Electrostatics Video

Uses of Electrostatics

Defibrillators – restarts a heart

Paint Spraying

Dust Precipitators

Photocopiers & Laser Printers

Paint Spraying

Spray Gun is charged – all of the paint gets the same charge

Like charges repel – paint particles spread out giving a fine spray

Object being painted is given the opposite charge – paint is attracted to object and sticks to it.

Advantages : less wasted paint; even coat of paint; awkward places are painted.

Electrostatic Dust Precipitators Removes harmful smoke particles from a chimney.

1. Metal grid/wires placed in chimney

2. Grid connected to high voltage supply

3. Dust particles attracted to metal grid

4. Dust particles stick together

5. Large particles fall down chimney

6. Soot used to make building blocks

Electrostatics & Uses of Electrostatics Tests!

Mains Electricity Video

Current Electricity

Electric Circuits – must be COMPLETE to allow electricity to flow from + to – terminals on a cell/battery.

A Cell provides POTENTIAL DIFFERENCE (Voltage) in VOLTS. This provides the FORCE to move the charge carriers (in wires – electrons; in electrolysis – ions)

The resulting flow of charge is called CURRENT measured in AMPS.

Current always flows from + to – (even though the electrons

flow from – to +!)

A Circuit to measure resistance

A variable resistor varies the current in

the circuit

The component which is having its resistance

measured

A voltmeter measures Potential Difference in Volts

An ammeter measures

Current in Amps

Resistance =

Potential difference ÷ Current

R = V ÷ I

Ohm’s Law

“For a metallic conductor at constant temperature, the ration of Potential difference (V) to Current (I) is constant”.

The constant is called Resistance, R measured in ohms, symbol Ω.

So R = V ÷ I, also V = I x R and I = V ÷ R

R I

V

x÷÷

Cover up what you

want to find

Mains Electricity Live (brown) brings supply

to house Neutral (blue) is return

path to power station Earth (green and yellow)

carries current to 0V if the casing becomes live. This blows the FUSE which cuts off the supply.

Fuse is in series with the Live wire.

Fuses come in various values; 2A,3A,5A,13A.A 13Amp fuse blows

when current through it exceeds 13Amps

A circuit-breaker is a re-settable fuse which can be re-set at the flick of a switchThese have replaced fuse wire in the main fuse box.

Electrical Circuits Test!

Ultrasound - sound waves beyond the human

hearing range

Range of human hearing is 20 – 20,000Hz so beyond that is Ultrasound

Sound waves are LONGITUDINAL – the vibrations of the particles are in the same direction as the wave

Compression – particles in wave squashed together

Rarefaction – particles in wave spread out.

Wavelength – distance occupied by one complete wave (unit – metres)

Frequency – number of complete waves per second (unit – hertz)

Amplitude – maximum distance a particle moves from its normal position.

Uses of Ultrasound Check the condition of a foetus Investigate heart and liver problems Look for tumours in the body Break down kidney stones and stones

elsewhere in the body Measure the speed of blood flow in vessels

when a blockage of a vein or artery is suspected

Cataract surgery

Non – medical uses : dentists shake plaque and dirt off teeth; jewellers clean delicate pieces of jewellery and watches

Ultrasound Test

Radioactivity Video

Radiotherapy and Diagnosis Gamma rays γ High frequency/energy

electromagnetic waves emitted from the nucleus of a radioactive isotope

Very penetrating – can pass into the body to treat internal organs

Can damage living cells – over exposure should be avoided

X rays High frequency/energy

electromagnetic waves emitted when high speed electrons are decelerated

Very penetrating Can damage living cells An X-ray machine can

produce and control X-rays of different energies – so some X-rays can have higher energy than γ rays

Alpha, Beta and Gamma emissions

Radiation Alpha Beta Gamma

Ionising Power Very strong Medium Weak

Range in air About 5cm About 1m Very large, its intensity decreases with distance

What stops it? paper A few mm of Aluminium

Reduced by lead and concrete

Ionisation – the ability to remove an electron from an atom, causing the atom to become charged.Alpha has 2 + charges so has a great ionising effect, beta has 1 – charge so a lesser ionising effect.

