G485 revision notes

OCR A Physics G485 revision notes by Robbie Peck

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G485 Fields, Particles and the Frontiers of Physics
Electric and Magnetic Fields
Electric Fields
Electric fields are created by electric charges. If a charged particle ventures into this electric field, then it
will experience a force. The magnitude of which depends of the charge and strength of the electric field.
Electric Field Strength, E, at a point is the force experienced per unit charge exerted on a positive charge
placed at that point.
Where F is the force experienced by a
positive charge of magnitude Q.
Electric field patterns can be mapped out
using electric field lines. The direction of the field at a point in space shows the direction of the force
experienced by a small positive charge. Hence electric fields point away from positively charged objects
and towards negatively charged objects.
A uniform electric field is created when two oppositely charged parallel plates mean the electric field
strength is constant and the electric field pattern has parallel lines which are evenly spaced. The p.d.
between the plates is V and they are separated by distance d. A positive charge Q moves across.
E is electric field strength.
Work done on charge = energy transformed × = ×
= but = so = (for parallel plates ONLY)
An electron is accelerated between 2 charged parallel plates. The p.d. is 450v between them and 1.8cm
apart. The electron goes from negative to positive. What is the force it experiences due to the electric field,
its acceleration and its final velocity?
E = V/d so E = 450 / (1.8 x10-2) = 2.5 x104 Vm-1. EQ = F => 2.5x104 x 1.6 x10-19 = 4 x 10-15 N. F/m = a => 4x10-
/(9.11x10-31) = 4.39 x1015ms-1 v2= 2x4.39x1015x1.8x10-2 => v=1.3x107 ms-1.
Coulomb's Law states two point charges exert an electrical force on each other that is directly
proportional to the product of the charges and inversely proportional to the square of separation
between them.
i.e. and
is the permittivity of free space. It has an experimental value of 8.85 x 10-12 F m-1.
Using = ; = ; hence

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Comparing electric and gravitational fields, the similarities are:
- Both electric and gravitational fields are to do with `action at a distance'
- Field strengths follow an inverse square law with distance.
- A point mass and point charge both produce radial fields
- Field strength is defined as force per unit mass or positive charge.
The differences are
- Electric field is created by charge, whilst a gravitational field by mass.
- Electric fields can be attractive or repulsive. Gravitational fields are always attractive.…read more

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Moving charges in a magnetic field
A current-carrying wire placed in a magnetic field experiences a force because each moving electron
within the wire experiences a tiny force.
Consider the force F on a positive particle moving is at right angles to the magnetic field.
= where B is the magnetic flux density, Q is the charge on the particle and v the speed of the
particle. A higher value of B has field lines closer together.…read more

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An e.m.f is induced in a circuit whenever there is a change in the magnetic flux linkage. The
magnitude of the induced e.m.f in a circuit can be determined using Faraday's law of
electromagnetic induction;
The magnitude of the induced e.m.f is equal to the rate of change of Magnetic flux linkage.
( )
= -
There are 3 ways in which an e.m.f may be induced in a circuit:
- Change the magnetic flux density B (e.g.…read more

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A capacitor consists of two metal plates separated by insulation. When connected to a source of e.m.f., the
plates acquire equal but opposite charges. The positive plate loses electrons and the negative plate gains
an equal number of electrons.
The magnitude of the charge Q on one of the plates is directly
proportional to the p.d. V across the capacitor. i.e.
The constant relies on the capacitor
i.e. = where is the capacitance.
Hence capacitance is charge stored per volt.…read more

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A 0.02F capacitor is charged to 9V. The fully charged capacitor is discharged through a lamp in a time of 40
ms. Find the average power dissipated by the lamp.
E = 0.5CV2 = 0.5x0.02x9 = 0.81J. Power= Work/Time => Power = 0.81/(40x10-2) = 20W
Capacitor discharging through a resistor
With the switch S at position X the capacitor is fully charged. The p.d.
across it is V0. When the switch is moved to Y, the capacitor will
discharge through the resistor.…read more

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Nuclear Physics
The nuclear atom structure
- Rutherford carried out the electron scattering experiment. Alpha particles were targeted towards a
thing gold foil. The number of alpha particles scattered at various angles were counted using a
alpha particle detector.
Most of the alpha particles weren't scattered The gold atoms are mostly empty space
Some of the alpha particles were scattered through large radius Small dense positive
Using coulombs law ( = ), we can work out the force at a given distance.…read more

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A proton has two up quarks and 1 down quark; (u u d)
- A neutron has one up quark and two down quarks; (u d d)
- A pi+ meson has one up quark and one down antiquark; (u )
You can hence work out the charge, baryon number and strangeness of the proton, neutron and pi+
meson. E.g. proton has Q=1, B=1, S=0; neutron has Q=0, B=1, S=0; pi + meson has Q=1, B=0, S=0.
Q,B,S are conserved in all nuclear reactions.…read more

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Stable nuclei lie on the line of stability. The nuclei in the shaded region are unstable. Decay of an unstable
nucleus brings it close to the line of stability.
Alpha decay
A helium nucleus is emitted;
232 228 4
90 88 + 2
Beta-minus decay
Electron-rich nuclei emit an electron
90 90 0
38 39 + -1 (+)
The weak nuclear force is responsible for this decay.…read more

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Nuclear Binding energy
The mass-energy equation is
= 2
where is the change in energy of the system, is the change in mass of the system and = 3x108ms-1.
Consequence of this:
- The mass of a system increases when external energy is supplied to the system.
- Energy is released from the system when its mass decreases.
The `system' could be decaying radioactive nuclei, an accelerated electron, a person etc. But the changes in
mass a really small.…read more


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