This topic is best taught together so that the similarities and differences between the three types of field are discussed.
Fields are regions of space in which susceptible particles feel a force:
• A gravitational field is a region of space in which a mass feels a force because of gravity.
• An electrical field is a region of space in which a charge feels a force because of another nearby charge.
• A magnetic field is a region of space where magnetic materials experience a force.
These three types of fields are studied together because they have similarities and the techniques for performing calculations are similar too, but there are also significant differences:
• Gravity is always attractive and there is only one type of mass.
• Electric forces can be attractive or repulsive and we can have negative or positive charge.
• Magnetic forces can also be attractive or repulsive and there are two types of magnetic pole (North and South) these always appear in pairs.
6.1 Gravitational force and field
6.1.1 State Newton’s universal law of gravitation.
Every point mass attracts every other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses:
6.1.2 Define gravitational field strength.
A gravitational field is a region of space in which a mass feels a force because of gravity. Gravitational field strength is the force per unit mass on a particle because of the gravitational field.
6.1.3 Determine the gravitational field due to one or more point masses.
Field strengths are vectors and therefore the gravitational field due to one or more point masses can be found by vector addition.
r
2F = GMm
Where
F is the magnitude of the gravitational force between the two point masses, G is the gravitational constant,
M is the mass of the first point mass, m is the mass of the second point mass,
r is the distance between the two point masses.
m g = F
Where
g is the gravitational Field strength in NKg-1 F is the force experienced in N
m is the mass of the object in kg
Allan Riddick 61.4 Derive an expression for gravitational field strength at the surface of a planet,
assuming that all its mass at concentrated at its centre.
The gravitational field strength at the surface of planet is more commonly known as “little g” or g (at the surface of the Earth it is 9.8 NKg-1)
The force experienced by a mass in a gravitational field is given by
r
2F = GMm
And the gravitational field strength is defined as
m g = F
By combining these formulas the gravitational field strength due to a point mass is given by
r
2g = GM
Allan Riddick 6.2 Electric force and field
6.2.1 State that there two types of electric charge There are two types of electric charge. Positive and Negative
6.2.2 State and apply the law of conservation of charge.
Charge can be added or removed from an object but it cannot be destroyed.
Charge conservation is the principle that electric charge can neither be created nor destroyed. The quantity of electric charge is always conserved.
6.2.3 Describes and explain the difference in the electrical properties of conductors and insulators.
Conductors allow electric charges to pass through them. In a metallic conductor the charges that flow are electrons. An insulator is a material that does not allow charges to flow.
6.2.4 State Coulomb’s law.
Coulombs law is used to calculate the force of attraction or repulsion between two point charges.
2
F is the magnitude of the force between the two charges in Newtons ε0 is the permittivity of free space - 8.85x10-12 C2N-1m-2
q1 is the charge of the first point charge in Coulombs, q2 is the charge of the second point charge in Coulombs, r is the distance between the two point charge in meters.
6.2.5 Define electric field strength.
Electric field strength is defined as the force experienced per coulomb by a small positive charge in an electric field.
q E = F
Where
E is the electric field strength in NC-1
F is the force experienced but the charge in N q is the charge on the object in C.
The electric field strength close to a sphere can be derived in the same way as gravitational field strength to be:
6.2.6 Determine the electric field strength due to one or more point charges.
Allan Riddick Field strengths are vectors and therefore the electric field due to one or more point charges can be found by vector addition.
6.2.7 Draw the electric field patterns for different charge configurations.
Field lines around a charged sphere
Field lines due to a dipoles
Field lines due to parallel plates
The region between a set of parallel plates is a uniform electric field. The field lines are parallel and equally spaced telling us that the electric field strength is constant.
6.2.8 Solve problems involving electric charges, forces and fields.
Allan Riddick 6.3 Magnetic force and field
6.3.1 State that moving charges give rise to magnetic fields.
When an electric current flows through a wire a magnetic field is created.
6.3.2 Draw magnetic field patterns due to currents.
The direction of the field lines surrounding a wire can be found using a right hand rule.
The magnetic field caused by the current can be increased by coiling the wire, creating a solenoid.
6.3.3 Determine the direction of the force on a current-carrying conductor in a magnetic field.
When a current carrying wire is placed between the poles of a magnet it experiences a force. This force causes the wire to “jump”
The direction that the wire will jump can be predicted using Fleming Left Hand Rule. Line your thumb, first and second fingers as shown below.
