PHYSICS
FORM 5
NOTES
Name: ______________________
Class: ___________
Teacher: Mr. Neil Briffa
Electricity – Theme 5 – Part 2 – Electricity in the Home
1
Electrical Power
Power is measured in Watts (W).
Power = Energy
Time
so 1 Watt = 1 Joule/ 1 second
Other units: kW (kilowatt), MW (megawatt)
(1W = 1 J/s)
James Watt
1736 -1819
(1kW = 1000W, 1MW = 1 000 0000W)
Formulae for Power:
P = V2
R
P = V I
P = I2 R
Example:
Find the power of a bulb if its voltage is 240V and its current is 0.5A. How many Joules of electrical energy
does it use every second?
________________________________________________________________________________________
Finding the power of a component in a circuit
The current (I) flowing through the lamp is
measured with an ___________________.
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The voltage (V) across the lamp is measured
with a __________________.
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The equation _________________ is then used.
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The heating effect of the current
The current passing through a wire produces a heating effect. (e.g. electric iron, electric blanket, electric
oven, electric water heater).
The power lost as heat can be found by using the formula P = I2 R.
Mr. N. Briffa B. Ed. (Hons)
Electricity – Theme 5 – Part 2 – Electricity in the Home
Rectification: (changing a.c to d.c)
Direct current (d.c.)
Alternating current (a.c.)
A current which ___________________
__________________________________.
A current which ___________________
__________________________________.
Supplies:
Supplies:
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If a diode is connected as shown, it will allow
current to flow in one direction only.
Therefore it will change _______________
current to ______________ current. This
process
is
known
as
_____________________________.
In our homes we use alternating current.
Mr. N. Briffa B. Ed. (Hons)
2
Electricity – Theme 5 – Part 2 – Electricity in the Home
3
Paying for Electricity
We pay for the electricity we consume and the units are kilowatt-hours (kWh).
Note that kW is ________________ while kWh is _____________________.
1kWh is the energy used by a 1kW appliance in 1 hour. This costs around 10c.
1 kWh is equivalent to 3,600, 000J and this costs about 10c.
Example 1:
An air conditioner of power 3600W is used for a total of 10hours. If electricity costs 8c per kWh, find:
a) the energy in kWh used.
_____________________________________________________________________________________
_____________________________________________________________________________________
b) the total cost.
_____________________________________________________________________________________
_____________________________________________________________________________________
Example 2:
A television set of power 100W is used for a total of 4 hours. If electricity costs 12c per kWh, find:
a) the energy in kWh used.
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_____________________________________________________________________________________
b) the total cost for one week.
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_____________________________________________________________________________________
_____________________________________________________________________________________
Example 3:
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A vacuum cleaner of power 1500W is used for a total of 30 minutes. At the same time a toaster of power
600W is used for 20 minutes. If electricity costs 10c per kWh, find:
a) the total energy in kWh used.
_____________________________________________________________________________________
_____________________________________________________________________________________
b) the total cost.
_____________________________________________________________________________________
_____________________________________________________________________________________
Mr. N. Briffa B. Ed. (Hons)
Electricity – Theme 5 – Part 2 – Electricity in the Home
4
Fuses
A fuse is a short length of wire which melts when a ________ current flows through it. When the current
through it is slightly greater than the amount it can withstand, it ___________ and breaks the ____________
protecting appliances.
Circuit symbols:
Example:
A television set uses a current of 4.2 A. Three different fuses are available: 1A, 5A, 13A.
• If the 1A fuse is used it would _________ when the television is switched on. The television would
not be damaged. However, it would not work.
• If the 13A fuse is used, it would work, but if the current becomes too ________, the television would
be damaged as the fuse would not melt.
• If the 5A fuse is used it would work but if the current becomes too high, the __________ melts
protecting the appliance.
