ELECTRICITY AND THE ATOM

physics

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  • Created by: katie
  • Created on: 07-05-12 15:24

static electricity

1) build up of static electricity is caused by friction - when two insulating materials are rubbed together, electrons will be scraped off one and dumped on the other, this will leave a positive static charge on one and a negative static charge on the other. It depends on the materials involved, which way the electrons are transferred.

2) only electrons move (never the positive charges) - both +ve and -ve electrostatic charges are only ever profuced by the movement of electrons. The positive charges DONT move! It is caused by electrons moving away elsewhere. The rate of flow of electical charge is called electric current.

3) like charges repel, opposites attract - when two things with the OPPOSITE electric charges, they are ATTRACTED to each other and when two things with the SAME electric charge, they will REPEL each other. These forces get weaker and further apart the two things are.

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static electricity - examples

STATIC ELECTRICTY BEING HELPFUL:

1) smoke precipitation - smoke is made up of tiny particles which can be removed with a precipitator. EXAMPLE: as smoke particles reach the bottom of the chimney, they meet a wire grid with a high negative charge and this charges the smoke negatively, the charged smoke particles are attracted to positively charged metal plates. The smoke particles stick together to form larger particles. When they are heavy enough the particle fall of the plates and the dust falls to the bottom of the chimney to be removed. The gases coming out of the chimnet have very little smoke in them

2) photocopier - EXAMPLE: the image plate is positively charged and whiter bits of the thing your copying make light fall on the plate and the charge leaks away in those places. The charged bits attract negatively charged black powder, which is transferred onto positively charged paper. The paper is heared so the powder sticks.

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static electricity - examples

clothing crackles - when synthetic clothes are dragged over each other (like in a tumble drier) or over your head, electrons get scraped off, leaving static charges on both parts, and that leads to the inevitable - attraction (they stick together) and little sparks/shocks as the charges rearrange themselves

STATIC ELECTRICTY A SERIOUS PROBLEM:

1) lightening - rain drops and ice bump together inside storm clouds, knocking off electrons and leaving the top of the cloud positively charged ans the bottom negative. This creates a huge voltage and a big spark.

 2) grain chutes, paper rollers & fuel filling nightmare - as fuel flows out of a filler pipe or paper drags over rollers or grain shoots out of pipes, then static can build up. This can leas to a spark and in dusty or fumy places - BOOM! The solution: make the nozzles or rollers out of metal so that the charge is conducted away, instead of building up. Its also good to have earthing straps between the fuel tank and the fuel pipe.

 

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circuits - the basics

1) current - is the flow of electrons round the circuit. UNIT = amps, A.

2) voltage - is the driving force that pushes the circuit round. UNIT = volt, V.

3) restistance - is anything in the circuit which shows the flow down. UNIT = ohms, Ω

4) 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:

  • if you INCREASE the voltage - then MORE CURRENT will flow
  • if you INCREASE the resistance - then LESS CURRENT will flow

the ammeter -> measures the current flowing through the component, must be placed in series, can be put anywhere in series in the main circuit but never in parallell like the voltmeter

the voltmeter -> measures the voltage across the component, must be places in parallell around the component under test - NOT around the variable resistor or the battery, the proper name for 'voltage' is POTENTIAL DIFFERENCE

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resistance and V=IxR

Three important voltage-current graphs

DIFFERENT RESISTORS - the current through a resistor (at constant temperature) is proportional to voltage. Different resistors have different resistances, hence they have different slopes. (all /)

FILAMENT LAMP - as the temperature of the filament lamp increases, the resistance increases, hence the curve.

DIODE - current will only flow through a diode in one direction.

For the straight-line graphs, the resistance of the component is steady and is equal to the inverse of the gradient of the line (the stepper the graph, the lower the resistance). If the graph curves, it means the resistance is changing.

RESISTANCE = POTENTIAL DIFFERENCE / CURRENT

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circuit symbols and devices

1) VARIABLE RESISTOR - a resistor whose resistance can be changed by twiddling a knob or something, the old-fashioned ones are huge coils of wire with a slider on them, the old fashioned ones are huge coil of wire with a slider on them, they're great for altering the current flowing through a circuit, turn the resistance up, the current drops and turn the resistance down, the current goes up.

2) SEMICONDUCTOR DIODE (DIODE) - special device made from silicon which lets current slow freely through it in one direction but not in the other.

3) LIGHT-DEPENDENT RESISTOR (LDR) - in bright light, the resistanc falls, the darkness, the resistance is highest.

4) THERMISTOR - in hot conditions, the resistance drops, in cool conditions, it goes up. They make useful temperature detectors, e.g. car engine temperature sensors and electronic thermostats.

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series circuits

SERIES CIRCUITS - all or nothing.

in series circuits, the different components are connected in a line, end to end, between the +ve and -ve of the power supply. If you move or disconnect one component, the circuit is broken and they all STOP.

