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A rough outline

PHYSICS

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SPARKS AND ELECTROSTATICS

Charging Isolators, Electric Sparks, Uses of Electrostatics, Dangers of Electrostatics

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Charging InsulatorsRubbing can charge insulators like wool, plastic and

rubberThere two types of charges:

Positive Negative

Charged insulators can attract dust, fibres and small pieces of paper

An insulator losing electrons will have a positive charge

An insulator gaining electrons will have a negative charge

Like charges repel and opposite charges attractA polythene rod rubbed with a duster acquires a

negative charge and the duster acquires an equal but opposite positive charge

The force between two charged objects are equal in size and act in opposite direction

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Electric Sparks You will get an electrostatic shock when:

You touch something that is charged You become charged by walking on a carpet/vinyl floor and become earthed

by touching a water pop or metal door handle Static charges are a nuisance as they cause synthetic clothes to ‘cling’ The transfer of charges between two objects creates an electric spark Electric sparks can generate high temperatures and therefore are

dangerous near inflammable gases and vapours (e.g. Refuelling aircraft)

The chance of receiving an electric shock can be reduced by correct earthing, using insulating mats or wearing shoes with insulating soles

The risk of electric sparks can be reduced by securing a metal strap between the object and the ground

The body acts as a good conductor of small amount of charge An electric shock is safe through the body and long as it doesn’t go

across the heart and cause a heart attack. If it goes through the hand down the leg it is unlikely to effect you but if it goes through one hand to the other hand as both are connected to earth then it can cause heart failure

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Static Shocks ExampleThe existence of repulsive forces between similar

charges explains why you experience a shock if you touch an earthed conductor such as the screw on a light switch after walking across a synthetic carpet:Synthetic carpets, for example those made of nylon, are

good insulatorsCharge builds up on the body when walking across the

carpet due to the friction forces between shoes and the carpet

The similar charges repel each other but they cannot leave the body through the carpet which is a very good insulator

When the body is placed in electrical contact with the earth, electrons move between the body and the earth to discharge the body, creating a current which causes a shock

The direction of electron movement depends on whether the body is positively or negatively charged

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Uses of Electrostatics Some uses of static electricity include:

Defibrillators Photocopiers Laser printers Removing dust/soot from smoke chimneys Spray painting

A defibrillator has two charged paddles. These are used to pass a charge through the patient to make the heart contract. The operator has to be careful not to be shocked. Good electrical contact is necessary between the patient and the paddles

Electrostatic dust precipitators are used in the chimneys of coal-burning power stations. They have metal plates/grids connected to high voltage. Dust particles are attracted to the plates/grids. When the dust particles are large enough, they fall down the chimneys

In spray painting the gun charged. The paint particles are repelled to produce a fine spray. The object to be painted is charged opposite to the paint. Attraction between the object and the paint ensures an even coat of paint. There is less waste in spray painting

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Dangers of Static ChargeElectrostatic charge is dangerous when it causes

lightning and sparks that can ignite fuelWhen an aircraft is being refuelled with kerosene

(paraffin) and when a car is being refuelled with petrol friction forces cause charge separation Charge separation result in the metal frame of an aircraft

gaining an opposite charge to the fuelThis could result in a build-up of static charge on the

metal frame of the aircraft or metal sleeve of the car refuelling pipe]if the voltage became high enough to cause a spark to earth, it could ignite the fuel

To prevent this, the framework of an aircraft is connected to earth before refuelling and the pipe leading to the petrol tank in a car is connected to the body of the car so that the charge can spread out, preventing the build-up of charge in a small area

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SAFE ELECTRICS

Circuits and Symbols, Resistance, Circuit Components, Safety

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CircuitsA complete loop is required for an electrical

circuit to workA resistor can be used in a circuit to change the

currentElectric current is the rate of flow of chargeIn a circuit, electric current is due to the

movement of electronsThe size of the current for a given circuit

depends on the resistance. Less resistance in a circuit means greater current and vice-versa

A variable resistor can be used to change the resistance in a circuit and hence the current

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Circuit Symbols

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ResistanceCurrent is measured using an ammeter placed in

series with a componentCurrent is measured in amperes (A)Voltage or potential difference (PD) is measured

using a voltmeter placed in parallel with a component

Potential difference in measure in volts (V)The resistance of a component is found using the

following equation: or

Resistance is measured in ohms (Ω)For a given PD, current decreases as resistance

increases and vice-versaFor a given resistor, current is increased as PD

increases and vice-versa

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Circuit ComponentsCommon circuit components include resistors, lamps and

diodes: The resistance of a resistor such as a metal wire does not change

provided that there is no significant change in its temperature; a graph of current against voltage shows that the current is proportional to the voltage

