Energy Transfer by Heating//Infared Radiation
- Heat can be transferred from place to place by conduction, convection and radiation.
- Dark, matt sufaces are better at absorbing heat energy than light shiny surfaces.
- Heat energy can be lost from homes in many different ways and there are ways of reducing these heat losses.
- All objects emit (give out) and absorb (take in) thermal radiation - Infared Radiation.
- This hotter an object is, the most infared radiation is emits.
- Infared radiation is a type of electromagnetic radiation which involves waves rather than particles.
- Unlike conduction and convection, radiation can even pass through the vaccum 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.
- They have a fixed shape and cannot flow- The particles cannot move from place to place
- They cannot be compressed or squashed - The particles are close together and have no space to move into.
- They flow and take the shape of their container- The particles can move around each other
- They cannot be compressd or squashed - The particles are close together and have no space to move into.
- They flow and completely fill their container- The particles can move quickly in all directions
- They can be compressed or squashed- The particles are far apart and have space to move into
- Heat energy can move through 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 a peice 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 tightly together and they vibrate continually.
- The hotter the metal the more kinetic energy these vibrations have.
- The 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 to 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
- Evaporation involves a liquid changing into a gas
- Condensation involves a gas changing into a liquid
- The particles in a liquid have different energies. Some will have enough energy to escape from the liquid and becomes be 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 is can continue to evaporate.
The particles in a gas have different energies. Some may not have enough energy to remain as separate particles, particulary 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 you skin can cause scalds.
- The surface area of the liquid is increased and air moving over the surface of the liquid can affect the rate of condensation and evaporation
Keeping warm or cool
The bigger different in the 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. Such as...
- Surface area and volume of the object
- Material used to make the object
- Nature of the surface that the object is touching.
- 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.
Engineers design heat transfer devices so they can gain or lose heat energy efficently. For example, car radiators are flat, with many small fins to provide a large surface area.
U-Values measure how effective a material is an a insulator. The lower the U-value is, the better the material is as a heat insulator.
- A cavity wall has a U-value of 1.6 W/m²
- A solid brick wall has a U-value of 2.0 W/m²
- A double glazed window has a U-value of 2.8 W/m²
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.
Payback time (years) = Cost of Installation (£) ÷. Savings per year in fuel costs (£)
Solar panels do not generate electricity, but rather they heat up water. They are often located on the roofs of buildings where they can recieve heat energy from the sun.
Cold water is pumped up to the solar panel where it heats up and is transferred to a storage tank.
A pump pushes cold water from the storage tank through pipes in the solar panel. The water is heated by heat energy from the sun and returns to the tank.
In some of the systems, a conventionl boiler may be used to increase the temperature of the water.
- Solar energy is a renewable energy resource
- No harmful polluting gases are produced
- Solar panels may only produce very hot water in very sunny climates and in cooler areas may need to have a conventional boiler as well. Solar panels do not work at night.
Specific Heat Capacity
- Temperature is a measure of how hot something is (measured in degrees)
- Heat is a measure of the thermal energy contained in an object (measured in J)
When heat energy is transferred to an object, its temperature increat depends upon the:
- Mass of the object
- The substance the object is made from
- The amount of energy transferred to the object
The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1kg of the substance by 1 degrees. Different substances have different specific heat capacitites.
Calculating 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