1) Potential different is SHARED: it is shared between the various components so the voltages round a series circuit always add up to equal the source voltage.

2) Current is the SAME everywhere:flows through all parts of the circuit and the size of the current is determind by the total potential difference of the cells and the total resistance of the circuit.

3) Resistance ADDS UP: in the series circuits, the total resistance is just the sum of all the resistances and the bigger the resistance of a component, the bigger the share of the total potential difference.

4) cell voltages ADDS UP: there is a bigger potential difference when more cells are in series, provided the cells are all connected the same way, for examplem when two batteries of voltage 1.5V are connected in series they supply 3V between them.

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parallel circuit

PARALLELL CIRCUIT - independence and isolation.

in parallell circuits, each components is separately connected to the +ve and -ve of the supply and if you remove or disconnect one of them, it will hardly affect the others at all.

1) Potential difference is the SAME across ALL components: all components get the full source potential difference so the voltage is the same across all components and this means identical bulbs connected in parallell will all be at the same brightness.

2) Current is SHARED between branches: the total current flowing around the circuit is equal to the total of all curents in the separate branches, there are junctions where the current either splits or rejoins. If two identical components are connected in parallell then the same current will flow through each component.

3) Resistance is TRICKY: the current through each component depends on its resisance, the lower the resistance, the bigger the current that'll flow through it. The total resistance of the circuit is tricky to work out, but its always less than that of the branch with the smallest resistance.

 

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series and parallel circuits - examples

VOLTMETERS AND AMMETERS ARE EXCEPTIONS TO THE RULE:

1) ammeters and voltmeters are exceptions to the series and parallel rules

2) ammeters are always connected in series even in a parallel circuit

3) voltmeters are always connected in parallell with a component even in a series circuit.

- Christmas fairy lights are often wired in series

- Everything electrial in a car is connected in parallel:

1) everything can be turned on and off separately

2) everything always gets the full voltae from the battery

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mains electricity

Mains supply is AC, battery supply is DC

  • the UK mains supply is approx 230V

  • it is an AC supply (alternating current), which means the current constantly changing direction

  • frequency of the AC mains supply is 50 hertz (Hz)

  • by contrast, cells and batteries supply direct current (DC). This just means that the current keeps flowing in the same direction

OSCILLOSCOPE - a trace on the screen shows how the voltage of the supply changes with time.

the GAIN dial contros how many volts each cm divison represents on the verticle axis and the TIMEBASE dial controls how many milliseconds each divison represents on the horizontal axis

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mains electricity

HAZARDS IN THE HOME!!!

- long cables, frayed cables, cables in contact with something hot or wet, water near sockets, shoving things into sockets, damaged plus, too many plugs in one socket, lighting sockets without bulbs in and appliances without their covers on.

PLUGS AND CABLES

get the wiring right: the right coloured wire to each pin and firmly screwed in, no bare wires showing inside the plud and cable grip tightly fastened over outer layer.

plug features: neutral wire = BLUE, earth wire = GREEN&YELLOW, live wire = BROWN, also a rubber or plastic case, cable grip and brass pins.

plug wiring errors: earth wire not connected, neutral and live wires wrong way round, bare wires showing, cable grip not holding cable in correct place.

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fuses and earthing

The BROWN live wire in a mains supply alternates between a high +ve and -ve voltage

The NEUTRAL wire is always at OV

The green and yellow EARTHWIRE is just for safety, and works together with a fuse to prevent fire and shocks,

Fuses prevent electric shocks - to prevent surges of current in electrical circuits and danger of electric shocks, a fuse is pplaces in the circuit. If the current in the circuit gets too big, the fuse wire heats up and the fuse blows which breaks the circuit and prevents any electric shocks. The fuse should always have the same value as the manufacturer recommends.

Earthing prevents fires and shocks - the EARTHWIRE and FUSE work together like this: the earth pin connected to the case via the earth wire, if a fault develops in which the live somehow touches the metal case, then because the case is earthed, a big current flows in through the live, through the case and out down the earth wire. This surge in current blows the fusem which cuts off the live supply. This prevents electric shocks from the case.

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energy and power in circuits

Energy is transformed from cells and other sources:

- MOTION (motors), LIGHT (light bulbs), HEAT (hairdriers/kettles), SOUND (speakers).

All resistors produce hear when a current flows through them:

1) whenever a current flows through anuything with electrical resistance, then electrical energy is converted into heat energy. 2) the more current that flows, the more heat energy is produced. 3) a bigger voltage means more heating because it pushes more current through. 4) you can measure the amount of heat produced by putting a resistor in a known amount of water, or inside a solid block and measuring the increase in temperare.

Power ratings of appliances -> the total energy transformed by an appliance depends on how long the appliance is on and its power rating. Formula for energy tranformed is ENERGY = POWER X TIME

Electrical power and fuse ratings -> Formula for energy tranformed is POWER = VOLTAGE X CURRENT. Most electrical goods show their power rating and voltage rating.