Allan Riddick 6.3.4 Determine the direction of the force on a charge moving in a magnetic field.
6.3.5 Define the magnitude and direction of a magnetic field.
6.3.6 Solve problems involving magnetic forces, fields and currents.
BIl F =
Where
F is the force experienced by the wire, measured in Newtons B is the magnetic field strength, measured in Tesla
I is the current flowing through the wire, measured in Amps
l is the length of wire between the poles of the magnet, measured in Meters.
Allan Riddick 7.1 The atom
7.1.1 Describe a model of the atom that features a small nucleus surrounded by electrons.
Facts and figures about the atom.
Diameter of a nucleus ≈10-15m Diameter of an atom ≈10-10m Mass of nucleus ≈10-27kg
Mass of a proton 1.673×10−27 kg Mass on an neutron 1.675×10−27 kg Mass of an electron 9.110×10−31 kg Charge on a proton -1.60x10-19C Charge on an electron -1.60x10-19C
7.1.2 Outline the evidence that supports a nuclear model of the atom.
The best evidence for the nuclear model of the atom is the Geiger-Marsden Gold leaf experiment. They fired a beam of charged particles at a single layer of gold molecules and observed what happened.
According to the JJ Thomson “Plum Pudding” model they were expecting the charged particles to pass straight through.
They were very surprised that some of the alpha particles where deflected as they passed through the gold. From this they deduced that there the atom was made up of a small massive positively charged nucleus surrounded by space.
7.1.3 Outline one limitation of the simple model of the nuclear atom.
The problem with this theory was that accelerating charges are known to lose energy. If the orbiting electrons were to lose energy they would spiral into the nucleus. The Rutherford model cannot explain to us how atoms are stable.
Allan Riddick 7.1.4 Outline evidence for the existence of atomic energy levels.
This model was developed further by Niels Bohr. He suggested that the electrons orbit the nucleus rather like a planet orbits the sun. The radius of Bohr’s electrons depended on the energy they had.
He also suggested that they could only move in certain orbits.
When the electrons moved from a high energy state to a lower energy state they emitted a photon of light. The frequency of the light depends on the difference between the energy levels.
As there are a fixed number of energy levels only a few wavelengths of light are given out. This results in a line spectrum. Each individual element has distinct energy levels and therefore the emission spectra can be used to identify them.
Nuclear structure
7.1.5 Explain the terms nuclide, isotope and nucleon.
Nuclide – protons and neutrons that form a nucleus
Isotope – nuclei that have the same number of protons but a different number of neutrons.
Nucleon – The collective name for particles that are found in the nucleus (protons and Neutrons) hf = E1-E2
Where
h Planks constant (6.02 x 10-34 m2 kg s-2) f Frequency of the emitted photon (Hz) E1 Energy level before emitting photon (J)
E2 Energy level after photon has been emitted (J)
Allan Riddick 7.1.6 Define nucleon number A, proton number Z and neutron number N.
Nucleon Number, A – The number of protons and neutrons that are in the nucleus.
Proton Number, Z – The number of protons that are in the nucleus.
Neutron Number, N – The number of neutrons that are in the nucleus.
7.1.7 Describe the interactions in a nucleus.
According to our knowledge of electrostatics a nucleus should not be stable. Protons are positive charges so should repel each other. There must be another force in the nucleus that overcomes the electrostatic repulsion and hold the nucleus together. This force is called the strong nuclear force.
Strong nuclear forces must be very strong to overcome the electrostatic forces. They must also have a very small range as they are not observed outside of the nucleus.
Neutrons have some involvement in strong nuclear forces. Small nuclei have equal numbers of protons and neutrons. Larger nuclei, which are harder to hold together, have a greater ratio of neutrons to protons.
Allan Riddick 7.2 Radioactive decay
7.2.1 Describe the phenomenon of natural radioactive decay.
7.2.2 Describe the properties of alpha and beta particles and gamma radiation.
7.2.3 Describe the ionizing properties of alpha and beta particles and gamma radiation.
7.2.4 Outline the biological effects of ionizing radiation.
7.2.5 Explain why some nuclei are stable while others are unstable.
Half-life
7.2.6 State that radioactive decay is a random and spontaneous process and that the rate of decay decreases exponentially with time.