Wiring a 3 pin plug
Colour of insulation of the wires :
Earth: _________________________
Neutral : _______________________
Live: _________________________
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Switches and fuses are connected to the ___________________ wire because it has the highest current and
voltage. The earth wire is a safety measure to prevent electric shock. If a live wire becomes disconnected
and touches the metal frame current flows down to Earth through the earth wire rather than through the person
because it finds less ________________. If the earth wire is not fixed properly, the current would pass
through the person. Appliances that are ______________________
, do not need an earth wire, as the
frame of the appliance is made of insulating material, so one cannot get a shock.
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Wires connected properly
Mr. N. Briffa B. Ed. (Hons)
Live touches metal case… but person is still safe.
Electricity – Theme 5 – Part 2 – Electricity in the Home
The __________ wire carries electricity FROM the power grid to
houses, schools, hospitals, factories etc.
The __________ wire returns electricity TO the power grid.
The __________ wire is a safety measure to prevent electric shock.
Power grid
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Mr. N. Briffa B. Ed. (Hons)
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Electricity – Theme 5 – Part 2 – Electricity in the Home
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Electricity – Theme 5 – Part 2 – Electricity in the Home
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Electricity – Theme 5 – Part 2 – Electricity in the Home
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Magnets and motors - Theme 6
1
PROPERTIES OF A MAGNET
•
Attracts only ___________________ materials ( __________ and ___________ )
•
Has magnetic poles: (____________ pole and ____________pole).
•
If suspended its ___________ pole points towards the magnetic North of the _________________.
THE EARTH’S MAGNETIC FIELD
Our planet Earth has its own magnetic field which is
caused by swirling motions of molten iron in Earth's
outer core.
The magnetic North Pole and
geographical North Pole are not on the same place
on Earth. The geographic North Pole is fixed (in the
middle of the Artic Ocean). However, the magnetic
North Pole moves slowly over time due to magnetic
changes in the Earth's core. In 2001, it was in northern Canada, while in 2009 it was moving toward Russia at
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around 50 km per year. At present, they are about 1000 km away from each other.
A plotting compass points toward the ____________________ North Pole.
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The Earth’s magnetic field protects us from the dangerous solar wind, which is a stream of
charged particles ejected from the Sun. and which would be dangerous to life on Earth.
plotting compass
The interaction between the solar
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Earth’s magnetic field protecting us from solar wind.
Mr. N. Briffa B.Ed (Hons.)
wind and the Earth’s magnetic
field sometimes results in a
spectacular natural light display in
the sky called auroras.
Aurora Borealis
Magnets and motors - Theme 6
THE LAW OF MAGNETIC POLES
Like poles (NN,SS) _______________ ,
unlike poles(NS) ___________________.
The closer the poles, the __________________ the magnetic force.
MAKING A MAGNET
By stroking: This method consists of stroking with a magnet a
piece of steel from end to end in the same direction for several
times.
Electrically:
The magnetic material is placed inside a coil of
wire having hundreds of turns of insulated copper wire connected
to a low voltage, high current supply. The polarity of the magnet
depends on the direction of the current and is given by the
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______________________________________. This states that
if the fingers of the right hand, grip the coil in the direction of the
ISOTOPES
current, the thumb will point _______________.
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How can you produce a strong magnet?
USING THE RIGHT HAND GRIP RULE
a)
b)
Mr. N. Briffa B.Ed (Hons.)
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Magnets and motors - Theme 6
3
Use the right hand grip rule to find the poles of the magnet produced.
MAGNETIC PROPERTIES OF IRON AND STEEL
___________ becomes more strongly magnetized than ____________ for the same current and number of
turns. However __________ loses all its magnetism when the current is switched off, while ________ retains
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its magnetism. This means that iron (or soft iron) can be used to make a __________________ magnet while
steel can be used to make a ______________________ magnet.
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Iron is _________________ magnetized and ________________ demagnetized.
Steel is _________________ to magnetize and __________________ to demagnetize.
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Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6
THEORY OF MAGNETISM
A magnet is made up of ‘tiny’ magnets called ______________ lined up with their North poles pointing in
the same direction.
