Physics P4

The build up of static is caused by friction:

• two insulating materials are rubbed together electrons will be taken off one and put on the other
• this will leave a positive static charge on one and a negative on the other
• which way depends which way electrons are transferred
• electrically charged objects attract small neutral objects
• The classic example is the polyethene rod being rubbed with a cloth
  • with the polyethene rod, electrons move from the duster to the rod
  • with the acetate rod electrons move from the rod to the duster

The electrons move not the positive or negative charge

A charged conductor can be discharged safely by connecting it to earth with a metal strap- if negative ground flow down strap, if positive ground flow up

Opposite electric charges attract

Same electric charges repel

The forces get weaker the further apart they are

Attracting Dust:

• Dust particles are charged and will be attracted to the opposite charge
• many objects around the house are made of insulators (e.g. tv)
• which means they get easily charged and attract dust

Clothing clings and crackles

• when synthetic clothes are dragged over each other or over your head electrons get removed leaving static charges on both parts
• leads to attraction- stick together and cling to you, and little sparks/shocks as the charges rearrange themselves

Shocks from door handles

• walking on a nylon carpet wearing shoes with insulating soles charge builds up on your body and if you touch a metal door hand the charges flow to the conductor and you get a shock.

A large charge can build up on clothes

• static charge can build up on clothes made out of synthetic materials, if they rub against other synthetic fabrics the charge can become large enough to produce a spark

Grain chutes, Paper rollers and fuel filling

• as the fuel flows out of the filler pipe or the paper drags over the rollers static can build up
• this can lead to a spark and might cause an explosion in dusty or fumey places- like when filling up a car
• all these problems can be prevented by earthing

• dangerous sparks can be prevented by connecting a charged object to the ground using a conductor- earthing provides an easy route for the static charges to travel to the ground. meaning no charge can build up to give you a shock or spark
• Fuel tankers must be earthed
• Static charges are also problems in places where sparks could ignite inflammable gases, or where there are high concentration of oxygen
• anti-static sprays and liquids work by making the surface of a charged object conductive- providing an easy path for the charges to move away

• 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 all the others because they all have the same charge
• the object being painted is given the opposite charge to the gun which attracts the fine spray
• gives an even coat and hardly any paint is wasted, no paint shadows

Smoke is made up of tiny particles which can be removed with a precipitator

• as smoke particles reach the bottom of the chimney they meet a wire grid with a high negative charge, which charges the particles negatively
• the charged smoke particles are attracted to positively charged metal plates, the smoke particles stick together to form larger particles
• when heavy enough the particles fall off or are knocked by a hammer, the dust falls to the bottom and can be removed
• the gases coming out of the chimney have very few smoke particles

• beating of the heart is controlled by tiny little electrical pulses so an electric shock to a stopped heart can make it start beating again
• machines called defibrillators can be used to shock the heart
• the defibrillator has 2 paddles connected to a power supply
• The paddles of the defibrillator are placed firmly on the patient's chest to get a good electrical contact and then the defibrillator is charged up
• everyone moves away except the operator who holds the insulated handles this is so only the patient gets a shock

Current= flow of electrons around the circuit- measured in amps, current will only flow through a component if there is voltage

Voltage= the driving force that pushes the current round- like electrical presure, measured in volts

Resistance= anything that slows the flow down, measured in ohms

There's 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- more voltage will be needed to keep the same current

If you break the circuit the current stops flowing- wire fuses and circuit breakers are safety features

The wiring must be right- the right colour wire connected to each pin and firmly screwed in, no bare wires, a cable grip

Plug features- metal parts are made of copper or brass because are good conductors, the case and cable grip and cable insulation are all made of rubber or plastic- good insulators and flexible

The Live wire alternates between a high +ve and -ve voltage of about 230V

The neutral wire is always at 0V

The earth wire and fuse are safety features

Earth wire provides a low resistance path to earth

• If a fault develops in the live and touches the metal case, a big current flows in through the case and out down the earth wire
• this surge blows the fuse
• this cuts of the power supply
• this isolates the whole appliance making it impossible to get an electric shock also prevents fire risk
• fuses should be rated as near as possible but higher than the normal operating current

An earthed conductor can never become live

if the appliance has a plastic casing and no metal parts which means its double insulated, anything that has double insulated doesn't need an earth wire