The wire in a filament lamp becomes hotter as the current in the filament increases, causing an increase in its resistance

A diode only allows current to pass in one direction (shown by the direction of the arrow on its symbol)

The direction of the current is always shown as being from positive to negative

The resistance of some circuits depend on their surroundings; these components are often found in electronic circuits used for switches and maintaining constant environmental conditions in, for example, greenhouses and incubators: The resistance of a light-dependent resistor (LDR) decreases with

increasing light level The resistance of a thermistor decreases with increasing

temperature

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Safety A mains cable has three coloured wires:

Brown – the live wire Blue – the neural wire Yellow/green – the earth wire

Charges can safely flow to the ground if a conductor is earthed Metal appliances are earthed in order to protect you from

accidental shocks An earthed conductor cannot become live Double-insulated appliances have plastic casing and do not

need to be earthed The fuse is a safety device. A large current will melt the fuse

and cut off the supply A fuse prevents large currents from starting a fire Circuit breakers are safety devices and can be described as ‘re-

settable fuses’ A fuse has to be replaced after a fault whereas circuit breakers

just have to be reset

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ULTRASOUND AND RADIATION

Ultrasound, X-rays and Gamma Rays, Treatment

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Ultrasound

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X-Rays and Gamma Rays

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TreatmentSome radioactive nuclei of atoms emit nuclear

radiation Alpha particles, beta particles and gamma rays

Gamma rays and X-rays are both used in medicine for treatment (therapy) and for finding out what is wrong (diagnosis)

In radiotherapy, several gamma ray sources producing wide beams are directed towards cancerous tissues to destroy the cancer cells

A radioactive tracer is a radioactive substance that is drunk or injected into the body

Only beta particles and gamma rays can pass through the skin. Hence a tracer is either a beta or gamma emitter

The function of some vital organs can be diagnosed using a radioactive tracer

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NUCLEAR PHYSICS

Radioactivity, Uses of Radioisotopes, Fission, Nuclear Waste

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Radioactivity

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Uses of RadioisotopesBackground radiation is always present and is

due to radioactive substances in rocks, soil, air and cosmic rays

Sound background radiation comes from man-made sourcesE.g. Nuclear and hospital waste

Radioisotopes are used as tracers in industry and hospitals

Gamma-emitting tracers are used to find leaks or blockages in underground pipes

Smoke detectors contain an alpha emitterCarbon dating is used to date bone, cloth,

wood and paper. It relies on the radioactivity isotope of carbon-14

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Nuclear FissionNuclear power stations use uranium as a fuel. Most

of the waste is radioactive for thousands of yearsReactions produce the heat to produce steam to

turn the turbines, which turn the generators to produce electricity

In a fission reaction, a neutron is captured by a uranium-235 nucleus and splits into two smaller (daughter) nuclei and either two or three neutrons

Energy is released when a uranium nucleus splitsMaterials become radioactive when they capture

neutrons in a nuclear reactorIn a chain reaction, the neutrons can cause further

fission reactionsOne destructive use of fission is the atomic bomb.

One peaceful use if electricity production

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The Process of Nuclear Fission In the process of the fission of uranium-235:

A uranium-235 absorbs a neutron, making it unstable It splits into two daughter nuclei, which are also unstable, and

two or three spare neutrons The fission products have a lot of kinetic energy, which is

removed by a coolant The coolant generates steam that turns a turbine The spare neutrons can cause further fission of other nuclei in

a chain reaction If each spare neutron were allowed to cause another

fission, the result would be a chain reaction which would be out of control. To maintain the reaction at a steady rate, on average just one of the neutrons released by each fission is allowed to go on and cause further fission

In the core of a nuclear reactor, control rods absorb the spare neutrons to control the rate of reaction. It is also a safety feature

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Nuclear WasteNuclear power has one major disadvantage which

is how to dispose of the nuclear waste. There are three main categories:Low-level waste such as laboratory clothing and

packaging materials; these are either buried either underground or at sea

Intermediate-level waste such as the casing used for nuclear fuel and reactor parts that have been replaced; these are kept in stores with thick concrete walls or buried in deep trenches with concrete linings

High-level waste such as spent fuel rods; these present a long-term disposal problem since they remain significantly radioactive for thousand of years; much of this waste is in temporary storage in tank of water until the problem of what to do with it can be solved