Alpha and Beta particles are not good inside the body – they cannot escape from inside – so don’t swallow any – remember Mr Litvinienko? – killed with Polonium 210, an alpha emitter!

Using Gamma radiation Treating cancer – large doses can kill and

destroy cancer cells. A ‘gamma knife’ is rotated around a cancer to give the cancer a high dose but the healthy tissue a low dose. Side effects of this kind of therapy can be unpleasant but slows down the growth of the cancer.

Sterilising hospital equipment – Gamma kills bacteria and prevents the spread of disease.

Tracers – some radioactive isotopes (in very low doses!) can be injected into the body to highlight places where a cancer may

be growing.

Common isotopes are

Technetium – 99 and Iodine - 123

Radiographers – carry out procedures

using X-rays and nuclear radiation.

Lead prevents tissue absorbing radiation.

Radioactivity - discovered by

Becquerel and researched by Pierre & Marie Curie (among others)

Some atoms naturally break up because the nuclear forces holding them together are not strong enough.

Radioactive substances decay naturally and give out

Alpha (α), Beta (β) and Gamma (γ) radiation. Radioactivity is a random process that cannot be

controlled by external conditions such as temperature, pressure etc. Neither can the decay be predicted.

Radioactivity is detected usually with a Geiger-Müller (GM) tube and a ratemeter.

Activity is measured by counting the average number of nuclei that decay every second, unit – Becquerels (Bq).

Half Life

The time taken for the activity of a sample to fall to one half of its original activity,

OR The time for half of

the atoms in a radioactive isotope to have decayed.

It is different for every radioisotope

A Typical Decay curve

Alpha Decay An alpha particle (or helium nucleus) contains 4 nucleons

(2p + 2n)

When α is emitted, A decreases by 4, Z decreases by 2 The new element formed is two places lower in the Periodic

table than the original radioisotope.

92U 2α + 90Th

Remember : A is the mass number = protons + neutrons Z is the proton number = number of protons OR electrons

238 4 234

Beta Decay -1β A beta particle is a high energy electron

emitted from the nucleus (!) [This is because a neutron decays to a proton, an electron and a bit of antimatter]

When β- is emitted, A does not change

Z increases by 1 (because it has an extra proton).

6C -1β + 7N A new element is formed that is one place higher in the periodic table

than the original radioisotope.

14 0 14

0

Background Radiation

This is ionising radiation that is always present in the environment.

The level of background radiation is low and does not cause harm.

E.g. Granite contains small amounts of Uranium which decays to Radon, a radioactive gas.

Sources (7): radioactivity in the air; radiation from Space (cosmic rays); rocks; food; medical uses; nuclear power; nuclear weapons testing (these last two make up just 1%).

Uses of Radioactivity Alpha is used in Smoke detectors Americium-241 Beta is used to monitor the thickness of paper – Strontium-90 Gamma is used to treat cancer; to search for leaks from

pipes; to check welds in castings.

Dating: •Uranium/Lead levels are used to date rocks – VERY OLD!•Potassium/Argon levels date rocks up to about 100,000 years old•Carbon 14/Carbon12 levels are used to check organic material up to 60,000 years old

What is radioactivity? Uses of radioisotopes &

Radiation treatment tests!

Nuclear Fission•U235 can become unstable when bombarded with neutrons. •It accepts a neutron, becomes U236, which decays readily to Kr92, Ba141 and extra neutrons. (These may go on to strike the nuclei of other atoms causing further fission reactions – a CHAIN REACTION).

•It also releases massive amounts of energy!!!

A Nuclear Power Station

The output of a Nuclear Reactor is controlled by:A graphite moderator between the fuel rods slows down fast-moving neutronsBoron control rods absorb neutrons and so control the rate of fission.

Control Rods in a reactor core

Nuclear Fission Test!