radiation. Colder objects absorb thermal energy from hotter objects through radiation. The following are the best emitters and absorbers of radiation: Are dull Are black Have large surface area The worst emitter and absorbers of radiation: Are shiny Are white or silver Have a small surface area Shiny, white or silver surfaces reflect radiation away.

6. Conduction, convection and radiation Classroom Task Using a properly baled diagram, explain how a vacuum flask is designed to reduce the effects of conduction, convection and radiation. Justus Ela Jumoke Liz zuhayb