Build up of static is caused by friction
- when two insulating materials are rubbed together, electrons are scraped off one and dumped on the other
- this leaves a postitive static charge on one due to the lack of electrons and a negative charge on the other
-which way the electrons are transferred depends on the materials involved
- charged objects attract small neutral objects near them
Example: polythene and acetate rods being rubbed with a duster
With the polythene rod, the electrons move from the duster to the rod
With the acetate rod, electrons move from the rod to the duster
An atom that has become charged is called an ion
ONLY ELECTRONS MOVE - not the charges. The charges are produced by the movement of electrons.
- if enough static charge builds up, it can suddenly move which can cause sparks or shocks
- a charged conductor can be discharged safely by connecting it to earth with a metal strip, this is called earthing
- the electrons flow down the strap to the ground if the charge is negative and up the strip if the charge is positive
opposite charges attract
same charges repel
forces get weaker the further apart
atoms or molecules that have becom charged are called IONS
Uses of static electricity
- Bikes and cars can be painted with electrostatic paint sprayers
- The spray gun gives each paint particle the same charge, so they repel each other, giving a fine spray.
- The object to be painted is given a positive charge, so the particles are attracted to it evenly.
- This gives an even coat, hardly any paint is wasted and everywhere is coated
Dust Precipitators - used to remove smoke in factories
- The dust particles are given a negative charge by a charged wire grid at the bottom of the chimney
- Metal plates on the sides of the chimney are given a postive charge so the particles are attracted to the plates and collect there
Defibrillators - restarting a heart
- an electric shock to a stopped/irregular heartbeat can make it regain natural rhythm
- placed firmly on patient for good electrical contact. everyone except the operator moves away so only the patient gets the shock
Plugs and circuits
current - the flow of electrical charge
voltage - the force pushing the current round
resistance - anything which slows the current down
Live wire - carries voltage (brown)
Neutral wire - completes circuit (blue)
Earth wire and fuse - for safety (green and yellow)
Earthing and fuses
1. fault develops in live wire, touches metal case. Big current flows through live wire and out the earth wire. Current surges to Earth
2. The surge blows the fuse, the wire inside melts. Cuts off supply because the circuit is broken.
POWER = VOLTAGE X CURRENT
RESITANCE = VOLTAGE/CURRENT
Double insulated appliances: the appliance is a non conductor and cannot become live.
Ultrasound treatments and scans
Compressions - bits under high pressure
Rarefactions - parts under low pressure
Wavelength - full cycle of a wave, (crest to crest, compression to compression)
Frequency - no of complete waves per second
ULTRASOUND - a sound at a higher frequency than we can hear
1. Getting rid of kidney stones. Concentrates high energy waves at the stone to break it down, can then be passed out in urine.
2. Body scanning - waves pass through body, whenever they reach a boundary between different media some of the wave is reflected back, returning back from different depths at different times. Timing and distribution of the echoes are processed by a computer to form an image
Ultrasound and x-rays
1. Ultrasound can image soft tissue.
2. safer, doesnt damage living cells
Alpha - a helium nucleus. Mass 4, charge +2.Two protons, two neutrons.
When a nucleus emits an alpha particle:
* Mass decreases by 4
* Atomic no decreases by 2... new element is formed
Beta radiation - a fast moving electron
no mass and a charge of -1.
When a nucleus emits a beta particle:
* Mass doesnt change - it loses an neutron but gains a proton
* Atomic no increases by 1 - one more proton ... basically a neutron changes into a proton
Gamma rays are electromagnetic radiation - no mass no charge
Radioactive decay is random
HALF LIFE - time taken for half the radioactive nuclei now present to decay
e.g Initial radioactivity of sample is 640bq. Two hours later it has fallen to 40bq.
640 - 320 - 160 - 80 - 40 ... 4 half lives, 2 hours/4 = 30mins. 1 half life = 30 mins
Medical uses of radiation and background radiation
1. radiotherapy- treating cancer
• gamma rays focused on tumour
• wide beam used
• rotated round the patient with tumour at centre
• limiting damage to non-cancerous tissue.
2. Tracers in medicine
beta or gamma emitter with a short half life
• drunk/eaten/ingested/injected into the body
• allowed to spread through the body
• followed on the outside by a radiation detector.
Sterilisation of surgical instruments - gamma rays
high doses of gamma rays kill all microbs. Better than boiling because it doesnt involve high temperatures, so plastic instruments can be cleaned too.
• the amount of Carbon-14 in the air has not changed for thousands of years
• when an object dies (eg wood) gaseous exchange with the air stops
• as the Carbon-14 in the wood decays the activity of the sample decreases
• the ratio of current activity from living matter to
the activity of the sample is used
Carbon-14 makes up 1/10,000,000 of carbon in the air and living things. .
It has a half life of 5730 years.
E.g axe handle has 1 part in 40,000,000 carbon-14
1/10,000,000 - 1/20,000,000 - 1/40,000,000
2 half lives old. 2 x 5730 = 11,460 years old.
Nuclear Power and fission
• uranium nucleus hit by neutron
• causes nucleus to split
• energy released
• more neutrons released.
This creates heat, heat water, steam, turn turbine, drives generator
Fusion starts a chain reaction e.g a nuclear bomb is a chain reaction that has got out of control
• when each uranium nucleus splits more than oneneutron is given out
• these neutrons can cause further uranium nuclei to split.
Scientists stop nuclear reactions going out of control:
• rods placed in the reactor
• to absorb some of the neutrons
• allowing enough neutrons to remain to keep the
Gamma and X-Rays
• gamma rays are given out: from the nucleus of certain radioactive materials
• X-rays are made: by firing high speed electrons at metal targets
• X-rays are easier to control than gamma rays.
Background radiation comes from unstable isotopes that are all around us. The majority comes from rocks and building materials, food and radon gas.
Radiation fro human activity includes waste from industry and hospitals
Different isotopes of hydrogen can undergo fusion to form helium:
The conditions needed for fusion to take place, to include:
• in stars, fusion happens under extremely high temperatures and pressures
• fusion bombs are started with a fission reaction which creates exceptionally high temperatures
• for power generation exceptionally high temperatures and/or pressures are required
and this combination offers (to date) safety and practical challenges
Cold fusion - not accepted yet as few scentists have managed to reproduce the results reliably
A smoke detector with an alpha source:
• smoke particles hit by alpha radiation
• less ionisation of air particles
• current is reduced causing alarm to sound.
Alpha particles make good ionisers because:
Theyre large - easy for them to collide with other particles
They're highly charged - easily remove electron from the atoms they collide with