Magnetic
information
In an unmagnetised bar we can imagine the dipoles pointing in all directions.
can
be
stored
for
example
on
__________________________
and
_____________________________.
DEMAGNETISING A PERMANENT MAGNET
Hammering
Hammering a magnet would cause the dipoles to lose their alignment.
Heating
Like hammering, heating a magnet would also result in it losing its magnetism. Heat energy would cause the
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atoms of a magnet to vibrate more rigorously and hence would cause the dipoles to lose their alignment.
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Electrical method using an alternating current (a.c.)
This is the most effective method of demagnetisation.
The magnet is placed inside a solenoid connected to an
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a.c. supply. An alternating current is a current which
changes direction.
The magnet is then slowly withdrawn in the East-West
direction with the alternating current still flowing
through the solenoid.
Mr. N. Briffa B.Ed (Hons.)
4
Magnets and motors - Theme 6
MAGNETIC FIELDS
The magnetic field is the space around a magnet where it produces a magnetic force.
A magnetic field is made up of __________________, ______________________ or _____________.
Examples of magnetic fields
Earth
Bar magnet
C shaped magnet
Unlike poles
Like poles
EXPERIMENT: PLOTTING FIELD LINES
Plotting compass method:
1. A bar magnet is placed on a sheet of paper.
2. The plotting compass is placed as shown.
3. The position of the compass is noted each
time.
4. The magnetic field line is formed by joining
the different positions.
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Draw the magnetic fields around the magnets:
Mr. N. Briffa B.Ed (Hons.)
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Iron filings method:
1. A sheet of paper is placed on top of a bar
magnet.
2. Iron filings are carefully sprinkled using
a pepper pot.
3. The paper is tapped gently.
4. The iron filings form patterns of the
magnetic field.
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Magnets and motors - Theme 6
Induced magnets
The iron nails and the paper clips become induced magnets that
that is temporary magnets as long as they are in contact with the
permanent magnet.
bar magnet.
THE RIGHT HAND GRIP RULE
If the fingers of the right hand grip a:
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1. coil in the direction of the current the thumb points _________________.
2. wire with the thumb in the direction of the current, the direction of the fingers gives the direction
of the ________________________________.
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Mr. N. Briffa B.Ed (Hons.)
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Magnets and motors - Theme 6
7
Question:
(a)
Consider the following materials:
wood, rubber, copper, steel, polystyrene, iron, PVC
(i) Name two materials from the above list that are attracted to a magnet.
___________________________________________________________________________________
(2)
(ii) State one main difference between the magnetic properties of the two materials named in
question (i) above.
___________________________________________________________________________________
(1)
(iii) Which one of them gets more easily magnetized?
___________________________________________________________________________________
(1)
(iv) Is the force acting between two like poles, one of attraction or repulsion?
___________________________________________________________________________________
(1)
(b)
Mary sprinkles some iron filings over a cardboard and places a magnet beneath it.
(i) Draw what she observes on the cardboard on Diagram 3.
cardboard
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magnet beneath cardboard
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(2)
Diagram 3
(ii) She replaces the iron filings with three small compasses. Draw arrows to show the direction of each
compass given in Diagram 4.
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compasses
cardboard
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magnet beneath cardboard
Diagram 4
Mr. N. Briffa B.Ed (Hons.)
(3)
Magnets and motors - Theme 6
ELECTROMAGNETISM
FIELD DUE TO A STRAIGHT WIRE
If a straight vertical wire is passed through a horizontal piece of
cardboard and _______________________ are sprinkled on the card,
they will set in concentric circles when the card is tapped. If a plotting
compass is used, it will also indicate the direction of the field lines (flux)
at different points. (The right hand grip rule indicates that the direction
of the field is clockwise.
Note that
means a wire carrying a current ___________ page.
Note that
means a wire carrying a current ____________ page.
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Magnetic field around wire
carrying current (towards page)
Mr. N. Briffa B.Ed (Hons.)
