P1 board notes

Stuff you know:

What is the unit of temperature?

What is the unit of heat energy?

Which way does heat flow?

Degrees Celsius (centigrade)

•The Joule

•From hot to cold

Stuff you know (2):

What happens to hot things?

What happens to cold things?

Why?

They cool down

• They heat up

• Heat flows into the surroundings or out of the surroundings

• So objects end up at room temperature.

Stuff you know (3):

Does everything need the same energy to heat it up?

What makes a difference?

Why?

No

• How big it is

• The bigger it is (more mass), the more energy it takes and different materials need different amounts of energy.

• What it’s made of

A Thermogram

Eating an ice lollyThe colours are added by computer

The coldest areas are colour-coded in blue, the hottest in white.

Thermograms - uses

A breast with cancerThe warm one has

the cancer

A power switch overheating because of a poor connection

Normal

This room has cold areas round the top of the window

A heavy security fire door losing heat

Can-do:39 – I can use a thermogram9 – I can measure temp. in C14 – I can calculate Sun exposure from

SPF and health recommendations

Heating things up

When you try to heat-up different materials, the amount of energy needed is different for every material.

Metals need small amounts of heat energy, water needs a lot.

Heating a metal - 1

Collect an aluminium block, a timer, a thermometer, a power pack, 5 wires and a heater.

Connect the heater to the power pack with an ammeter and voltmeter as shownThen put the heater in the block

Heating a metal - 2

Put the thermometer in the block Reset the timer.Take the starting temperatureTurn on the power pack and start the

timer.Heat for 5 minutes.Record the new temperature, you may

need to wait a bit for it to settle.

Heating a metal - 3

Calculate the rise in temperatureCalculate the electrical energy as:(volts x amps x time in seconds)This is the electrical energy in Joules

Calculate how much heat energy it took per degree for the 1kg block (joules temperature change in degrees)

This is how much heat energy was needed per degree per kilogram.

Definition:

The name for this value is specific heat capacity defined as:

The amount of heat energy needed to raise the temperature of 1kg of a substance by 1°C

Write-up the experiment.

Practice

There is an equation for specific heat capacity:

Energy = Mass x Specific heat capacity

x Change in temperature

Latent heat

This is the energy change involved when 1 kg of a substance changes state – Melting / freezingBoiling / condensing

Melting and boiling need heat energy, freezing and condensing release heat energy

The energy change depends on which you are doing.

Can-do P2c - the energy transferred in a circuit

Insulation

Heat can move by:Conduction (touching)Convection (warm gases / liquids

moving)Radiation (heat “shining”)

Insulation has air-gaps in itGases are poor

conductorsThe insulation

material stops air from moving around

The effect of insulation

Take two metal beakers, one with insulation and one plain

Put the same amount of hot water in each

Take both temperatures every minute for 10 minutes

Draw a graph of both temperatures

So how does heat travel?

3 key words:ConductionConvectionRadiation

Conduction

The particles in a hot material are vibrating and the vibration spreads to a colder material if they can touch.

Gases and liquids are poor conductors because their particles are not strongly connected together.

Convection

Heating-up part of a liquid or gas makes it expand, but it isn’t any heavier, so it rises above the cooler partsSo hot air risesAnd so do all hot gases and liquidsThey will circulate if they have a chance.

How early coal mines were ventilated

Convection currents

(Use overlay mode to draw currents)

Radiation

“Heat travelling like light” “Infra-red” light rays Straight lines mean you can get heat

shadows like light shadows Reflected like light, especially from light-

coloured materials Absorbed best by dark/black materials “Shines-out” best from black materials

Says it all, really....

More infra-red

Some other uses of infra-red radiation:Toaster (see it glow red)Iron (see it doesn’t glow red)Grill pan (heat energy absorbed by food)Remote controls for TVs etc.

The whole spectrum

The spectrum we can see:

is only a small part of the whole spectrum of electromagnetic waves:

Microwaves

Microwaves are another form of radiation energy, like visible light and infra-red.

They vibrate water molecules in the outer 20mm of food

Vibration = heatThe heat has to conduct to the rest of the

food, so you usually stir it or rotate it.They do not really “cook from the inside”!

Or, used for mobile phones,Hazards:All forms of radiation are bad for you Microwaves have been connected to cell

damage causing cancer

How different ovens cook (because it’s given people big problems!)Conventional Microwave

Hot air circulates by convection

Touches food, heat conducts into food outer surface

Heat spreads through food by conduction and (sometimes) convection

Microwaves vibrate water molecules

This means they are hotter

This heats the outer 1½ cm of food

Heat spreads through food by conduction and (sometimes) convection

Ultraviolet and Sunburn

Sunburn is caused by ultra-violet rays in sunlight

U.V. damages living cells This is good for sterilising medical

instruments or bacterial culturesIt’s bad when it’s your skinToo much skin damage triggers skin

cancer.

Sunscreen creams

You need a physical barrier to stop the U.V. from reaching your skin

Clothes, hats, sunscreenSun protection factor, “how many times

longer” you can stay in the SunE.g. if you would normally burn after an

hour, factor 10 cream will make that 10 hours

You get different amounts and intensities of sunlight in different parts of the World.

Try these:

If you would normally burn after ½ an hour in the sun, how long can you stay out when wearing:

Factor 10Factor 20Factor 60 for sensitive skinDoes everybody’s skin burn after the

same time?

Beware!

The numbers on the suncream bottle assume you have laid it on thickly

Most people don’t.

The greenhouse effect

The Sun sends out short-wave infra-red rays (high energy from a hot object)

These come into the greenhouse through the glass

Long wave infra-red (from cooler objects) is reflected back into the greenhouse by the glass and can’t get out

So the greenhouse stays hotter than it should do

Global Warming

Water vapour and carbon dioxide in the atmosphere have the same effect as the glass

So the atmosphere traps heat and keeps us all warmer

This is good unless…We have been making extra carbon dioxide

from burning fossil fuels for the past 200 years, mostly in the last 40 years.

Probable Effects

More extreme weather :Hot, wet countries get hotter and wetter

(e.g. India – more floods, more dangerous)

Hot dry countries get hotter and dryer (e.g. Australia – drought)

England just gets warmer rain. (Crops rot in wet fields, cars rust faster)(and the lawn is too long and wet to cut with a mower.)

Efficiency

Mostly of power stations, heaters, motors, hair dryers etc.

Defined as useful energy obtained (x 100) total energy supplied

(The x 100 is for a percentage, which is optional but makes some people more comfortable)