Heat transfer methods

Learning objectives Explain the 3 ways that heat can be transferred Describe how heat is transferred in common situations

How does heat move? Passing vibrations between molecules Moving as waves through space Moving fluid

What about these situations? Touching a hot cup to coffee The sun A radiator (Heizkörper)

Conduction (Leitung) Heat is passed on through the vibration of particles Heat always passes from a hot body to a cold one Example: touching a cup of hot coffee

Questions 1. Name 3 good conductors 2. Name 3 poor conductors 3. What is another name for a poor conductor? 4. Why will a balloon filled with water not burst if a match is under it?

Questions 1. Name 3 good conductors 2. Name 3 poor conductors 3. What is another name for a poor conductor? Insulator 4. Why will a balloon filled with water not burst if a match is under it? The water conducts the heat away from the balloon before it can melt

Convection (Konvektion) Heat is transferred through a hot fluid (Flüssigkeit) rising The fluid expands, making it less dense than its surroundings Example: a radiator heating a room

Questions 1. Why can convection not occur in solids? 2. What would cause the speed of a convection current to increase? 3. Why is a radiator not a radiator? 4. How can a “radiator” be made to give out more heat into a room?

Questions 1. Why can convection not occur in solids? The molecules are not free to move 2. What would cause the speed of a convection current to increase? A greater temperature difference between the hot and cold sides 3. Why is a radiator not a radiator? It heats by convection more than radiation 4. How can a “radiator” be made to give out more heat into a room? Paint it black, give it more surface area

Radiation (Strahlung) Heat is transferred by radiation or infra- red light Radiated heat can travel through a vacuum Example: heat reaching the Earth from the sun

Questions 1. How fast does radiated heat travel? 2. Why does glass allow radiated heat to pass through, and plastic does not? 3. If a black bulb and a clear bulb are used for heating, which one would give out most heat? Why?

Questions 1. How fast does radiated heat travel? At the speed of light 2. Why does glass allow radiated heat to pass through, and plastic does not? Heat is carried as infra-red waves. This is very close to light, and so passes through most things that light will pass through 3. If a black bulb and a clear bulb are used for heating, which one would give out most heat? Why? The black bulb, as no energy is wasted as light