Work and Power

Work (W) is said to be done when a force moves an object. Work is the measured by multiplying the force by the distance. The distance moved in the direction of the force is called displacement and is given the symbol s.

Work = Force x Displacement W = F ´ s The unit of work is joule (J)

Energy is the capacity for doing work and its symbol is E. Energy is also measured in joules (J)

The release of energy does work. Energy is added when work is done. Work is done when a force moves.

Some Simple Calculations with Work

a) Calculate the work done when an 8 Newton weight falls a distance of 10 metres.

W = Fs

= 8 N ´ 10 m

= 80 Nm

= 80 J

b) Calculate the work done when a 70 kg man climbs a ladder to a height of 4 metres.

W = Fs F = mg => W = m ´ g ´ s

= (70n kg ´ g) ´ 4 m

= (70n kg ´ 10 m/s/s) ´ 4 m

= 700 N ´ 4 m

= 2,800 Nm

= 2,800 J

Power (P) is the amount of work done per second or the energy converted per second. Power is the work rate

Power = Work ÷ Time The unit of power is the Watt.(W)

One watt is equivalent to one joule of energy converted to other forms of energy every second.

A 100 W bulb converts 100J of electrical energy to other forms of energy every second

A 1,500 W hair dryer does 15 times more work per second than a 100 W bulb.

Calculations on Work and Power

a) Calculate the power of a 70 kg man who on climbing a ladder to a height of 4 metres in 10 seconds uses 2,800 J of energy.

Power =Work


= 2,800 J

10 s

= 280 Watts

b) Calculate the work and power of a crane that lifts a 200 kg load through a vertical height of 5 metres in 20 seconds.

(i) Work = Fs

= (200 kg x g) x 5 m

= (200 kg x 10 m/s/s) x 5 m

= 2,000 N x 5 m

= 10,000 J

(ii) Power = Work¸ Time

= 10,000 J

20 s

= 500 Watts

Principle of Conservation of Energy

Energy cannot be created or destroyed but can be converted to other forms of energy.

Forms of Energy

1. Potential The energy a body has due to its position or state of stress.

2. Kinetic The energy a body has due to its motion.

3. Chemical The energy stored in the chemical bonds that link atoms or ions.

4. Light Energy transmitted as visible electromagnetic waves.

5. Sound Energy transmitted as pressure changes from vibrating objects.

6. Heat Energy that transfers form region of higher to lower temperature.

7. Electrical Energy from moving or static electric charges.

8. Magnetic Energy due to a magnetic field of a magnet.

9. Nuclear The energy stored in the nuclei of atoms.

Sources of Energy

1. Non-renewable Energy Sources: fossil fuels – coal, oil, natural gas and peat.

2. Renewable Energy Sources: solar, hydroelectric, wind, tidal, wave, geothermal, biomass.

3. Nuclear: nuclear fission reactors and in the future nuclear fusion reactors.

Advantages and Disadvantages of Different Energy Sources

1. Fossil Fuels

  • Costly and dangerous to extract except for peat (turf).
  • Costly to transport.
  • Short term reserves.
  • Pollution problems: smoke, CO2 and acid rain.
  • Relatively cheap in comparison to some renewable forms of energy.

2. Renewable Energy Sources

  • Very expensive – large subsidies make it profitable for private companies.
  • Inconsistent – solar, wind and tidal.
  • Very low pollution impact.
  • Safe to set up and run.
  • Expected to expand greatly in the future.

3. Nuclear

  • Very efficient and not as expensive as some renewable sources.
  • Adverse public opinion in Ireland but very favourable in other countries.
  • Modern reactors very safe.

The Sun is Our Primary Source of Energy

a) The sun is the source of all the energy needed by living organisms to grow and reproduce. Plants convert sunlight energy into chemical energy (food). The food is transferred along food chains to all other living organisms.

b) The sun is a major energy source for production of electricity from renewable sources of energy. Light energy directly to produce electricity and indirectly by wind, waves, and along with the moon’s gravity tidal.

c) The sun is a major energy source for production of fossil fuels – non-renewable energy sources.

Importance of the Sun in Food Production Plants use light energy to make their food by photosynthesis. All our food comes from plants. Directly from plants when we eat plants or their parts. Indirectly when we eat meat – the flesh of plant eaters. Therefore our food is sunlight energy converted to chemical energy by plants.

National energy Needs

Over 90% of our energy needs in Ireland is supplied by fossil fuels. Very little energy comes form renewable energy sources. Each year our energy consumption increases – we have an expanding population and the economy is thriving with the demand for electricity more than doubling over the last 15 years.

Need for Energy Conservation

Energy conservation means getting the most value from our energy sources by using them more efficiently and limiting the loss of energy. This will reduce the amount of fossil fuels we need to use. More efficient engines and fossil fuel burning stations are needed.

Better insulation to reduce heat loss from homes, schools and office buildings would greatly reduce our consumption of fossil fuels – one third of our energy needs in winter is for the heating of our homes.

Home insulation is improved by using good attic insulation, lagging jacket around the hot water cylinder, cavity walls especially if filled with insulation material, heavy curtains, double glazing and carpets.

Energy Conversion Energy cannot be created or destroyed but can be converted to other forms of energy.

Examples 1. Respiration: chemical energy to heat energy.

2. Photosynthesis: nuclear energy to light energy to chemical energy.

3. Walking up stairs: chemical energy to kinetic energy to potential energy.

4. Falling down the stairs: potential energy to kinetic energy and sound energy.

5. Light bulb: electrical energy to heat energy to light energy.

6. Electric kettle: electrical energy to heat and sound energy.

7. Door Bell: electrical energy to magnetic energy to kinetic energy to sound energy.

8. Solar calculator: light energy to electrical energy.

9. Washing Machine: electrical energy to heat, kinetic and sound energy.

10. Car: chemical energy to kinetic and heat energy.

Be able to show how you could demonstrate a set of energy conversions in your Laborotory

Heat is a Form of Energy & can be converted to other forms of energy & other forms of energy can be converted to heat. Heat is energy that transfers from hotter to colder regions, by conduction, convection & radiation.

Heat Can be Converted into Other Forms of Energy

a) Heat energy converted to electricity in fossil fuel power stations.

b) Heat energy converted to light in a bulb, steel heated to white hot; glowing hot lava and fire.

c) Heat energy to kinetic energy e.g. convection currents in water and air.

Other Forms of Energy Can be Converted to Heat

a) Electrical to heat e.g. light bulb and an electric fire.

b) Chemical to heat e.g. the burning of fossil fuels, respiration – keeping our body temperature at 37°C

c) Kinetic to heat e.g. heat produced by friction.

Heat has Many Applications in Everyday Life

a) Production of electricity.

b) Maintaining our correct body temperature, 37°C.

c) Cooking, Washing dishes and clothes.

Three Major Effect of Heat on Substances

1. Change in Volume: generally expand on heating and contract on cooling.

2. Change in Temperature: gain of heat raises the temperature and loss of heat causes a drop in temperature

3. Change in State: gain of heat leads to melting & vaporisation and loss of heat in condensation & solidification.