Energy Formulae summary

Here is a list of all the formula mentioned in this topic:

 

Quantities and Units

• Energy = Joules (J)
• Wd = Work done (J)
• GPE = Gravitational Potential Energy (J)
• KE = Kinetic Energy (J)
• F = force (N, Newtons)
• d = distance moved in the direction of the force (m, metres)
• m = mass (kg)
• g = gravitational field strength (N/kg) NB: G on earth = 10N/kg
• h = height (m)
• v = velocity (m/s)
• P = power (W, Watts)
• t = time (s)

End of Energy Topic

4.17 Energy Advantages

Describe the advantages and disadvantages of methods of large-scale electricity production from various renewable and non-renewable resources.

Energy Source	Advantages of Use	Disadvantages of Use
Fossil Fuels	Fuels readily available, easy to make electricity.	Fuels are non-renewable, produce C02 and so add to global warming, acid rain. Oil spills damage environment.
Nuclear	Creates large amounts of energy, is reliable and relatively cheap to set up. Produces no C02.	Expensive to decommission power stations after use. Waste is produced that is dangerous for long periods. Reactors that are not cooled will overheat and release waste.
Wind	Produces no C02. Once setup takes little maintenance. Renewable.	Expensive to set up. Installations may be noisy and seen as ugly. Unreliable as you cannot count on the wind all the time.
Wave	Produces no C02.  Small devices can produce large amounts of electricity. Out to sea means out of sight. There are large areas unused across the globe. Renewable.	Easily destroyed by bad weather. Electricity produced needs to be transferred over long distances. Expensive to set up.
Tidal	Reliable as the tides are predictable as they depend on the Sun and Moon. Produces no C02. Renewable.	Few areas are suitable of being dammed. The dam restricts movement of sea life. Silt will build up. tides vary in strength throughout the day. Expensive to set up.
Hydroelectric	Produce no C02. Once set up have little maintenance cost. Create useful water resources. Can be turned on and off instantly. Renewable.	Expensive to build. Destroy ecosystems. If dams break damaging flooding will occur. Most areas in the world that can be used are being used.
Geothermal	Virtually emission free. Some reduce sulphur emissions. Simple systems are easy to maintain. No fuel costs. Renewable.	Expensive to set up. Expensive drilling through hot rocks. Limited areas in the world where hot areas are close to the surface.
Biomass	Produces no long term C02. Renewable as trees can be regrown	Limited areas of land available. Reduces the production of food.
Solar Heating	Produce no C02. Renewable. Light is free.	Unreliable as the sun does not shine all the time. No sun at night.
Solar (Photovoltaic) Cells	Produce no C02. Renewable. Light is free. Can be small to be used instead of batteries.	Unreliable as the sun does not shine all the time. No sun at night. Expensive to set up.

4.16 Generating Electricity

Understand the energy transfers involved in generating electricity using:

• Wind
• Water
• Geothermal resources
• Solar heating systems
• Solar cells
• Fossil fuels
• Nuclear power

Wind power stations

Fossil Fuel power stations

Hydroelectric power stations

Geothermal power stations

Solar Heating system

Solar Cell

Nuclear power stations

4.15 Power Formula

Use the relationship between power, work done (energy transferred) and time taken:

P = Power (Watts, W)
Wd = Work Done (Joules, J)
E = Energy transferred (Joules, J)
t = Time (s)

Example

A girl does 1000 joules of work in 5 seconds. What power does she develop?

4.14 Power

Describe power as the rate of transfer of energy or the rate of doing work

Remember, ‘Rate’ simply means ‘divided by time’ so…

or

4.13 The Link

Understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work

Conservation of Energy
As a reminder; the conservation of energy states that the total amount of energy in a system is constant over time (i.e. if you put 5J in you get 5J out). A consequence of this is that energy can neither be created nor be destroyed; and can only be transformed from one state to another.

We can write the principle of conservation of energy as a formula:

Calculation

The Diagram below shows a man at the top of a building, calculate how much energy he has at the start and hence show how much energy he has at the end.

Energy at the top GPE = mgh
GPE = 50 x 10 x 20
GPE = 10,000J

KE = 0J (He isn’t currently moving)

Total Energy = KE + GPE
Total Energy = 0 + 10,000
Total Energy at start = 10,000J

Energy at the bottom GPE = 0J

Total energy Start = Total energy End

Total energy end = KE + GPE
10,000J = KE + 0J

KE = 10,000J

KE = ½ mv²
10,000 = ½ x 50 x v²
v = 20m/s

4.12 Kinetic Energy

Recall and use the relationship:

KE = Kinetic energy [Joules, J]

m = mass [Kilograms, kg]

v = velocity [metres per second, m/s]

Using the formula…

Calculate the Kinetic Energy of a 10 tonne lorry driving along the expressway at 100km/hr
(NB: 1 tonne = 1000kg; 1km/hr = 0.28m/s)