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Magnetic field around wire
carrying current (out of page)
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Magnets and motors - Theme 6
FIELD DUE TO A COIL (OR SOLENOID)
A solenoid is a long cylindrical coil of wire. It produces
magnetic flux similar to that of a bar magnet. In the
diagram end A behaves like a ____________ pole and
end B behaves like a ____________ pole. (The polarity
can be checked with the right hand grip rule).
The magnetic field can be made stronger by :
a)____________________________________________
b)____________________________________________
c)____________________________________________
_____
ELECTROMAGNETS
An electromagnet is made up of a coil of insulated wire wound
on an __________ core.
An electromagnet is a temporary
magnet when current is flowing and so cannot be made up of
steel as this becomes permanently magnetized.
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An electromagnet can be made stronger by:
a)________________________________________________
b)________________________________________________
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Electromagnets are used e.g. in scrap yards to pick up scrap
iron. An electromagnet cannot pick up copper, silver, gold,
aluminium
etc.
because
they
_____________________________ materials.
Mr. N. Briffa B.Ed (Hons.)
are
not
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Magnets and motors - Theme 6 10
DOOR CHIMES
When the bell push is pressed, the coil becomes
___________________ and _________________
the iron rod inside the coil, making it hit chime plate
_______.
When the bell push is released, the
____________ pulls the iron making it hit chime
plate ______.
http://home.howstuffworks.com/home-improvement/repair/doorbell3.htm
ELECTRIC BELL
When the switch is closed current starts to flow. The
_________________________ becomes magnetized.
This attracts the __________________________,
making the hammer hit the gong. At this point the
contact screw does not make contact and the
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electromagnet is _______________________.
The
springy metal strip pulls back the iron armature
making contact once again.
The circuit is again
complete. As a result, the hammer will keep hitting
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the gong repeatedly as long as the switch is on.
http://freezeray.com/flashFiles/electricBell.htm
Other uses of electromagnets:
magnetic door locks, toasters, scrapyards, transformers, loudspeakers.
Mr. N. Briffa B.Ed (Hons.)
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Magnets and motors - Theme 6 11
RELAYS
A relay is a mechanical switch that works by using an electromagnet. Its main advantage is that direct contact
with a second circuit which can have high current or high voltage is avoided and so it is safer.
Explain how the relay works.
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
Explain how the relay works.
_____________________________________________
_____________________________________________
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_____________________________________________
_____________________________________________
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_____________________________________________
http://freezeray.com/flashFiles/relay.html
Mr. N. Briffa B.Ed (Hons.)
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Magnets and motors - Theme 6 12
Describe an experiment to investigate how the
Describe an experiment to investigate how the strength
strength of an electromagnet depends on the
of an electromagnet depends on the current.
number of turns.
Diagram:
Diagram:
Method:
Method:
_____________________________________
_________________________________________
_____________________________________
_________________________________________
_____________________________________
_________________________________________
_____________________________________
_________________________________________
_____________________________________
_________________________________________
_____________________________________
_________________________________________
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Table:
Table:
Graph:
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Graph:
_____________________________________
_____________________________________
Precaution:
Precaution:
_____________________________________
_____________________________________
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 13
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battery
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Magnets and motors - Theme 6 14
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Magnets and motors - Theme 6 15
Describe an experiment to investigate which of iron or steel is more easily magnetized for the same current.
Diagram:
Method:
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
Table:
Precaution:
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______________________________________________________________________________
Prediction of result:
______________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 16
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Magnets and motors - Theme 6 17
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Magnets and motors - Theme 6 18
FLEMING’S LEFT HAND RULE
This is used to find the direction of the CATAPULT FORCE.
If a wire carrying a current is placed PERPENDICULAR to a magnetic field,
it experiences a ___________________________________.
FIRST FINGER – Field
SECOND FINGER – Current
THUMB – Motion (catapult force)
The FORCE or THRUST increases if :
a) the current is _________________.
b) the strength of the magnet is__________________________.
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Draw and/or state the direction of the CATAPULT FORCE in each case
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Direction
the force
is ____________________
Mr. N.ofBriffa
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Direction of the force is ___________________
Magnets and motors - Theme 6 19
Draw and/or state the direction of the CATAPULT FORCE in each case
1.
