P4 OCR Gateway

Charged Objects

• The build-up of static is caused by friction.
• When two insulating materials are rubbed together, electrons will be scraped off one and dumped on the other; this leaves a positive negative charge on one and a negative static charge on the other.
• The way the electrons are transferred depends on the two materials involved.
• Electrically charged objects attract small neutral objects placed near them.
• When you rub a duster on a polythene rod, electrons move from the duster to the rod, so the polythene rod becomes negatively charged.
• When you rub a duster on an acetate rod, electrons move from the acetate rod to the duster, so the acetate rod becomes positively charged.

Discharging

• A charged conductor can be discharged safely by connecting it to earth with a metal strap; the electrons will move accordingly to make the charges neutral.

What Static Electricity Does

1. Attracts dust - Dust particles are charged and will be attracted to anything with the opposite charge. Insulators get easily charged and are everywhere around the home.

2. Clings to Clothes - When synthetic clothes are rubbed against each other, like in a tumble drier or over someone's head, electrons get scraped off, leaving both materials charged; therefore they are attracted to each other and cling. There are little sparks or shocks as the charges rearrange themselves.

3. Shocks from Door Handles - Walking on a nylon carpet wearing insulating soles causes a charged body; so touching a metal door handle causes a discharge so you get a shock.

Dangers

1. Charge on Clothes - For example, moving around on a car seat causes two synthetic materials to rub against each other and this charge can build up enough to produce a spark - dangerous near inflammable gases or fuel fumes.

2. Grain Chutes; Paper Rolling; Fuel Filling - As fuel shoots out of a filler pipe, paper drags over rollers or grain shoots out of pipes, static can build up enough to cause a spark leading to an explosion if at a petrol station.

Preventing Sparks

• Earthing is when a charged object is connected to the ground using a conductor, like copper wire, and it provides an easy route for the charges to travel into the ground; therefore, no charge can build up.
• Fuel tankers and hospital operating theatres (with inflammable gases or high concentrations of oxygen) must be earthed.
• Anti-static sprays and liquids make the surface of a charged object conductive.

Uses of Static Electricity

• Paint sprayers - To paint bikes and cars; the spray gun is charged so each drop of paint gets charged; they repel each other so the spray is fine. The object to be painted is given an opposite charge, attracting the spray -> even coat; not much wastage; there are no paint shadows.
• Dust precipitators - As smoke particles reach the bottom of a chimney, they are negatively charged by a wire grid and so are attracted to positively charged metal plates, where the smoke particles stick together to form larger particles; once heavy enough, they either fall or are knocked by a hammer, and they can be collected and removed. This cleans up emissions as the gases coming out of the chimney have few smoke particles in them.
• Defibrillators - In hospitals and ambulances. The heart is controlled by electrical pulses, so can be restarted by an electric shock. It consists of two paddles and a power supply, which are placed on the patient's chest to get a good electrical contact so the defibrillator is charged up; only the patient gets a shock - everyone moves away except the operator, holding insulated handles.

Definitions

• Current - The flow of electrons around the circuit; in amps, A. Current only flows through a component if there is a voltage across it, unless it's a superconductor. Can be measured using an ammeter connected in series.
• Voltage (aka Potential Difference) - The driving force that pushes the current round; in volts, V. Can be measured using a voltmeter connected in parallel.
• Resistance - Anything in the circuit which slows the flow down; in ohms, Ω.

Plugs

• Earth wire - Green and yellow; in middle; just for safety.
• Neutral wire - Blue; on left; always 0V.
• Live wire - Brown; connected to fuse; alternates between +230V and -230V.
• The metal parts are made of copper or brass - good conductors.
• The case, cable grip and cable insulation are made of rubber or plastic - they're flexible and good insulators.

Fuses

• If a fault develops where the live touches the metal case, a big current flows in through the live, through the case and out down the earth wire; this surge in current blows the fuse, which cuts off the live supply. This isolates the whole appliance, preventing electric shock or a fire from the heat of a large current.
• Fuses should be rated as near as possible but just higher than the normal operating current.
• All appliances with metal cases must be earthed to reduce the danger of electric shock.
• An appliance with a plastic casing and no metal parts showing then it's double insulated; doesn't need an earth wire.

Variable resistors

• A variable resistor used to be a coil of wire with a slider; they alternate the current flowing through a circuit.

Longitudinal Waves e.g. Sound

• One wavelength - crest to crest or compression to compression.
• Frequency - how many waves pass a certain point per second, Hz. 1 Hz is one wave per second.
• Amplitude - how much energy the wave is carrying; can see it on an oscilloscope (CRO) which displays sounds as transverse waves; half of height.
• In longitudinal waves, the vibrations are along the same direction as the wave.
• In transverse waves, the vibrations are at 90 degrees to the wave.

Ultrasound - Frequency>20 kHz - Uses

1. Kidney stones - high energy waves -> sand-like particles; go out of the body in urine.

2. Pre-natal scanning - some of the US wave is reflected back and detected - the echoes' timing and distribution are detected and shown as a video image of the foetus.

3. Measuring the speed of blood flow.

Dangerous X-Rays

• X-rays pass through soft tissue so are only useful for viewing bone.
• If exposed to a high dose of X-rays you may get cancer - ionising radiation.
• X-rays and gamma rays are both high frequency, short wavelength EM.
• Gamma rays are released from some unstable atomic nuclei when they decay - nuclear decay is random so there's no way to control when they're released.
• X-rays are produced by firing high-speed electrons at a heavy metal - tungsten.
• When radiographers take X-rays of people with suspected broken bones, they wear lead aprons and stand behind a lead screen or leave the room.
• Lower doses of radiation cause cancer; higher dosescause radiation sickness.
• Alpha - very ionising; not penetrating; helium nuclei; two protons and two neutrons; big, heavy, slow moving.
• Beta - not ionising; penetrating; fast-moving electrons; very small.
• Gamma - not at all ionising; most penetrating; EM radiation; no mass/charge.