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charge, voltage and energy change

Total change through a circuit depends on current and time:

- Current is the flow of electrial charge (in coulombs, C) around a circuit. TOTAL CHARGE = CURRENT X TIME. More charge passes around the circuit when a bigger current flows.

The voltage is the energy transferred per charge passed:

- When electrical charge (Q) goes through a change in voltage (V), then energy (E) is transferred, energy is supplied to the charge at the power source to 'raise' is through a voltage. The charge gives up this energy when it 'falls' through any voltage drop in components elsewhere in the circuit. ENERGY TRANSFORMED = CHARGE X POTENTIAL DIFFERENCE. The bigger the change in voltage, the more energy is transferred for a given mount of charge passing through the circuit. A battery with a bigger voltage will supply more energy to the circuit for every coulomb of charge which flows round it, because the charge is raised up 'higher' at the start.

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atomic structure

Rutherford Scattering and the Demise of the Plum Pudding: 1804, John Dalton agreed with Democritus that matter was made u of tiny spheres (atoms) that couldnt be broken up but each element was made up of different types of 'atom'. J J Thomson discovered 100 years later that electrons could be removed from atoms. Atoms COULD be broken up. He suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them like plums in a plum pudding. in 1909, Ernest Rutherford tried firing alpha particles at thin gold foil. Most of them went through but the odd one came straight back at them which was a shock. Rutherford realised this meant that most of the mass of the atom was concentrated at the centre in a tiny nucleus with a positive charge and that most of the atom is just empty space. He came up with the Nuclear Model of the Atom.

Isotopes are different forms of the same element:

1) The atomic number of an element is the total number of protons in one atom of it. The mass number is the total number of protons and neutrons in one atom.

2) Isotopes are atoms with the same number of protons but a different number of neutrons - so different isotopes of an element have the same atomic number but different mass number.

3) Unstable isotopes are radioactive which means they decay into other elements and give out radiation. This is where all radioactivit comes from - unstable radioactive isotopes undergoing nuclear decay and spitting out high-energy particles.

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radioactive decay processes

ALPHA PARTICLES are helium nuclei:

- relatively big and heavy and slow moving, dont penetrate very far into materials but are stopped quickly, because of size they are strongly ionising which means they bash into alot of atoms and knock electrons off them before they slow down which creates lots of ions.

BETA PARTICLES are electrons:

- these are in between alpha and gamma in terms of properties, they move quite fast and are quite small (they're electrons), they penetrate moderately before colliding and are moderately ionising too. For every beta particle emitted, a neutron turns to a proton in the nucleus.

GAMMA RAYS are very short wavelength EM waves:

- they are the opposite of alpha particles, they penetrate a long way into materials without being stopped, they are weakly ionising because they tend to pass through rather than colliding with atoms. Eventually they hit something and do damage.

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background radiation

Background radiation comes from many sources, we receive it from:

- Radioactivity of natually occuring unstable isotopes which are all around us (in the air, food, building materials and in the rocks under our feet). Radiation from space which is known was cosmic rays, these come mostly from the sun. Radiation due to human activity e.g fallout from nuclear explosions or dumped nuclear waste (this is a tiny proportion of total background radiation)

The level of background radiation changes depedsning on where you are:

- At high altitudes (jet planes) the background radition increases because of more exposure to cosmic rays. That means commercial pilots have an increased risk of getting some types of cancer. Underground in mines, it increases because of the rocks all around. Certain underground rocks (granite) can cause higher levels at the surface, especially if they release radioactive radon gas, which tends to get trapped inside people's houses.

RADON GAS -> radon concentration in people's houses varies widely across the UK depending on what type of rock the house is built on. Exposure to high doses of radon gas can cause lung cancer - greater concentration, higher the risk. 1 in 20 deaths from lung cancer caused by radon exposure. New houses in areas where high levels of radom gas might occur must be designed with good ventilation systems.

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nuclear fission and fusion

Nuclear Fission - the splitting up of big atomic nuclei: - Nuclear power stations and nuclear submarines are both powered by nuclear reactors.

1) If a slow-moving neutron gets absorbed by a uranium or plutonium nucleus, the nucleus can split.

2) Each time is splits, it spits out two or three neutrons, one of which might hit another nucleus causing it to split also (chain reaction going).

3) When a large atom splits in two it will form two new lighter elements. These new nuclei usually radioactive because they have the 'wrong' number of neutrons in them.

4) Each nucleus splitting (fission) gives out a lot energy - a lot more energy than you get with a chemical bond between two atoms.

Nuclear Fusion - the joining of small atomic nuclei

Two light nuclei can combine to create a larger nucleus - nuclear fusion. It releases a lot of energy, ALL energy  released in stars comes from fusion. It doesnt leave alot of radioactive waste and theres plently of hydrogen knocking about to use as fuel. It can only happen at really high temperatures (10000000°C)

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