2.
N Í S
N
6.
N
S
7.
N
S
S
9.
8.
3.
S
4.
N
S
S
S
5.
N
N
N
S
S
Í
N
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11.
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Magnets and motors - Theme 6 20
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8.
Magnets and motors - Theme 6 21
A wire AB hangs from A. End B dips in a tray filled with mercury.
clip
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d.c. supply
S
B
switch
mercury
a)
When switch S is closed, the wire AB moves. Explain why this happens.
_________________________________________________________________________________
b)
In which direction does the wire move?
_________________________________________________________________________________
c)
Name the rule which was used to determine the direction in which the wire moves.
_________________________________________________________________________________
d)
Suggest two changes which increase the force acting on wire AB.
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e) Suggest two changes which change the direction of the force acting on wire AB.
_________________________________________________________________________________
f)
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The d.c. supply is removed and an a.c. supply is connected instead. State the difference between a.c.
(alternating current) and d.c. (direct current).
_________________________________________________________________________________
_________________________________________________________________________________
g)
What effect is observed on the wire?
_________________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 22
The simple DIRECT CURRENT (d.c.) MOTOR
Fig. 1
Fig. 2
How does the motor work?
When
the
coil
is
horizontal
____________________________.
(Fig.1),
the
sides
_____
and
_____
experience
a
This creates a _________________________ which makes the
coil rotate. When the coil is _________________ (Fig. 2), the brushes are no longer in contact with the
_____________________. However, the coil keeps turning due to inertia, creating continuous rotation.
The coil rotates faster if :
a) __________________________________________________
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b) __________________________________________________
c) __________________________________________________
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The moving coil loudspeaker
As current passes into the coil, it experiences a
catapult force inwards and outwards according to the
direction
of
the
current.
This
creates
_____________________and____________________
of the surrounding air particles (i.e. sound).
Mr. N. Briffa B.Ed (Hons.)
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Magnets and motors - Theme 6 23
Electromagnetic Induction
When the magnet is inserted into the coil, the pointer of the galvanometer gives a _______________. This
happens because while pushing the magnet in, the ___________ cuts the field lines of the _____________
and so there is an ________________________. This is observed by a deflection on the galvanometer. If the
magnet is not moved inside the coil, there is no ______________________ because there is no
___________________________.
However, if the magnet is pulled out of the coil there is an
________________ deflection because there is again _______________________________.
The induced current can be increased if:
a) ________________________________________________
b) ________________________________________________
c) ________________________________________________
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Faraday’s law: The induced current is directly proportional to
the rate at which the magnetic field lines are cut.
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Michael Faraday
1791 - 1867
Law of conservation of energy: Energy can neither be _______________ nor _________________.
What energy change is present during electromagnetic induction ?
___________________
Energy
Mr. N. Briffa B.Ed (Hons.)
___________________
Energy
Magnets and motors - Theme 6 24
Lenz’s law:
The coil _____________ the motion of the magnet.
Heinrich Lenz
1806 - 1865
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When the North pole of a magnet is brought near the
coil as shown in the figure below, a current flows in
the coil in an aNti-clockwise direction, making the end
of the coil become a North pole. Two Norths repel
and so the coil opposes the motion of the
magnet.
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When the North pole is inside the coil and is withdrawn
as shown in the figure below, the current flows in a
clockwiSe direction making the end a South pole. A
North and a South attract and so the coil opposes
the motion of the magnet in the opposite
way.
Mr. N. Briffa B.Ed (Hons.)
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Magnets and motors - Theme 6 25
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Magnets and motors - Theme 6 26
If a metal wire cuts the magnetic field there is an induced current.
The induced current is greater if:
a) the wire is moved _________________.
b) _________________ magnets are used.
c) more _____________ of wire are used.
