GCSE OCR Gateway Science - P1 A-D

Energy Flow

Heat energy flows from hot regions to cooler ones. Therefore :

• An object that is hotter than its surroundings will lose heat energy and cool down.
• An object that is cooler than its surroundings will gain heat energy and warm up.

Thermograms

A thermogram is simply a picture that indicates different temperature gradients.

e.g. what's hot and cold.

White and red - hotter, blue and purple - colder

Heat Energy and Temperature

Temperature is a measure of how hot something is, this is measured using a thermometer.

Heat is a measure of the thermal energy contained in an object it is measured using a device like a joule-meter.

Specific Heat Capacity - All objects heat up if energy is given to them. The amount of energy that is needed to change an object's temperature depends on :

• mass
• material its made from
• how much of a temperature rise is needed

Specific Heat Capacity (cont'd)

All substances need different amounts of energy to increase their temperature by 1 C.

This is called the Specific Heat Capacity.

Energy Transferred (to change temp.) (J) = SHC (J/kg/C) * Mass (kg) * Temp. Change (C)

Specific Latent Heat - is a measure of how much heat energy is needed to melt or boil it of 1kg of a substance.

Energy Transferred (to change state) (J) = Specific Latent Heat (J/kg) * Mass (kg)

Investigating Melting and Boiling

If we look at a material changing state, such as water, the temperature does not change during a change of state because :

• the energy is being used to break the intermolecular forces that exist between the particles.
• the temperature remains the same even though the substance may be gaining heat energy from the source like a Bunsen or heater.
• the temperature will not change until the intermolecular forces have been overcome enough for the particles to be in their new arrangement for that state.

Payback Time - is the time it takes for the cost of installing insulation to be equaled by the savings made from reduced energy cost.

payback time (in years) = cost of insulation / saving each year

Energy Efficiency - is a measure of how good a device is at changing energy from one form to another. All devices waste energy, so the efficiency of a device will never be 100%.

Conduction

In solids, particles are held together by strong forces but vibrate continuously about fixed positions and heating on end of a solid gives the particles more kinetic energy so they vibrate more. This causes neighbouring particles to vibrate more, and in this way kinetic energy is passed between the particles.

Convection

When a gas is heated it expands, becomes less dense and rises. This change of density causes fluid flow as the rising warm gas is replaced by falling colder gas, which has a higher density. In this way a convection current is set up that transfers heat throught he gas. Convection also takes place in liquids, for the same reason.

Radiation

All objects lose heat energy through waves called infrared radiation. This radiation does not need a medium to travel through and is the way that heat energy from the Sun reaches us through space.

Cavity walls reduce energy loss by conduction, but as the air inside them flows, heat is lost by convection instead. We can reduce this heat loss by adding cavity wall insulation, which traps the air in pockets, preventing convection.

Cooking with Microwaves

• The microwaves produced in a microwave oven penetrate about 1cm into the food. They are absorbed by water molecules in the outside layers of the food, increasing their kinetic energy and raising the temperature of the food.
• The nergy is transferred to the centre of the food from the outside by conduction or convection.
• Microwaves pass through glass and plastic but are reflected by metal, so most of the microwaves stay inside the oven.

Cooking with Infrared

• Infrared radiation is the heat source in toaster, cookers and grills. The infrared radiation is absorbed by all particles on the surface of the food, increasing their kinetic energy, and this increases the temperature of the food by conduction or convection.

Energy in Waves

The amount of energy carried by an electromagnetic wave depends on its frequency. The higher the frequency, the greater the enrgy and the more dangerous the radiation. Visible light has a higher frequency that infrared radiation, which has a higher frequency than microwaves.

Microwave Communication

Microwaves are used for movbile phones and long-distance communication. Transmitters and receivers are usually on high building or towers as they have to be in the 'line of sight'. This is because microwaves cannot bend round corners or over mountains. Places that are not in line of sight with a mast will get a poor signal.

Diffraction occurs because when waves pass through a gap of similar wave lenght and spread out on the opposite side.