The higher the resistance the harder it is for electricity to flow so the lower the current

Variable resistors:

• is 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
• They're good for altering the current flowing through the circuit- turn the resistance up, the current drops, turn the resistance down the current goes up

The ammeter:

• measures the current (in amps) through the component
• can be put anywhere in series in the main circuit but never in parallel

The Voltmeter:

• measures the voltage (in volts)
• must be placed in parallel- not around variable resistor or battery
• Proper name is potential difference

If you increase the voltage across a resistor the current increases as well

Resistance= Voltage/Current

Sound is a longitudinal wave

the features of a longitudinal wave are:

• sound waves squash up and stretch out the material they pass through making compressions and rarefactions
• The WAVELENGTH is a full cycle of the wave (crest to crest)
• FREQUENCY is how many complete waves there are per second
• the AMPLITUDE tell you how much energy the wave is carrying or how loud the sound is- can see it on an oscilloscope

Longitudinal waves the vibrations are along the same direction as the wave is travelling

In Transverse waves the vibrations are at 90' to the direction of the wave

Ultrasound is sound with a higher frequency than we can hear

Breaking down kidney stones:

• an ultrasound beam concentrates high energy waves at the kidney stone and turns it into sand like particles
• these particles can pass out the body in urine

Pre-natal scanning of a foetus:

• Mothers skin is smeared with jelly to provide good contact
• ultrasound waves enter the body and bounce off different layers
• the waves reflect back to the probe and the information is fed to a computer

• x-rays are produced when electrons strike a metal target- ionising radiation
• x-rays can show bone but not soft tissue
• x-rays can damage human cells
• ultrasound doesn't damage human cells
• using ultrasound you can see movement and soft tissue

X-rays and Gamma rays are electromagnetic waves

• both have high frequency and short wavelength
• similar wavelengths so similar properties but made in different ways:
• gamma rays are released from an unstable atomic nuclei when they decay and it is completely random so there is no way to control when they're released
• X-rays can be produced by firing high speed electrons at a heavy metal like tungsten, they are much easier to contol

Dangers of x-rays

• Radiographers in hospitals take x-rays of people with broken bones
• x-rays pass through flesh but not easily through bones or metal (denser)
• X-rays can cause cancer so radiographers have to wear lead aprons and stand behind a lead screen or leave the room to keep their exposure to x-rays at a minimum

Nuclear radiation and x-rays enter living cells and collide with molecules

These collisions cause ionisation which damages or destroys the molecules

lower doses tend to cause minor damage without killing the cell, this can give rise to mutant cells which divide uncontrollably- cancer

Higher doses tend to kill cells completely which causes radiation sickness

Outside the body beta and gamma are most dangerous as they can get into the delicate organs, alpha is much less dangerous as it cannot penetrate through skin

Inside the body alpha is the most dangerous as they do there damage in a very localised area whereas beta and gamma they pass straight out and are less ionising

Alpha particles are helium nuclei           They're big, heavy and slow moving

Beta Particles are fast moving electrons           quite fast and quite small

Gamma rays are electromagnetic radiation           no mass no charge

Radioactivity is a totally random process:

unstable nuclei will decay and give out radiation at random

it happens spontaneously and is unaffected by physical conditions like temperature

When the nucleus does decay it will spit out one or more of the 3 types of radiation and in the process the nucleus will normally form a new element

Alpha emission:

alpha particle has mass of 4 and charge of +2

e.g.

Beta emission

beta particle has no mass and charge of -1

e.g.

Gamma emission:

gamma particle is a photon with no mass and no charge, after an alpha or beta emission it usually has extra energy to get rid of, does this by emitting a gamma ray e.g.

