Transcript P4 General revision

Electrostatics Video
Uses of Electrostatics
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
Grid connected to high voltage supply
Dust particles attracted to metal grid
Dust particles stick together
Large particles fall down chimney
Soot used to make building blocks
2.
3.
4.
5.
6.
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
Resistance =
Potential difference
÷ Current
R=V÷I
The component
which is having
its resistance
measured
A
An ammeter
measures
Current in Amps
V
A voltmeter
measures Potential
Difference in Volts
Ohm’s Law
Cover up
what you
want to find
V
÷ ÷
R x I
“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
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 switch
These 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

X rays

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

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
Remember : A is the mass number = protons + neutrons
Z is the proton number = number of protons OR electrons

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.


238
4
92
2
U
α + 90Th
234
Beta Decay - β
0
1

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).
14
6

C
0
-1
β + 7N
14
A new element is formed that is one place higher in the periodic table
than the original radioisotope.
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
Control Rods in
a reactor core
The output of a Nuclear Reactor is controlled by:
A graphite moderator between the fuel rods slows down fast-moving neutrons
Boron control rods absorb neutrons and so control the rate of fission.
Nuclear Fission Test!