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# Physics Revision

Extracts from this document...

Introduction

Physics Revision

Collisions and Explosions

• Momentum has size and direction
• When 2 objects are at rest, their momentum is zero
• When 2 objects push each other apart, they move apart with equal and opposite momentum

Changing Momentum

• The more time an impact takes, the less the force exerted
• This is used in cars. Crumple zones and air bags slow down the collision and therefore reduce the forces

Electrical Charges

• + + +   - Repel
• + + -- Attract
• - + -- Repel
• Insulating materials that lose electrons when rubbed become positively charged
• Insulating Materials that gain electrons when rubbed become negatively charged.
• Electrons have Negative forces

Charge on the Move

• Electric current is the rate of flow charge.
• A metal object can only hold charge if it is isolated from the ground
• A metal object is earthed by connecting it to the ground
• Metals – good conductors because they contain free, conduction electrons that are not confined to a single atom.
• Insulators cannot conduct because all the electrons are held in atoms

Electric Circuits

Resistance

• Resistance (ohms, Ω) =
• Current – potential difference graphs are used to show the current through a component varies with the potential difference across it.
• Current is measured with an ammeter
• Potential Difference is measured with a voltmeter

• 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

Middle

• 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

Conclusion

Rutherford, Geiger and Marsden devised an alpha particle scattering experiment in which they fired alpha particles at thin gold foilMost 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.

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