Question:
The copper swing can be made to oscillate in and out of the page.
a) Describe what you would observe if the swing is:
i) moved outwards. _______________________________________
ii) moved inwards. ________________________________________
iii) made to oscillate in and out. _______________________
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iv) what type of current would be induced in this case.
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v) state two ways to increase the strength of the induced current.
________________________________________________________
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b) If the galvanometer was changed with a constant d.c. supply, state
what
would
be
observed
and
why?
__________________
________________________________________________________
c) If the galvanometer was changed with an a.c. supply, state what
would
be
observed
and
why?
_______________________
________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
________________________________________________________
Magnets and motors - Theme 6 27
Question:
The diagram below shows a permanent magnet.
(a) Name a material it can be made of. ____________________________________________________
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(b) Draw the magnetic field pattern of the magnet indicating the direction of the field lines.
coil
magnet
When the magnet is pushed into the coil, the reading of the ammeter deflects. Explain.
(c) Suggest two ways of making the deflection of the needle larger.
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_________________________________________________________________________________
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diaphragm
coil
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magnet
(
(d) The diagram above shows the inside of a microphone. When a person talks into the microphone, a current
flows through the coil. Explain.
________________________________________________________________________________________
________________________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 28
Generators
A generator generates current by electromagnetic induction.
(i.e. by cutting field lines and generating an induced current).
The dynamo (simple direct current (d.c,) generator)
The dynamo is connected to e.g. a ____________, and as
you turn the coil, cutting the ______________ of the
magnet, you generate a _____________________ which
makes the bulb light.
The strength of the current generated depends on:
a)________________________________________
b)________________________________________
c)________________________________________
The dynamo has exactly the same construction of a d.c.
motor.
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Dynamo – you turn the coil, you get ________________.
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Motor - you give current, you get _________________.
This means that a motor can be used as a __________________ and a dynamo can be used as a _________.
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The alternator (simple alternating current (a.c,) generator)
The altenator is connected to e.g. a ____________, and as you turn the coil cutting the ______________ of
the magnet, you generate an _____________________ which makes the bulb light.
We use alternating current in our homes. At the power stations ___________ is burnt to produce steam which
turns the turbines of the alternators producing electricity (a.c.).
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 29
Question:
a) What do you observe when the switch is closed? Explain.
________________________________________________
________________________________________________
________________________________________________
________________________________________________
b) What do you observe when the switch is opened? Explain.
_____________________________________________________________________________________
_____________________________________________________________________________________
c) What do you observe if the switch is opened and closed continuously? What type of current is being
produced in the secondary circuit ?
_____________________________________________________________________________________
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_____________________________________________________________________________________
_____________________________________________________________________________________
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d) What can be done to get a current in the secondary coil without staying switching on and off the primary
circuit?
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____________________________________________________________________________________
____________________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 30
TRANSFORMERS
A transformer is used to ________________ or ____________________an alternating voltage.
A transformer works only with a.c. (not constant d.c) because only
a.c. produces a
____________________________________________.
A step down transformer is used in a _____________________.
A step up transformer is used in a ___________________________.
VOLTAGE of
VOLTAGE of
coil
coil
V1
=
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Number of turns in ____________coil
Number of turns in
coil
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N1
=
V2
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N2
How does a transformer work?
The changing magnetic field of the _______________ coil is continuously cut the _________________
coil, inducing a current and e.m.f. in it. The voltage changes according to the ______________of turns.
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 31
Efficiency of Transformer
Not all energy or power in the primary circuit is transferred to the secondary circuit.
So a transformer which is 80% efficient has ______% energy losses.
EFFICIENCY
=
POWER OUTPUT
POWER INPUT
X
100%
AN IDEAL TRANSFORMER IS _________% EFFICIENT
AND HAS NO ______________________.
SO FOR A TRANFORMER WHICH IS 100% EFFICIENT
POWER (PRIMARY) = POWER (SECONDARY)
V1I1
=
V2I2
Example 1:
The primary coil of a transformer has 20 turns, a voltage of 6 V and a current of 1 A. If the
number of turns in the secondary coil is 1500 and the transformer is 100% efficient, find the :
a i) voltage in the secondary coil.
