P1 1.1

Infared Radiation

• Infared radiation is energy transfer by electromagnetic waves
• All objects emit infared radiation
• The hotter an object is, the more infared radiaition it emits in a given time

Radio waves, microwaves, infared radiation, ultraviolet rays, x-rays and visible light are parts of the electromagnetic spectrum. Electromagnetic waves are eletric and magnetic waves that travel through space.

The sun emits all types of electromagnetic radiation. The earths atmosphere blocks most of the radiation but not infared.

The earths atmosphere acts like a greenhouse made of glass:

• short wavelength infared radiaiton from the sun can pass through the glass to warm the objects inside the greenhouse.
• infared radiation from these warm objects is trapped inside by the glass becuase the objects emit infared radiation of longer wavelengths that can`t pass through the glass so the greenhouse stays warm.

All the gases in our atmosphere contribute to trapping the infared radiation.

Surfaces and Radiaition

• Dark, matt surfaces emitt more infared radiation than light, shiny surfaces
• Dark, matt surfaces absorb more infared radaition than light, shiny surfaces
• Light, shiny surfaces reflect more infared radiaiton than dark, matt surfaces

A matt surface has lots of cavities. The radiation reflected from the matt surface hits the surface again, The radiation reflected from the shiny surface travels away from the surface.

States of Matter

• Flow, shape volume and density sre the properties used to describe each state of matter
• The particles in a solid are held next to each other in fixed positions
• The particles in a liquid move about at random and are in contact with each other
• The particles in a gas move about randomly and are much further apart than particles in a solid or liquid

Changing of state:

When water in a kettle boils the water turns to steam. Steam is water in its gaseous state. When solid carbon-dioxide or dry ice warms up, the solid turns into a gas directly. When steam touches a cold surface the steam condenses and turns to water.

Kinetic theory of Matter

When the tempreture of a substance is increased, the particles move faster:

• the particles in a solid vibrate about fixed positions
• the particles in a liquid move about at random, in contact with each other
• the particles in a gas move about at random much faster
• the particles in all 3 states of matter have diffrent amounts of energy, gas > liquid > solid

Conduction

• Metals are the best conductors of energy
• Materials such as wool and fibreglass are the best insulators
• Conduction of energy in a metal is due mainly to free electrons transferring energy inside the metal
• Non-metals are poor conductors because they do not contain free electrons

Testing rods are rods with a bit of wax on one end. These are used to show and investigate which materials are the best conductors and insulators.

Copper is a better conductor than steel. Wood conducts better than glass.

Conduction in Metals

Metals contain lots of free electrons. These move about randomly inside the metal and hold all the positive metal ions together. They collide with each other and the positive ions. Eg When a metal rod is heated electrons diffuse and collide with other free elctrons and ions in the cooler ports of the metal. As a result kinetic energy is transferred to these electrons and ions. Ie The cool end becomes hot.

In a non-metallic solid all elctrons are held in the atoms. Energy transfer takes place because the atoms vibrate and shake each other.

Convection

• Convection is the circulation of a fluid caused by heating it
• Convection takes place only in liquids and gases
• Heating a liquid or gas amkes it less dense so it rises and causes circulation

Convection is when circulation currents are caused because fluids rise where they are heated (making them less dense). Then they fall where they cool down (makes them more dense). Convection currents transfer energy from the hotter parts to the cooler parts.

EG A hot water tank, the hot water from the boiler rises and flows into the tank where it rises to the top. When you use a hot water tap you draw off the hot water from the top of the tank.

Sea breezes keep you cool. When its sunny the ground heats up faster than the sea. So the air above the ground warms up and rises. Cooler air from the sea flows in as a sea breeze to take the place of the rising warm air.

Fluids expand when heated because the particles move about more, taking uo more space so the density decreases because the same mass of fluid now occupies a bigger volume. So heating part of a fluid amkes that part less dense and therfore it rises.

