Physics Slide

7/27/2019 Physics Slide

1/24

Contents

A rough outline

PHYSICS


2/24


3/24

Contents

SPARKS AND ELECTROSTATICS

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


4/24

Contents

Charging InsulatorsRubbing can charge insulators like wool, plastic and rubberThere two types of charges:

PositiveNegative

Charged insulators can attract dust, fibres and small piecesof paper An insulator losing electrons will have a positive charge An insulator gaining electrons will have a negative chargeLike charges repel and opposite charges attract A polythene rod rubbed with a duster acquires a negativecharge and the duster acquires an equal but oppositepositive chargeThe force between two charged objects are equal in sizeand act in opposite direction


5/24

Contents

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 earthedby 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 sparkElectric sparks can generate high temperatures and therefore aredangerous near inflammable gases and vapours (e.g. Refuelling aircraft)The chance of receiving an electric shock can be reduced by correctearthing, using insulating mats or wearing shoes with insulating solesThe risk of electric sparks can be reduced by securing a metal strapbetween the object and the groundThe body acts as a good conductor of small amount of charge An electric shock is safe through the body and long as it doesnt go acrossthe heart and cause a heart attack. If it goes through the hand down theleg it is unlikely to effect you but if it goes through one hand to the otherhand as both are connected to earth then it can cause heart failure


6/24

Contents

Static Shocks ExampleThe existence of repulsive forces between similar chargesexplains why you experience a shock if you touch anearthed conductor such as the screw on a light switchafter walking across a synthetic carpet:

Synthetic carpets, for example those made of nylon, are goodinsulatorsCharge builds up on the body when walking across the carpetdue to the friction forces between shoes and the carpetThe similar charges repel each other but they cannot leavethe 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 dischargethe body, creating a current which causes a shock

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


7/24

Contents

Uses of ElectrostaticsSome uses of static electricity include:

DefibrillatorsPhotocopiersLaser printersRemoving dust/soot from smoke chimneys

Spray painting A defibrillator has two charged paddles. These are used to pass a chargethrough the patient to make the heart contract. The operator has to becareful not to be shocked. Good electrical contact is necessary betweenthe patient and the paddlesElectrostatic dust precipitators are used in the chimneys of coal-burningpower stations. They have metal plates/grids connected to high voltage.Dust particles are attracted to the plates/grids. When the dust particles arelarge enough, they fall down the chimneysIn spray painting the gun charged. The paint particles are repelled toproduce a fine spray. The object to be painted is charged opposite to thepaint. Attraction between the object and the paint ensures an even coat of

paint. There is less waste in spray painting


8/24

Contents

Dangers of Static ChargeElectrostatic charge is dangerous when it causeslightning and sparks that can ignite fuel

When an aircraft is being refuelled with kerosene (paraffin)and when a car is being refuelled with petrol friction forcescause charge separation

Charge separation result in the metal frame of an aircraft gainingan opposite charge to the fuel

This could result in a build-up of static charge on the metalframe of the aircraft or metal sleeve of the car refuellingpipe]if the voltage became high enough to cause a spark to

earth, it could ignite the fuelTo prevent this, the framework of an aircraft is connected toearth before refuelling and the pipe leading to the petroltank in a car is connected to the body of the car so that thecharge can spread out, preventing the build-up of charge ina small area


9/24

Contents

SAFE ELECTRICS

Circuits and Symbols, Resistance, Circuit Components, Safety


10/24

Contents

Circuits A complete loop is required for an electrical circuit to work A 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 onthe resistance. Less resistance in a circuit meansgreater current and vice-versa A variable resistor can be used to change the resistancein a circuit and hence the current


11/24

Contents

Circuit Symbols


12/24

Contents

ResistanceCurrent is measured using an ammeter placed in series with acomponentCurrent is measured in amperes (A) Voltage or potential difference (PD) is measured using a voltmeter placed in parallel with a componentPotential difference in measure in volts (V)The resistance of a component is found using the followingequation:

or

Resistance is measured in ohms ( )For a given PD, current decreases as resistance increases and vice-versaFor a given resistor, current is increased as PD increases and vice-versa


13/24

Contents

Circuit ComponentsCommon circuit components include resistors, lamps and diodes:

The resistance of a resistor such as a metal wire does not change providedthat there is no significant change in its temperature; a graph of currentagainst voltage shows that the current is proportional to the voltageThe wire in a filament lamp becomes hotter as the current in the filamentincreases, causing an increase in its resistance A diode only allows current to pass in one direction (shown by the directionof the arrow on its symbol)

The direction of the current is always shown as being from positive to negativeThe resistance of some circuits depend on their surroundings; thesecomponents 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 withincreasing light levelThe resistance of a thermistor decreases with increasing temperature


14/24

Contents

Safety A mains cable has three coloured wires:

Brown the live wireBlue 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 liveDouble-insulated appliances have plastic casing and do not need to beearthedThe 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 fireCircuit 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 tobe reset


15/24

Contents

ULTRASOUND AND RADIATION

Ultrasound, X-rays and Gamma Rays, Treatment


16/24

Contents

UltrasoundUltrasound is a longitudinal waveFor a longitudinal wave, the particles vibrate in the same direction as the waveFor transverse waves, the particles vibrate at right angles to thedirection of the waveLongitudinal waves cannot travelthrough a vacuumUltrasound creates regions of highpressure called compressions and

regions of low pressure calledrarefactionsThe amplitude of a wave is themaximum distance of a particle fromits normal (or equilibrium) positionThe wavelength of a wave is the

distance occupied by one complete wave

The frequency of a wave is thenumber of complete waves in onesecondFrequency is measured in hertz (Hz)Ultrasound is used for seeing insideour bodies without surgery Ultrasound scans are used to checkthe condition of the foetus, breakingdown kidney and other stones andmeasuring the speed of blood f lowin the body

Ultrasound is reflected by differentlayers of tissueUltrasound is inaudible to thehuman earUnlike x-rays, ultrasound scans canbe used to produce images of soft

tissues and they do not damageliving cells


17/24

C


18/24

Contents

TreatmentSome radioactive nuclei of atoms emit nuclear radiation

Alpha particles, beta particles and gamma raysGamma rays and X-rays are both used in medicine fortreatment (therapy) and for finding out what is wrong(diagnosis)In radiotherapy, several gamma ray sources producing widebeams are directed towards cancerous tissues to destroy thecancer cells A radioactive tracer is a radioactive substance that is drunk orinjected into the body Only beta particles and gamma rays can pass through the skin.Hence a tracer is either a beta or gamma emitterThe function of some vital organs can be diagnosed using aradioactive tracer


19/24


20/24

C t t


21/24

Contents

Uses of RadioisotopesBackground radiation is always present and is due toradioactive substances in rocks, soil, air and cosmicraysSound background radiation comes from man-madesources

E.g. Nuclear and hospital wasteRadioisotopes are used as tracers in industry andhospitalsGamma-emitting tracers are used to find leaks orblockages in underground pipesSmoke detectors contain an alpha emitterCarbon dating is used to date bone, cloth, wood andpaper. It relies on the radioactivity isotope of carbon-14


22/24


23/24

Contents


24/24

Contents

Nuclear WasteNuclear power has one major disadvantage which is howto dispose of the nuclear waste. There are three maincategories:

Low-level waste such as laboratory clothing and packaging

materials; these are either buried either underground or at seaIntermediate-level waste such as the casing used for nuclearfuel and reactor parts that have been replaced; these are keptin stores with thick concrete walls or buried in deep trenches with concrete liningsHigh-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 intemporary storage in tank of water until the problem of whatto do with it can be solved

Documents

Physics Slide