____________________________________________________________________________________
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____________________________________________________________________________________
ii) power in the primary coil.
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____________________________________________________________________________________
iii) power in the secondary coil. Explain.
____________________________________________________________________________________
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iv) current in the secondary coil.
____________________________________________________________________________________
b) What type of transformer is this? Comment about what happens to voltage and current.
_____________________________________________________________________________________
_____________________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 32
Example 2:
primary coil
secondary coil
6V
12 W
240 V (a.c.)
S
core
a) What do you observe when switch S is closed?
___________________________________________________________________________________
b) What is the current in the secondary coil, given that the lamp is at its normal brightness?
____________________________________________________________________________________
____________________________________________________________________________________
c) Calculate the resistance of the secondary coil.
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____________________________________________________________________________________
____________________________________________________________________________________
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d) If the primary coil has 2000 turns, calculate the number of turns in the secondary.
____________________________________________________________________________________
____________________________________________________________________________________
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e) Calculate the current in the primary coil of the transformer, assuming it to be 100% efficient.
____________________________________________________________________________________
____________________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Example 3:
Magnets and motors - Theme 6 33
Complete the table below assuming the transformer is 90% efficient.
Voltage (V)
Primary coil
Secondary coil
240
12
a)
200
No. of turns
b)
c)
Power (W)
d)
0.5
Current (A)
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
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_____________________________________________________________________________________
_____________________________________________________________________________________
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_____________________________________________________________________________________
_____________________________________________________________________________________
Turns
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Step down transformer
(100% efficient)
Step up transformer
(100% efficient)
Step down transformer
(70% efficient)
Step up transformer
(70% efficient)
Mr. N. Briffa B.Ed (Hons.)
Voltage
Current
Power
Magnets and motors - Theme 6 34
ENERGY LOSSES IN A TRANSFORMER
a) Resistance of winding wires:
The coil wire is usually long and thin. It has some resistance. Heat losses result. This can be decreased
by using a material with __________ resistance (e.g. copper).
b) Leakage of field lines:
Not all the field lines of the primary coil are cut by the secondary. This can be decreased by winding the
___________________ coil on top of the ____________________.
c) Eddy currents:
The soft iron core also ends up cutting the field lines. Circular electric currents called eddy currents are
induced in it. Heat losses result. These can be decreased by using a ___________________ core.
Example 4: The number of turns of a primary coil of a transformer is 50 while that in the secondary coil
is 1000. If the voltage in the primary coil is 2 V and the current is 0.1 A, find the:
a) i) voltage in the secondary coil.
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_____________________________________________________________________________________
_____________________________________________________________________________________
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ii) power in the primary coil.
_____________________________________________________________________________________
iii) efficiency of the transformer if the power of the secondary coil is 0.15 W.
_____________________________________________________________________________________
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_____________________________________________________________________________________
iv) current in the secondary coil.
_____________________________________________________________________________________
b) What type of transformer is this? Comment about what happens to voltage and current.
_____________________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Magnets and motors - Theme 6 35
Transmission of Electrical Power
Power stations produce alternating current because it can be easily and cheaply decreased and increased in
values using ____________________. This is why in our homes we have alternating current; at a voltage
of about 240 V and a frequency of 50 Hz. Electricity is produced in the power stations at about 25 kV and
stepped up at once with a ___________ transformer to 400 kV to be sent over long distances in a high
voltage – low current format to decrease _____________ losses. It is then reduced at sub-stations by
using ______________ transformers for local use.
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Mr. N. Briffa B.Ed (Hons.)
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Radioactivity – Theme 7 – Radiation and its uses
1
THE STRUCTURE OF THE ATOM
Marie Curie
(1867-1934)
•
All matter is made up of very small particles called ____________.
•
The diameter of an atom is about ________.
•
The centre of the atom is called the __________________.
•
The nucleus contains ______________ and ________________.
•
Since both the protons and the neutrons are in the nucleus they are
also known as NUCLEONS.
