Here we will jump straight in with the most common form of electricity production worldwide - the fossil fuel power station. While quite difficult to draw (see below!), you will learn about the physics principles that govern the four key sections:
- Fuel
- Boiler
- Turbine
- Generator
Key Concepts
Specific energy is the ratio of the energy contained in a body to its mass, \(E\over m\). The unit of specific energy is J kg-1.
Like density, specific energy is an example of a material property where the dimensions of the body are irrelevant. If an energy requirement of a process is known, the amount of fuel required can be calculated.
There is also a concept termed energy density, for the ratio of the energy contained in a body to its volume, \(E\over V\). The unit of specific energy is J m-3.
What relationships link the energy produced by a fuel with the possible temperature rise of water?
Heat engines convert thermal energy into mechanical work.
The principle behind the operation of a heat engine can be described using a hot air balloon:
On the way up, fuel is combusted (burned in oxygen) to heat the air. The air molecules gain kinetic energy and spread out, so the air becomes less dense and rises. This lifts the balloon so it gains gravitational potential energy.
When the balloon is higher, the surroundings are lower in temperature. This cools the air inside the balloon, making it more dense and so it falls.
A cycle can form:
- When a gas is hot it, can do work.
- When the gas is cold, work done is done on it.
Pressure and force are proportional. Where there is a pressure difference across a lamina, a force will be exerted:
\(F=\Delta P.A\)
When water is boiled to produce steam, these gas particles exert a pressure on the walls of their container. The force exerted can be converted into rotational kinetic energy (a form of mechanical energy) using turbine blades.
When the steam has passed through the turbine, it has lost kinetic energy and the molecules come together. The gas condenses into liquid water and can return to the boiler tank.
As discussed in the Magnetic fields section, there is a link between magnetic fields, current and movement.
When a conductor (such as a wire) moves in a magnetic (B) field, the electrons in it experience a force. The overall effect of this is an electro-motive force (\(\epsilon\)), which is a potential difference converting an energy source in to electrical energy.
This how the generator section of a power station works. The spinning turbine is connected to a large coil of wire. The rotation of the coil within a magnetic field, converts mechanical energy into electrical energy. At this point the aim of the power station is complete.
Think you know your power stations? Describe the purpose and energy changes in the sections of this sketch.
Check your answers below.
Use quizzes to practise application of theory.
START QUIZ!
The diagram represents a piece of wood of density ρ.
The wood is burnt and the heat used to raise the temperature of a mass m of water by ΔT degrees
The specific energy of the wood is
energy produced/mass
The diagram represents a piece of wood of density ρ.
The wood is burnt and the heat used to raise the temperature of a mass m of water by ΔT degrees
The energy density of the wood is
energy produced/mass
Wood has a specific energy of 20 MJkg-1. If burnt on a 50% efficient stove how much would would be burned per second to be equivalent to a 1 kW electric fire
50% efficient so 10 MJ per kg go to heat the room
1kW is 1000 J so would 1000/10 x 106 kg = 0.1g
The diagram shows the fusion process on the sun
The yellow particle is a
a proton changes to a neutron by beta + decay
This is a Sankey diagram for a wood burning stove:
The efficiency of the stove is
the width of the arrows are proportional to the energy flow
A hot air balloon burns 100g of fuel of energy density 50 MJkg-1 to lift 100 kg to a height of 100m.
What is the efficiency of this system?
energy in = 5 x 106 J
useful work = mgh = 105 J
Efficiency = (105/5x106 ) x 100% = 2%
This is the Sankey diagram for a heat engine with heat being lost in two stages.
If 240 MJ of useful work are done, how much heat is lost?
Each square represents 40 MJ
Which device does not convert mechanical energy to electrical energy
turbine converts translational KE into rotational
How much of Power stations have you understood?
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