Ohne Titel

Seite 1

1.Energy transfer by heating

1.1 Infrared radiation

Every object emits infrared radiation

The hotter an object is, the more infrared radiation it emits in a given time

The Earth’s atmosphere blocks out harmful electromagnetic radiation from the sun

The Earth’s atmosphere acts like a greenhouse

Short wavelengths, such as infrared and light, can pass through the atmosphere and warm up the earth

Infrared radiation from the warm objects in the earth cannot escape because they emit infrared radiation of longer wavelengths

Gases in the atmosphere such as carbon dioxide and methane trap the infrared radiation from the Earth making the Earth warm

The earth is becoming too warm causing polar ice caps to melt and sea levels to rise

1.2 Surfaces and radiation

A light, shiny surface is a better reflector of infrared radiation because of its smooth surface

A dark, matt surface is a better absorber of infrared radiation because of its cavities in the surface so the radiation reflected from the matt surface hits the surface again

A dark, matt surface is also a better emitter of radiation that a light shiny surface

1.3 States of matter

→

When the temperature of a substance is increased, the particles move faster

Particles in a solid are held next to each other in a fixed position and vibrate about in their fixed position

Particles in a liquid are in contact with each other but move about at random

Particles in a gas move about at random and much more faster thus gases have low densities

Particles in a gas have more energy than those in a liquid and solid

1.4 Conduction

Metals are better conductors than non-metals

Copper is a better conductor than steel

Metals contain free electrons which collide with other free electrons and the positive ions and hold the positive ions together

When one end of a metal rod is heated, the free electrons gain kinetic energy and move faster

They diffuse and collide with other free electron and ions in the cooler part

Thus transferring kinetic energy to those electrons and ions

Non-metallic solids don’t have free electrons and therefore, energy transfer only takes place because of atoms vibrating and shaking each other

1.5 Convection

Convection occurs when a fluid (gas/liquid) is heated

Heat is applied to the fluid meaning the particles gain energy

The fluid then expands and becomes less dense and therefore rises

The fluid cools, the particles lose energy and becomes more dense and therefore falls

Convection currents transfer energy from the hotter parts to the cooler parts

1.6 Evaporation and condensation

Evaporation is when a liquid turns into a gas

Particles in the liquid when heated, absorb more energy

Therefore they move faster

Some particles from the surface escape and enter the air in the room

The remaining liquid is cooler

The remaining liquid is cooler because weak attractive forces exist between the molecules in the liquid

The faster molecules, which have more kinetic energy break away from the attraction of other molecules and escape from the liquid

The liquid is cooler because the average kinetic energy of the remaining molecules in the liquid has decreased

Increasing the surface area of the liquid meaning more if the liquid is contact with dry air

Increasing the temperature of the liquid increases rate of evaporation

Creating a draught of air across the liquid’s surface

Gases condense when it hits something cooler

Increasing the surface area increase the rate of condensation

Increasing the surface temperature can increase the rate of condensation

1.7 Energy transfer by design

Energy transfer needs to be controlled

Cooling fins increase the surface area of the engine in contact with the air so the engine transfers energy to its surroundings faster than if it had no fins

Radiators in cars have large surface areas increasing the rate of energy transfer through convection in the air and through radiation

The vacuum between the container cuts energy transfer by conduction and convection

This is because in a vacuum there are very few particles of air and conduction and convection need particles to occur

Glass is a poor conductor so there is little energy transfer by conduction

Glass surfaces are silvery to reduce radiation but also infrared radiation is reflected

Plastic/Rubber spring at the bottom for supports is a good insulator

Plastic cap stops cooling by evaporation as it stops vapour loss from the flask

Plastic cap also stops conduction

The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred

Design factors such as the materials an object is in contact with, the object’s shape and surface area also affect the rate at which an object transfer energy

1.8 Specific heat capacity

When a substance is heated, its temperature rise depends on

The amount of energy supplied to it

The mass of the substance

What the substance is

The specific heat capacity of a substance is the energy needed or energy transferred to 1 kg f the substance to raise its temperature by 1℃

The unit of specific heat capacity is J/kg ℃

E = m × c × θ

E = energy transferred

M = the mass in kg

C = specific heat capacity in J/kg ℃

Θ = temperature change in degrees

Storage heaters use electricity at night (off-peak) to warm up concrete blocks in the heater.

The blocks have a high specific heat capacity so they store lots of energy

They warm up slowly and cool down slowly

The greater the mass of an object, the more slowly its temperature increases when heated

1.9 Heating and insulating buildings

Loft insulation such as fibreglass reduces the rate of energy transfer through the roof as fibreglass is a good insulator

Cavity wall insulation reduces energy loss through the outer walls

The space between the two layers of brick that make up the wall is pumped with insulation

It is a better insulator than the air it replaces and it traps the air in small pockets reducing convection currents

Aluminium foil between and a radiator panel and the wall reflects radiation away from the wall

Double glazed windows have two glass panes with dry air or a vacuum

This prevents conduction and convection

Dry air is a good insulator so it reduces the rate of energy transfer by conduction

The U value is the energy per second that passes through one square metre of material when the temperature difference across it is 1℃

The lower the u-value the more effective the material is as an insulator

Solar heating panels do not use fuel to heat water but they are expensive to buy and install

Payback time for solar heating panels is too long