1 feroskhan m. 2 objectives: understanding modes of heat transfer. applications of heat transfer in...

1

FEROSKHAN M

2

Objectives:

•Understanding modes of Heat Transfer.

•Applications of heat transfer in everyday phenomena.

Question

• If a cup of coffee and a ice cream were left on the table in this room what would happen to them? Why?

• The cup of coffee will cool until it reaches room temperature. The Ice Cream will melt and then the liquid will warm to room temperature.

4

Heat energy only flows when there is a temperature difference from a w______ area to a c______ area.armer ooler

Heat is a type of energy called thermal energy.

Heat can be transferred (moved) by three main processes:

1. conduction

2. convection

3. radiation

During heat transfer, thermal energy always moves in the same direction:

HOT COLD

5

Conduction

Conduction is the method of heat transfer in solids. If you touch the handle of a metal saucepan that has been heated, the heat from the saucepan will conduct along the metal handle into your hand.

The heat energy conducts along the

handle to your hand

Why does metal feel colder than wood, if they are both at the same temperature?

Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

7

Convection

8

Convection currenthot water

cools

cool water sinks

hot water rises

cool water warms

This is called a convection current.

9

10

Convection currentsThe air reaching land is heated, the air becomes hotter, the air particles move faster, the air expands, becomes less dense and rises.

The air then cools, its particles move slower, it contracts, becomes more dense and falls.

11

12

•In sunshine, the land warms up more quickly than the sea

•Heat is conducted from the land to the air layer immediately next to it, so this air becomes warmer than that over the sea

•The warmer air over the land rises, because it is now less dense, and the cooler air above the sea moves inland, forming a convection current or breeze, blowing in from the sea - this is called a sea breeze

•In sunshine, the land warms up more quickly than the sea

•Heat is conducted from the land to the air layer immediately next to it, so this air becomes warmer than that over the sea

•The warmer air over the land rises, because it is now less dense, and the cooler air above the sea moves inland, forming a convection current or breeze, blowing in from the sea - this is called a sea breeze

Sea breeze

13

•At night, the land cools more quickly than the sea

•The warmer air is now over the sea

•This air rises and the cooler air moves from the land towards the sea - this is called a land breeze

•At night, the land cools more quickly than the sea

•The warmer air is now over the sea

•This air rises and the cooler air moves from the land towards the sea - this is called a land breeze

14

Why is the freezer compartment at the top of a fridge?

freezer compartment

The freezer compartment is at the top because cool air sinks.

The freezer cools the air at the top and this cold air cools the food on the way down.

It is warmer at the bottom of the fridge.

This warmer air rises and so a convection current is set up.

15

Why are boilers placed beneath hot water tanks in people’s homes?

Hot water rises.

When the boiler heats the water, and the hot water rises, the water tank is filled with hot water.

Why does hot air rise and cold air sink?

Cool air is more dense than warm air.

The cool air ‘falls through’ the warm air and so the warm air rises to the top.

16

Radiation

17

Radiation

Radiation is the only method of heat transfer that does not require particles as a medium. This means it is the only

method of heat transfer that can travel through a vacuum.

Heat radiation is emitted by all objects above absolute zero and travels in straight lines.

Emission experiment

Four containers were filled with warm water. Which container would have the warmest water after ten minutes?

Shiny metal

Dull metal

Dull black

Shiny black

The __________ container would be the warmest after ten minutes because its shiny surface reflects heat _______ back into the container so less is lost. The ________ container would be the coolest because it is the best at _______ heat radiation.

shiny metalradiation

dull blackemitting

Absorption experiment

Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes?

The __________ container would be the warmest after ten minutes because its surface absorbs heat _______ the best. The _________ container would be the coolest because it is the poorest at __________ heat radiation.

dull blackradiation

shiny metalabsorbing

Shiny metal

Dull metal

Dull black

Shiny black

20

Why are houses painted white in hot countries?

White reflects heat radiation and so keeps the house cooler.

Why are shiny foil blankets wrapped around marathon runners at the end of a race?

The shiny metal reflects the heat radiation from the runner back in to their body, which stops the runner from getting cold.