Heat

Keep Your Cool

• Humans live in a number of different places, some of which can be very hot or very cold.

• Inuit people can live in very cold climates, because their clothing and homes help to keep them warm.

• People living in the desert need to wear special clothing in order for them to keep cool.

Body Temperature

• The average body temperature is 37oC.

• If the body gets a few degress warmer or colder, we die.

• The body has a way of keeping constant body temperature.

• Example: – Sweating

– Shivering

• Clothes slow down the transfer of heat energy between our bodies and the surroundings.

Hot Stuff

• Heat and temperature are related, BUT they are not the same thing.

• Temperature: Describes how hot or cold an object is and is usually measured in degrees Celsius (oC).

• Heat: A form of energy and is measured in Joules (J). It is also known as thermal energy.

• The amount of heat energy stored in something depends on:

–Its temperature

–Its material

–Its mass

Hot Stuff

• Heat energy always flows from a hot object to a cool one.

– The larger the temperature difference, the faster the rate of heat transfer.

• The cool object becomes hotter and the hot object becomes cooler until they are both at the same temperature.

Heating Solids

• A conductor is something that lets energy flow through it.

• When something allows heat energy to flow through it, it is called a conductor.

• Some materials are better conductors of heat than others, such as metals.

• A bad conductor of heat is called an insulator.

– Example: Wood, Plastic

Example

• Do your feet feel colder on tiles or on a carpet?

• Tiles feel colder as they are better conductors of heat than carpets.

• Heat is conducted away from your feet to the tiles.

Heating Solids

• The particle model can be used to explain why some substances allow heat to flow through them and some don’t.

• When a solid is heated, the particles in it gain energy and start to vibrate more.

• The particles bump into each other more, and thus can pass the heat energy on.

• Conduction happens best in solids as the particles are so close together.

• Conduction doesn’t take place in liquids or gases as the particles are far away from each other.

Heating Solids

• The extra vibrations in a hot solid mean that the particles need more space to move around, so the solid expands (gets bigger).

• When the solid cools down again, the particles slow down and the solid contacts (gets smaller).

• When a solid expands, it has the same mass, but it takes up more space. Its density decreases.

Heating Fluids

• Liquids and gases are all fluids.

• Heat doesn’t travel to fluids by conduction, but travels by convection.

• When a fluid is heated, the particles move around faster.

• The fluid expands and becomes less dense. This is because the same ass of fluid is taking up more space.

• If only a part of the fluid is being heated, that part starts to rise because it is less dense than the fluid around it. Cooler fluid moves in to take its place and a convection current forms.

Convection Currents

• Convection currents play a large part in the weather.

• Some parts of the Earth are warmed more than others by the sun.

• Warm places on the Earth heat the air above them and convection currents form.

• We feel these currents as wind.

• Convection currents also form when part of a fluid is colder than the surroundings.

Radiation

• There is nothing between the sun and the Earth, so heat cannot travel from the sun to Earth by conduction or convection.

• All the heat that comes from the sun travels as radiation, sometimes called infrared radiation.

• Infrared radiation is similar to light.

• It does not need a medium to travel through, and can pass through transparent substances such as air and glass.

Radiation

• All hot things emit (give out) radiation.

• Black things are the best absorbers and emitters of heat.

• White, shiny objects reflect most of the radiation that lands on them.

Thermal Imagers

• Thermal imagers are instruments that create pictures of heat rather than light.

• They measure infrared radiation and convert the data into map of temperature.

• Thermal imaging can be used for filming things at night, and for finding the temperature of remote parts of the Earth by taking photos from space.