Physics Revision

        

• Potential Difference = Current x Resistance (Ω)
• If the temperature of the resistor increases the resistance increases
• Resistance – Opposition to current flow
• A diode allows current through in one direction only.

Series Circuits

• For components in series:

• The current is the same in each component.
• The potential differences add to give the total potential difference
• The resistance adds up to give the total resistance

• In a series circuit the components are connected one after another so if there is a break in the circuit, charge stops flowing

• A = V/R
• The potential difference of the supply is shared between all the components in the circuit.

Parallel Circuits

• For components in parallel:

• The potential difference is the same across each component
• The total current is the sum of the currents through each component.
• The bigger the resistance of a component, the Smaller its current is.

• In a parallel circuit each component is connected across the supply, so if there is a break in one part of the circuit charge can still flow in the other parts.

Alternating Current

• Batteries supply current that goes round the circuit in one direction – this is called ‘direct current’ (d.c.)
• Current from the main supply passes in one direction, then reverses and passes in the other direction – this is called alternating current (a.c.)
• Frequency of UK mains supply = 50Hz this means it alternates direction 50 times every second.
• Voltage of UK mains supply = 230V
• The potential of the neutral terminal is zero.

Cables and Plugs

• Earth – Green and Yellow wire
• Neutral – Blue Wire
• Live – Brown Wire
• Cable grip – bit that holds the main wire

Fuses

• Fuses must always be on the live (Brown) wire
• The rating of the fuse should be slightly higher than the normal working current of the appliance
• Circuit breaker can be used in place of a fuse. This is an electromagnetic switch that opens and cuts off the supply if the current is bigger than a certain value.

Electrical Power and Potential Difference

• The power supplied to a device is the energy transferred to it each second.
• Power (watts, W) = Current (amps, A) x potential difference (Volts, V)

Electrical Energy and Charge

• Energy transformed (Joules, J) = p.d (Volts, V) x charge (coulombs, C)
• Charge (C) = Current (A) x Time (s)

Nuclear Reactions

• Gamma rays are electromagnetic waves and therefore have no charge and no mass.
• An atom has a nucleus, made up of protons and neutrons surrounded by electrons.

• An alpha particle consists of 2 protons and 2 neutrons
• E.g.

• Beta is a high speed electron from the nucleus.
• It is emmited when a neutron in the nucleus changes to a proton and an electron
• Atomic number goes up by one and the mass number is unchange

• Background radiation goes on all the time  it comes from cosmetic rays, from rocks, or from nuclear power stations.

Discovery of the Nucleus

• Plumb Pudding model – what people called the atom, they called it this because they thought atoms consisted of spheres of positive charge with electrons stuck to them like plums in a pudding.
• Rutherford, Geiger and Marsden devised an alpha particle scattering experiment in which they fired alpha particles at thin gold foil
• Most of the particles passed through which means that most of the foil was empty space.
• Some of the particles were deflected because of the very large mass and positive charge of the nucleus. (Alpha particles have a + charge)

Nuclear Fission

• Nuclear Fission is the splitting of an atomic nucleus.
• Nuclear fission occurs when a neutron collides with and splits a uranium-235 nucleus or plutonium-239 nucleus.
• A chain reaction occurs when neutrons from the fission go on to cause further fission.
• In a nuclear reactor one fission neutron per fission on average goes on to produce further fission.
• Enriched uranium – Uranium containing a higher percentage of uranium 235 than occurs naturally

Nuclear Fusion

• Nuclear fusion is the joining of 2 atomic nuclei to form a single, larger nucleus.
• During the process energy is released
• A fusion reactor needs to be at a very high temperature before nuclear fusion can take place.
• It has to be very hot to overcome the repelling positive charges of the two atomic nuclei.
• Nuclei are contained in a fusion reactor by a magnetic field.