OCR Gateway P4


Static Electricity

Build up of electricity is caused by Friction - Electrons have a negative charge, so when two insulating materials are rubbed together, electrons will be scraped off one and dumped on the other. This leaves a positive static charge on one due to the lack of electron. The other one will be left with a negative static charge due to the excess of electrons as it gained all the electrons from the other one. Electrically charged objects attract small neutral objects - a good example of this is polythene and acetate rods being rubbed with a cloth duster

Only Electrons Move and never positive charges - a positive static charge is always caused by electrons moving away elsewhere. Positve charges definitely do not move

Like charges repel, opposite charges attract - two things with opposite electric charges are attracted to each other. Two things with the same electric charge will repel each other. These forces get weaker the further apart the two objects are.

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More on Static Electricity

Static Electricity can be a nuisance - Objects can attract dust because dust particles are charged and can attract to enything with an opposite charge to them (TV screen can attract dust). Clothing can cling and crackle because when synthetic are dragged over each other or over your head, electeons get scraped off leaving static charges. Shocks from door handles can alse be a nuisance, this can happen because if you walk ona nylon carpet, charge builds up in your body then if you touch a metal door handle, the charge flows via the conductor.

Static electricity can be dangerous - a lot of electricity can build up on clothes and if it becomes large enough it can spark and if you are near inflammable gases an explosion could happen. Fuel filling and static charge can be dangerous - As fuel flows out of a filler pipe static can build up, this can lead to a spark and might cause an explosion.

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Objects can be earthed or insulated to prevent sparkes - Dangerous sparks can be prevented by connecting a charged object to the ground using a conductor (copper wire) - this is called earthing and it provides an easy route for the static charged to travel into the ground. This means no charge can build up to give you a shock or make a spark. Fuel tankers must be earthed to prevent sparks. Refueling aircraft are bonded to their fuel tankers using an earthing cable to prevent sparks.

Anti static sprays - anti static and liquids work by making the surface of a chraged object conductive - this provides an easy path for the charges to move away and not cause a problem.

Anti static cloths - Anti static cloths are conductive, so they can carry charge away from objects they are used to wipe

Insulating Mats - insulating mats and shoes with insulating soles prevent static electricity from moving from them, so they stop you from getting a shock

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Uses of Static Electricity

Paint Sprayers for getting an even coat - bikes and cars are painted using electrostatic paint sprayers. The spray gun is charged, which charges up the small drops of paint. Each paint drop repels each other, since they've got the same charge, so you get a very fine spray. The object being painted is given an opposite charge to the spray gun. This attracts the paint. This method gives a nice even coat and hardly any paint is wasted.

Dust Preciptators - Smoke particles do through a negative charged wire grid. The particles become negatively charged. They pass positively charged metal plates so they get attracted to them, where they stick together to form larger particles. When heavy enough the particles fall off the metal platesand fall to the bottom of the chimney where they are cleaned up.

Defibrillators - the beating of your heart is controlled by tiny electrical pulses, so an electric shock can stop your heart and start it again as well. Defibrillator consists of two paddles connected to a power supply. The paddles are placed firmly on the patients chest to get a good electrical contact and then the defibrillator is charged up. Everyoen must move away from the patient except from the operator so only the patient gets a shock. The charge passes through the paddles to the patient to make the heart contract

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Charge in Circuits

Current - is the flow of electrical charge around a circuit - basically the flow of electrons. its measured in amps. More charge passes around a circuit when a higher current flows. Current will only flow through a component if there is a voltage across that component.

Voltage - is the driving force that pushes the current around - kind of like electrical pressure. Voltage is measured in volts

Resistance - is anything in the circuit which slows the flow down. Resistance is measured in Ohms

Theres a balance - the voltage is trying to push the current round the circuit, and the resistance is opposing it - the relative sizes of the voltage and resistance decide how big the current will be.

breaking the circuit - current only flows in a circuit as long as theres a complete loop for it to flow around. Break the circuit and the current stops. Wire fuses and circuit breakers are safety  features that break a circuit of there is a fault.

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Plugs and Fuses

Colour coded wires - the live wire is brown and it carries the Voltage. The neutral wire is blue and it is the wire that completes the circuit - electricity normally flows in throught the live wire and leaves through the neutral wire. The earth wire is yellow/green, this and the fuse are for safety and work together. ALL apliances with metal casing must must be earthed to reduce the danger of an electric shock. If the appliance has a casing that non-conductive (like plastic) then it is said to be double insulated.

Earthing and fuses - if a fault develops in the live wire and somehow touches the metal case, then because the case is earthed, a big current flows in through the live wire, through the case and out down the earth wire. The surge in current blows the fuse and causes the wire inside it to melt. This cuts off the live supply because it breaks the circuit - the appliance will no longer work and it is impossible to get a electric shock from.

Electrical power - the formula: power = voltage X current (P = V X I) 

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Variable Resistors - A variable resistor is a resistor whose resistance can be changed by twiddling the knob or something. They're great for altering the current flowing through a circuit. Turn the resistance up, the current drops. Turn the resistance down, the current goes up.

Calculating Resistance - Resistance = voltage/Current

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Ultrasound Treatments and Scans

Sound is a longitudinal wave - sound waves squash up and stretch out the arrangement of particles in material they pass through, making compressions and rarefactions. Compressions are bits under high pressure (lots of particles) and rarefractions are the parts under low pressure (fewer particles).

Ultrasound is used by Hospitals - ultrasound can be used to to break down accumulations in the body, getting rid of kidney stones: an ultrasound beam concentrates high energy waves at the kidney stone and turns it into sand like particles, these particles then pass out of the body in urine. Ultrasound can also be used for body scanning - ultrawaves can pass through the body, but whenever they reach a boundary between two different media some of the wave is reflected back and detected, returning back from different depths at different times. The exact timing and distribution of these echoes are processed by a computer to produce a video image of whatever is being scanned.

ultrasound had advantages over xray - x-rays pass easily through soft tissues like muscle and skin, so they can only be used for hard things like bone unlike ultrasound. The other thing is that ultrasound is alot safer than x-rays (its ionising so can damage living cells causing cancer)

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Radioactive Decay

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