Energy, Work and Fuels (KS3)

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Definitions of Energy, Work and Fuels

Energy is the ability to do work (in life processes, in machines). Work is said to be done when a force move an object through a distance (energy is used up). Fuels are substances which have energy 'locked up' in their molecules.

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Types of Energy

There are several types of energy which can be used to do work. In doing work, energy is convert from one form to another. Types of energy are:

  • Chemical - energy produced as a result of chemical reaction (burning of fuel, human uses food, battery produces electricity)
  • Electrical - energy due to electrical charges moving (electric current in a wire)
  • Nuclear - energy due to changes in the nuclei of atoms (radioactive fuel in a nuclear power station)
  • Sound - energy carried by sound wave (sound waves from a loudspeaker)
  • Thermal (heat) - energy due to fast-moving particles in hot objects (electric kettle - thermal energy is transferred from the heating element to the cold water)
  • Radiated energy - energy carried as electromagnetic waves (light from the Sun, radio waves from an aerial)
  • Potential energy (mechanical) - energy stored in materials that have been bent or stretched (tightly wound springs in clocks - also known as strain energy)
  • Potential energy (gravitational) - energy stored in materials that have been moved upwards so that they can now fall down
  • Kinetic energy - energy in objects that are moving (moving car)

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Work and Energyy

We use chemical energy from food to do work. Work is done when a force is used to make something move. The amount of work done is calculated from

Work Done = Force x Distance Moved  (WD = F x d)

The unit of work is joule (J). Energy is used to do work hence work and energy have the same unit. 1 J of work done means 1 J of energy has been transferred.

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Energy from Fuels

Energy is released from fuels by a chemical reaction called combustion. The fuel reacts with oxygen to give thermal energy (heat). Generally a small amount of energy is needed to start the reaction, but once the fuel has been ignited, the reaction keeps going until either the fuel or the oxygen has been used up.

When a fuel is lit (ignited), the energy is used to break the bonds holding the atoms in the fuel molecules together. In a natural gas molecule (methane CH4), the bonds between the carbon and hydrogen atoms are broken. Once free, the atoms can react with oxygen in the air to form new molecules - the products from burning methane are water (H2O) and carbon dioxide (CO2). Energy is needed to break bonds and making bonds in the new molecules releases lots of energy (heat and light).

Common fuels are wood, charcoal, coal, coke, natural gas, liquified petroleum gases (lpg) and oil products. 

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Fossil Fuels

Fossil fuels are formed from living things that died over millions of years. The main fossil fuels are coal, oil and natural gas. Coal was formed from the remains of plants on land. Oil and natural gas were formed from plants and animals lived in the sea.

Coal is an impure form of the element carbon. Different types of coal are formed at different pressures and temperatures under the Earth's surface: anthracite conatins 90% carbon and formed at great depth; bituminous contains 60% carbon formed at lower pressure than anthracite; lignite contains 40% carbon and are soft and sometimes brown in colour.

Oil and natural gas were formed initially under high pressure and temperature below sedimentary rocks. Because sedimentary rocks are porous, the oil/gas moved slowly through them to the surface or trapped under impermeable rocks.

The burning of fossil fules is releasing the Sun's energy that has been stored as chemical energy underground for millions of years.

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Problems with Fossil Fuels

All fossil fuels contain carbon (in fact they are hydrocarbons). When carbon compounds burn, they release energy. The chemical reaction for natural gas (methane) burning in air is

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g) + energy

Natural gas is a very clean fuel but the carbon dioxide is a greenhouse gas which trap heat in the atmosphere. The greenhouse effect has been beneficial in the past allowing Earth to reach the present temperature of 12°C, but further greenhouse effect will casue global warming leading to melting polar ice caps and uneven warming of the atmosphere causing powerful storms.

Fossil fuels like coal and oil contain the element sulphur as an impurity. When they burn the sulphur reacts with the oxygen in the air to form sulphur dioxide SO2. The sulphur dioxide dissolves in the water in air to become sulphurous acid H2SO3 and falls as acid rain. Acid rain kills vegetation, fish in the lake and damages limestone buildings.

S (s) + O2 (g) → SO2 (g)

SO2 (g) + H2O (l) → H2SO3 (aq)

Petrol engines give out nitrogen dioxide (NO2) in their exhaust gases which also dissolves in water to give acidic rain water.