7.2.7 Define the term radioactive half-life.
7.2.8 Determine the half-life of a nuclide from a decay curve.
7.3 Nuclear reactions, fission and fusion Nuclear reactions
7.3.1 Describe and give an example of an artificial (induced) transmutation.
7.3.2 Construct and complete nuclear equations.
7.3.3 Define the term unified atomic mass unit.
7.3.4 Apply the Einstein mass–energy equivalence relationship.
7.3.5 Define the concepts of mass defect, binding energy and binding energy per nucleon.
7.3.6 Draw and annotate a graph showing the variation with nucleon number of the binding energy per nucleon.
7.3.7 Solve problems involving mass defect and binding energy.
Fission and fusion
7.3.8 Describe the processes of nuclear fission and nuclear fusion.
7.3.9 Apply the graph in 7.3.6 to account for the energy release in the processes of fission and fusion.
7.3.10 State that nuclear fusion is the main source of the Sun’s energy.
7.3.11 Solve problems involving fission and fusion reactions.
Allan Riddick
Allan Riddick 8.1 Energy degradation and power generation
8.1.1 State that thermal energy may be completely converted to work in a single process, but that continuous conversion of this energy into work requires a cyclical process and the transfer of some energy from the system.
8.1.2 Explain what is meant by degraded energy.
The second law of thermodynamics states that “it is impossible to take heat from a hot object and use it without losing some heat to the surroundings”. Energy becoming more spread out is known as the degradation of energy.
Whenever thermal energy is converted into mechanical energy some of the energy is degraded (lost) to the environment.
8.1.3 Construct and analyse energy flow diagrams (Sankey diagrams) and identify where the energy is degraded.
Sankey diagram for a petrol engine.
8.1.4 Outline the principal mechanisms involved in the production of electrical power.
Mechanical energy can be converted into electrical energy using a generator of dynamo. A coil is turned in a magnetic field. As the coil cuts the field lines, electrons move round the coil. The
movement of electrons causes a potential difference which results in a current flowing. A current has been induced in the coil.
A more detailed explanation of electromagnetic induction can be found in Unit 12, Electromagnetic induction.
8.2.1 Identify different world energy sources.
Modern society requires a lot of energy. Most of the energy recourses used by hum make electricity or to make things move.
8.2.2 Outline and distinguish between renewable and non A renewable source of energy cannot be used up. A non
and will eventually run out.
Renewable Energy
8.2.3 Define the energy density
Energy density is the amount of energy that c
8.2.4 Discuss how choice of fuel is influenced by its energy density.
The cost of transporting fuels is dependent on the fuel density. A fuel with a low fuel density will be expensive to transport.
8.2 World energy sources 8.2.1 Identify different world energy sources.
Modern society requires a lot of energy. Most of the energy recourses used by hum make electricity or to make things move.
8.2.2 Outline and distinguish between renewable and non-renewable energy sources.
A renewable source of energy cannot be used up. A non-renewable source of energy can be used up
Renewable Energy Non-Renewable Energy
Solar Coal
Wind Oil
Hydroelectric Gas
Wave Nuclear (Uranium)
Tidal
Biofuels (Wood, ethanol) Geothermal
energy density of a fuel.
Energy density is the amount of energy that can be obtained per kilogram of fuel.
8.2.4 Discuss how choice of fuel is influenced by its energy density.
cost of transporting fuels is dependent on the fuel density. A fuel with a low fuel density will be
Allan Riddick Modern society requires a lot of energy. Most of the energy recourses used by humans are used to
renewable energy sources.
renewable source of energy can be used up
an be obtained per kilogram of fuel.
cost of transporting fuels is dependent on the fuel density. A fuel with a low fuel density will be
Allan Riddick 8.2.5 State the relative proportions of world use of the different energy sources that are available.
Worldwide Energy resources
8.2.6 Discuss the relative advantages and disadvantages of various energy sources.
Allan Riddick 8.3 Fossil fuel power production
A fossil fuel fired power plant is and based on somewhat ancient methods of energy production. The fossil fuel is placed in a combustion chamber and burnt to produce heat. In order to turn the heat energy to electric energy, water is pumped around the combustion chamber and the heat from the chamber heats the water and it turns to steam. In the case of a coal fired plant this heats the water to 1000ºC. The steam is used to turn a turbine which is attached to a generator. The generator
generates converts the kinetic energy into electrical energy. Meanwhile the used steam goes to a condenser where it is cooled and turns to liquid water again; this lets off the big clouds often seen coming from these power plants. These clouds are nothing more than steam. The water is then pumped back into the combustion chamber to begin the cycle again.