Radioactivity of a sample decreases over time

Each time a decay happens, an alpha, beta or gamma is given out which means one radioactive nucleus has disappeared

As the unstable nuclei disappear the activity as a whole will decrease so the older a sample becomes the less radiation it will emit

Half life is the time taken for half the radioactive atoms now present to decay

A short half life means the activity falls quickly because lots of nuclei decay quickly

A long half life means activity falls slowly because most of the nuclei don't decay for a long time

Work it out step by step:

Keep dividing by 2 untill the desired number is obtained, then count the number of times you have divided by 2

Finding the half life from a graph:

The half life is found by finding the time interval on the bottom axis corresponding to a halving of the vertical axis

Comes from:

• unstable isotopes- in air, food, building materials and rocks
• from space- cosmic rays
• from human activity- fallout from nuclear explosions

Level changes depending where you are:

• at high altitude the background radiation increases as there is more exposure to cosmic rays
• underground in mines as there are rocks everywhere
• certain underground rocks (e.g. granite) can cause higher levels at the surface especially if they release radioactive radon gas

• Radon concentration varies across the uk depending on what rock hoses are built on
• high doses of radon gas can cause lung cancer
• evidence suggests the risk of getting lung cancer from radon gas is higher for smokers
• new houses in areas of high levels of radon gas are designed with good ventilation systems

=the treatment of cancer using gamma rays

high doses of gamma rays can kill all living cells they can be used to treat cancers

have to be directed carefully and at the right dosage to kill the cancer cells

To treat cancer:

• The gamma rays are focused on the tumour using a wide beam
• This beam is rotated around the patient with the tumour at the centre
• This minimises the exposure of normal cells to radiation, and so reduces the chances of damaging the rest of the body

Always short half life gamma emitters

Certain radioactive isotopes can be injected into people and their progress around the body and can be followed using external detector- a computer converts the reading to a display showing where the strongest reading is coming from

e.g. iodine-131

All isotopes which are taken into the body must be gamma or beta, so that the radiation passes out of the body and they should only last a few hours, so that the radioactivity inside the patient quickly disappears

Sterilisation of surgical instruments using gamma rays

• medical instruments can be sterilised by exposing them to a high dose of gamma rays which will kill all microbes
• the great advantage of irradiation over boiling is that it doesn't involve high temperatures so heat sensitive things like thermometers and plastic instruments can be totally sterilised without damaging them

• Radioisotopes are used to track the movement of waste materials, find the route of underground pipe systems or detect leaks or blockages in pipes
• To check a pipe you put in the radiation and go along the outside with a detector
• If the radioactivity reduces or stops after a certain point, there must be a leak or blockage
• The isotope must be a gamma emitter so that the radiation can be detected through the metal or earth which may be surrounding the pipe
• It should also have a short half life so it is not a hazard if it collects somewhere

A weak radioactive source is placed in the detector close to 2 electrodes

the source causes ionisation and a current flows

if there is a fire then smoke will absorb the radiation- current falls and alarm sounds

• can accuratlely work out the age of some rocks and archaeological specimens
• by measuring the amount of radioactive isotope is left in a sample and knowing its half life you can work out how long its been around

Radiocarbon dating:

• carbon-14 calculations
• carbon-14 makes up 1/10000000 of carbon in the air, the level stays fairly constant in the atmosphere, the same proportion is found in living things
• when they die carbon-14 is trapped inside wood, wool etc and it gradually decays with a half life of 5730 years

Dating rocks-relative proportions calculations

• uranium isotopes have very long half lives and decay via a series of short-lived particles to produce stable isotopes of lead

=the splitting up of uranium atoms

nuclear power stations are powered by nuclear reactors- its a controlled chain reaction

uranium or plutonium atoms split up and release energy in the form of heat, this heat is used to drive a steam turbine

The splitting of Uranium-235 needs neutrons

• uranium-235 is quite stable so it needs to be made unstable before it splits
• this is done by firing slow moving neutrons at the atom
• the neutron joins the nucleus to create U-236 which is unstable
• the U-236 then splits into a smaller atom plus 2 or 3 fast-moving neutrons
• there are different pairs of atoms that U-236 can split into

You can split more than one atom

To get loads of U-235 atoms have to be split. So neutrons released from the previous fissions are used to hit other U-235 atom

These cause more atoms to split releasing even more neutrons which hit more U-235 atoms- chain reaction

The fission of an atom of uranium releases loads of energy in the form of kinetic energy of the two new atoms

• free neutrons in the reactor 'kick-start' the fission process
• the two fission fragments then collide with surrounding atoms, causing the temperature to rise
• control rods made of boron limit the rate of fission by absorbing excess neutrons
• a gas, typically carbon dioxide is pumped through the reactor in order to carry away the heat generated
• the gas is passed through the heat exchanger where it gives its energy to water, this water is heated and turned into steam which is then used to turn the turbines generating electricity