Evaporation and Condensation

• Evaporation is when a liquid turns into a gas
• Condensation is when a gas turns to a liquid
• Cooling by evaporation of a liquid is due to the faster moving molecules escaping from the liquid
• Evaporation can be increased by increasing the surface area of the liquid, by increasing the liquids tempreture, or by creating a draught of air across the liquids surface
• Condensation on a surface can be increased by increasing the area of the surface or reducing the tempreture of the surface 

Condensation example: In a steamy bathroom a mirror is often covered by a film of water. There are lots of water molecules in the water. Some of them hit the mirror, cool down and stay there. We say water vapour in the air condenses on the mirror. Effected by increasing the surface area and reducing the surface tempreture.

Cooling by Evaporation example:Injections might numb your skin by dabbing it with a liquid that easily evaporates. As the liquid evaporates your skin becomes to cold to feel any pain. 

Drying off example: Hanging wet clothes on a washing line in fine weather they will gradually dry. The water in the wet clothes evaporates. Effected by increasing the surface area of the liquid, increasing the tempreture of the liquid, creating a draught of air across the liquids surface.

Energy Transfer by Design

• The rate of energy transferred to or from an object depends on:
• the shape and size and type of the object
• the materials the objects in contact with
• the tempreture diffrence between the object and its surroundings

Cooling by Design

The colling system of a car engine transfers energy from the engine to a radiator. The radiator is shaped so it has a large surface area. This increases the rate of energy transfer through convection in the air and through radiation.

A motorcycle engine is shaped with fins on its outside surface. The fins increase the surface area of the engine in contact with air so the engine transfers energy to its surroundings faster than if it had no fins.

Most cars also have a cooling fan that switches on when the engine is too hot. This increases the flow of air over the surface of the radiator.

Another example is a vacum flask look at AQA Physics Textbook Page 36

Specific Heat Capacity

• The greater the mass of an object, the slower its tempreture increases when it is heated
• The rate of tempreture change of a substance when it is heated depends on:
• - the energy supplied to it
• - its mass
• - its specific heat capacity
• Storage heaters use off-peak electricity to store energy in special bricks.

Specific Heat Capicity is the energy needed or energy transferrred to 1kg of the substance to raise its tempreture by 1 degree clecius. The unit of Specific Heat Capacity is the joule per kilogram per degree celcius.

Energy = mass x Specific Heat Capacity x tempreture change (in degrees celcius)

Specific Heat Capacity = Energy / Mass (kg) x change in tempreture (in degrees celcius)

Storage Heaters

Energy transfer from the bricks keeps the room warm. The bricks have a high specific heat capacity so they store lots of energy. They warm up slowly when the heater element is on and cool down slowly when it is off.  Electricity consumed at off-peak times is sometimes charged for at cheaper rate, so storage heaters = cost effective.

Heating and Insulating Buildings

• Energy transfer from our homes can be reduced by fitting :  - loft insulation - cavity wall insulation - double glazing- draught proofing - aluminium foil behind radiators
• U-Value`s tell us how much energy per second passes through diffrent materials
• Solar heating panels do not use fuel to heat water but they are expensive to buy and install

We can compare diffrent insulating materials if we know their U-Values. This is the energy per second that passes through one square metre of material when the tempreture diffrence across it is 1 degree celcius. The lower the U-Value, the more effective the material is an insulator. EG replacing a single glazed window with a double galzed window that has a U-Value 4 times smaller would make the energy loss through the window 4 times smaller.

A solar panel uses solar energy to heat water. The panel is usually fitted on a roof that faces south, making the most of the suns energy. They are a flat box containing liquid filled copper pipes on a matt black metal plate. The pipes are connected to a heat exchanger in a water storage tank in the house. The top of the panel is transparent so the suns energy can get through but the back is insulated so the heat does not go through the back.

Payback time = time taken (in years) to recover the up-front costs from the savings on fuel bills