Coal Fired Power Station
8.3.1 Outline the historical and geographical reasons for the widespread use of fossil fuels.
8.3.2 Discuss the energy density of fossil fuels with respect to the demands of power stations.
8.3.3 Discuss the relative advantages and disadvantages associated with the transportation and storage of fossil fuels.
8.3.4 State the overall efficiency of power stations fuelled by different fossil fuels.
8.3.5 Describe the environmental problems associated with the recovery of fossil fuels and their use in power stations.
8.4 Non Nuclear power
8.4.1 Describe how neutrons produced in a fission reaction may be used to initiate further fission reactions (chain reaction).
In a nuclear reactor a large nuclei, e.g. Uranium
smaller nuclei. The daughter nuclei have less mass than the parent and so energy is released.
During the reaction two or 3 neutrons are released. They can move on and collide with other uranium nuclei and create a chain reaction. A cha
a moderator and if there is a large enough piece of fissional material. The minimum amount of material needed for a chain reaction to take place is called the critical mass.
8.4.2 Distinguish between controlled nuclear fission (power production) and uncontrolled nuclear fission (nuclear weapons).
In a nuclear power station it is important that the chain reaction is controlled. Only one neutron from each reaction can be allowed to make fissio
reactor.
In a nuclear weapon the chain reaction is not controlled. The fissionable material and a moderator are mixed together.
8.4.3 Describe what is meant by fuel enrichment.
8.4 Non-fossil fuel power production
8.4.1 Describe how neutrons produced in a fission reaction may be used to initiate further fission reactions (chain reaction).
In a nuclear reactor a large nuclei, e.g. Uranium-236, is bombarded with a neutron and spl smaller nuclei. The daughter nuclei have less mass than the parent and so energy is released.
During the reaction two or 3 neutrons are released. They can move on and collide with other uranium nuclei and create a chain reaction. A chain reaction will only occur if the neutrons are slowed down by a moderator and if there is a large enough piece of fissional material. The minimum amount of
material needed for a chain reaction to take place is called the critical mass.
between controlled nuclear fission (power production) and uncontrolled nuclear fission (nuclear weapons).
In a nuclear power station it is important that the chain reaction is controlled. Only one neutron from each reaction can be allowed to make fission. The other neutrons are absorbed by control rods in the
In a nuclear weapon the chain reaction is not controlled. The fissionable material and a moderator are
8.4.3 Describe what is meant by fuel enrichment.
Allan Riddick 8.4.1 Describe how neutrons produced in a fission reaction may be used to initiate further
236, is bombarded with a neutron and splits into two smaller nuclei. The daughter nuclei have less mass than the parent and so energy is released.
During the reaction two or 3 neutrons are released. They can move on and collide with other uranium in reaction will only occur if the neutrons are slowed down by a moderator and if there is a large enough piece of fissional material. The minimum amount of
between controlled nuclear fission (power production) and uncontrolled
In a nuclear power station it is important that the chain reaction is controlled. Only one neutron from n. The other neutrons are absorbed by control rods in the
In a nuclear weapon the chain reaction is not controlled. The fissionable material and a moderator are
Allan Riddick 99.3% of the uranium dug out of the ground is Uranium-238. Uranium-238 will absorb neutrons but will not fission so its presence in a nuclear reactor can hinder the chain reaction. The fuel in the reactor (or weapon) needs to have a much higher concentration of Uranium-235. The raw uranium must be enriched before it can be used. Commercial reactors use fuel with 5% Uranium-235. Weapon grade Uranium has over 85% Uranium-235.
8.4.4 Describe the main energy transformations that take place in a nuclear power station.
8.4.5 Discuss the role of the moderator and the control rods in the production of controlled fission in a thermal fission reactor.
Moderator The moderator slows down the neutrons. If the neutrons have too high an energy they will pass straight thorough the uranium nuclei and fission will not occur.
Moderator The moderator slows down the neutrons. If the neutrons have too high an energy they will pass straight thorough the uranium nuclei and fission will not occur.