Physics

Heat can be transferred from place to place by conduction, convection and radiation. Dark matt surfaces are better at absorbing heat energy than light shiny surfaces.

Infrared radiation

All objects emit (give out) and absorb (take in) thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits.

Infrared radiation is a type of electromagnetic radiation, which involves waves rather than particles. This means that, unlike conduction and convection, radiation can even pass through the vacuum of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the Earth.

The kinetic particle theory explains the properties of the different states of matter. The particles in solids, liquids and gases have different amounts of energy. They are arranged differently and move in different ways.

Solids ~ Close together, regular pattern, vibrate around a fixed position. They have a fixed shape and cannot flow. They cannot be squashed or compressed because they have no space to move into.

Liquids ~ Close together, random pattern and move around each other. They flow and take the place of their container. They cannot be squashed or compressed because the particles are close together and have no space to move into.

Gas ~ Far apart, random pattern and move quickly in any direction. They flow and completely fill their container. They can be compressed and squashed.

Heat energy can move through a substance by conduction. Metals are good conductors of heat but non-metals and gases are usually poor conductors of heat. Poor conductors of heat are called insulators. Heat energy is conducted from the hot end of an object to the cold end.

Heat conduction in metals

The electrons in piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now charged metal ions. The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons. These move through the structure of the metal, colliding with ions as they go.

Liquids and gases are fluids. The particles in these fluids can move from place to place. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy. Heat energy is transferred from hot places to cooler places by convection.

Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size.

The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up.

Evaporation and condensation are changes of state:

• evaporation involves a liquid changing to a gas
•  condensation involves a gas changing to a liquid.

Evaporation is the reason why damp clothes dry on a washing line. Condensation is the reason why windows become foggy on a cold day.

Evaporation ~ The particles in a liquid have different energies. Some will have enough energy to escape from the liquid and become a gas. The remaining particles in the liquid have a lower average kinetic energy than before, so the liquid cools down as evaporation happens. This is why sweating cools you down. The sweat absorbs energy from your skin so that it can continue to evaporate.

Condensation ~ The particles in a gas have different energies. Some may not have enough energy to remain as separate particles, particularly if the gas is cooled down. They come close together and bonds form between them. Energy is released when this happens. This is why steam touching your skin can cause scalds: not only is the steam hot, but energy is released into your skin as the steam condenses.

Factors affecting the rate of condensation and evaporation

The rate of condensation increases if the temperature of the gas is decreased. On the other hand, the rate of evaporation increases if the temperature of the liquid is increased. It is also increased if:

• the surface area of the liquid is increased
•  air is moving over the surface of the liquid.

The bigger the difference in temperature between an object and its surroundings, the greater the rate at which heat energy is transferred. Other factors also affect the rate at which an object transfers energy by heating. These include the:

• surface area and volume of the object
•  material used to make the object
• nature of the surface that the object is touching.

Animal adaptations ~ Small animals like mice have a large surface area compared to their volume. They lose heat to their surroundings very quickly and must eat a lot of food to replace the energy lost. Large animals like elephants have a different problem. They have a small surface area compared to their volume. They lose heat to their surroundings more slowly and may even have difficulty avoiding overheating.

U-values ~ U-values measure how effective a material is an insulator. The lower the U-value is, the better the material is as a heat insulator.

Payback Time ~ Homeowners may install double glazing or extra insulation to reduce heat energy losses and so save money. However, these energy-saving solutions cost money to buy and install. The payback time of an energy-saving solution is a measure of how cost-effective it is. Here is the equation to calculate payback time:

payback time (years) = cost of installation (£) ÷ savings per year in fuel costs (£)

The payback time will be shortest if the cost of installation is low compared to the savings made each year.

Solar panels do not generate electricity, but rather they heat up water. They are often located on the roofs of buildings where they can receive heat energy from the sun.

Advantages

• solar energy is a renewable energy resource
•  no harmful polluting gases are produced.

Disadvantages

• solar panels may only produce very hot water in very sunny climates, and in cooler areas may need to be supplemented with a conventional boiler
• although warm water can be produced even on cloudy days, solar panels do not work at night.

The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1 kg of the substance by 1°C.

Calculating specific heat capacity

Here is the equation relating energy to specific heat capacity:

E = m × c × θ

• E is the energy transferred in joules, J
•  m is the mass of the substances in kg
•  c is the specific heat capacity in J / kg °C
• θ (‘theta’) is the temperature change in degrees Celsius, °C

Magnetic ~ Energy in magnets and electromagnets

Kinetic ~ Energy in moving objects

Heat ~ Thermal energy

Light ~                                        Most Kids Hate Learning GCSE Energy Names

Gravitational Potential ~ Stored energy in raised objects

Chemical ~ Stored energy in fuel, food and batteries

Sound ~ Energy released by vibrating objects

Electrical ~ Energy in moving charges or static electric charges

Elastic ~ Stored energy in stretched or squashed objects

Nuclear ~ Stored in the nuclei of atoms

Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved.

The efficiency of a device is the proportion of the energy supplied that is transferred in useful ways. You should be able to calculate the efficiency of a device as a decimal or as a percentage.

Calculating efficiency

The efficiency of a device such as a lamp can be calculated:

efficiency = useful energy out ÷ total energy in (for a decimal efficiency)

efficiency = (useful energy out ÷ total energy in) × 100 (for a percentage efficiency)

The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh. 

 E = P × t

• E is the energy transferred in kilowatt-hours, kWh
•  P is the power in kilowatts, kW
•  T is the time in hours, h.

Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000. Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600.

Joules, watts and seconds

You also use the equation E = P × t when:

·          E is the energy transferred in joules, J

·          P is the power in watts, W

·          T is the time in seconds, s.

Joules, watts and seconds

You also use the equation E = P × t when:

• E is the energy transferred in joules, J
•  P is the power in watts, W
• T is the time in seconds, s.

Electricity meters measure the number of units of electricity used in a home or other building. The more units used, the greater the cost. The cost of the electricity used is calculated using this equation:

total cost = number of units × cost per unit

For example, if 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.

Remember that the number of units used can be calculated using this equation:

units (kWh) = power (kW) × time (h) … so …

total cost = power (kW) × time (h) × cost per unit