unit c - 2.4 & 2.5 -- conduction, convection, and radiation

2.4 & 2.5 CONDUCTION,

CONVECTION, AND RADIATION

WHAT IS CONDUCTION? In the previous sections we looked at

heat energy.This is energy that transfers from high

temperature to low temperature

Conduction is an example of heat energy taking place

Conduction – is the transfer of energy between two substances in contact

EXAMPLE Imagine you have a metal spoon in a

mug of hot chocolate

HOW THIS WORKS Because the particles inside the hot

chocolate are moving faster than those in the spoon –energy gets transferred

This energy transfer will increase the speed of the spoons particles increasing its own temperature

The parts of the spoon not touching the hot chocolate will slowly warm up as well

CONDUCTION AND CONDUCTORS Heat transfer only occurs in one

directionHot Cold or High kinetic energy low kinetic energy

Kinetic energy means the amount of movement

Conduction is most common in solidsLess common in liquidsRARE in gases

CONDUCTORS Conductors – are materials that allow

easy transfer of heat

Different materials allow different amounts of heat to be exchangedThe better the conductor the more heat that

can be transferred

Metal is a very good example of a GOOD conductor

INSULATORS Many solid items conduct heat, but not

all allow a lot to be transferred.

Insulators are materials that Do Not allow easy transfer of heat from a hot substance to a cold substance

Plastics, Cork and Wood are good examples of INSULATORS

HOW DO WE USE THEM Have you ever touched a pot on the

stove and burnt your hand?

Curled your hair and touched the metal?

To avoid injury we commonly combine Insulators & Conductors in order to create tools we can safely use

FIND THE CONDUCTOR AND INSULATOR

CONVECTION Heat is transferred when liquid or gas

particles move from one area to another.

In convection the particles move, in conduction only heat moves

Convection ONLY occurs in liquids and gases

CONVECTION CURRENTS Heat transfer by convection occurs

when particles in a liquid or gas move in a circular pattern.

These currents form when heat is transferred to the liquid or gas

EXAMPLE – BOILING WATER When pot of water is placed on a hot stove the

heat from the stove travels through the conductors and speeds up the particles in the water

The increased particle speed of the water on the bottom causes the water to move to the surface

This pushes the cooler water down to be heated by the stove

The water that moves away from the heat slightly cools and the process repeats itself

CONVECTION CURRENTS IN AIR Heat moves only one direction

From hot to cold

With the air the circular motion still is present

The warmer air will rise pushing the colder air down towards the heat and the cycle continues

RADIATION The third form of heat transfer, only one that

does not rely on movement of particles

Radiation is the transfer of energy through invisible waves Some travel great distances, such as; The Sun

Energy transferred from its source is called Radiant Energy

Heat is one type of energy, another is invisible infrared waves

HOW DOES RADIATION WORK? When these waves hit an object the

kinetic energy (movement) increases

The particles move faster and the object becomes hotter.

Every hot object gives off some radiant energyThis is how you feel the heat of a stove

without touching it

EXAMPLE – HOT CAR Have you every gone into a car that has

been sitting in the sun on a very hot summer day?

What does it feel like?

What happens when you touch the dashboard?

This is a result of the radiant energy from the sun, transferring heat to the car

REFLECT OR ABSORB Can anyone tell me why people tend to

wear more black clothes in the winter compared to summer?

Why does sand burn your feet when you walk on it?

Shiny or Light color objects reflect the radiant energy

Dark or dull color objects absorb radiant energy