•
Moving on the outer surface of an atom, there are very small particles called _________________.
A proton has a _________________ charge.
An electron has a ________________ charge.
A neutron is neutral and has _____________________.
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In an atom the NUMBER OF ELECTRONS = NUMBER OF PROTONS.
Therefore the atom is overall uncharged. It is NEUTRAL.
However, an atom may become charged when it loses or gains electrons.
A charged atom is called an _________.
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the
The
electrons
can
protons
and
This atom has a lack of electrons.
move.
It lost ____ electrons and so it is
neutrons are ‘imprisoned’ in
a ______________ ion.
the __________________.
The mass of a proton and
neutron are almost the same
This atom has an excess of
but
electrons. It gained ____ electron
__________ times lighter.
and so it is a _____________ ion.
Mr. N. Briffa B.Ed (Hons.)
Only
an electron is about
Radioactivity – Theme 7 – Radiation and its uses
2
Nucleon number (mass number):
This is the number of ____________ and ______________ (each known as nucleon) in the nucleus.
Proton number (atomic number):
This is the number of ________________ in the nucleus.
Example1:
An atom of chlorine is represented as shown
Example2:
An atom of uranium is represented as shown
This atom of uranium has:
This atom of chlorine has:
___________ protons
___________ neutrons.
___________ protons
___________ neutrons.
____________ electrons
___________ nucleons.
____________ electrons __________ nucleons.
ISOTOPES
35
17
Cl and 1737 Cl are two isotopes of chlorine
They each have ____ protons and therefore _____ electrons.
neutrons and the second has ______neutrons.
Definition of isotopes:
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However, the first isotope has ______
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Isotopes are atoms of the same element having the same _________________ number but a different
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__________________ number.
Example: The following symbols represent six nuclei.
Which nuclei are isotopes of each other? ____________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Radioactivity – Theme 7 – Radiation and its uses
3
The nature of Radioactivity
Some elements in nature are not stable and spontaneously emit invisible radioactive radiation
which can be harmful to human beings. This radiation may consist of:
a) ________________ radiation
b) ________________ radiation
c) ________________ radiation
Examples of radioactive elements are uranium, plutonium, thorium, protactinium, radon and polonium.
Each of these radioactive elements may emit a single type of radioactive radiation (e.g. alpha) or a
combination of them (e.g. alpha and beta). Radioactive elements may be solids, liquids or gases.
Consists of:
Symbol:
Charge:
Range in air:
Stopped by:
Ionising
power:
(knocking off
electrons)
Alpha
particle
Beta
particle
Gamma
radiation
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Only gamma radiation is electromagnetic and travels at the speed of light (3x 108 m/s).
Mr. N. Briffa B.Ed (Hons.)
Radioactivity – Theme 7 – Radiation and its uses
4
RADIOACTIVE DECAY
A radioactive element decays when it changes into another radioactive element to get rid of excess
energy and become more stable. For example the radioactive element uranium decays to form
thorium. When the element decays (or disintegrates), its nucleus breaks up, emitting an _____________
particle or a ____________ particle and may be some _____________ radiation.
Detecting radioactive radiation
Radioactive radiation i.e. (α, β, and γ) radiation is invisible. However it can be detected by using:
a)
_______________________________________.
b)
_______________________________________.
The Geiger-Müller tube (G–M tube)
and the Rate meter
The G-M tube is connected to a ratemeter so that the
radiation that enters the tube is recorded by the
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ratemeter as a __________________ in counts per minute or counts per second.
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Using Photographic film
film badge
(closed)
When radioactive radiation falls on photographic film
it causes it to darken. Workers in the nuclear industry
wear ________________________________ , (small
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squares of film in plastic cases with various “windows”).
film badge
(open)
When they open it, the workers know whether or not they
were exposed to radiation.
Mr. N. Briffa B.Ed (Hons.)
different materials
(e.g. paper, aluminium, lead)
Radioactivity – Theme 7 – Radiation and its uses
5
Example: To show how a film badge works, a student decides to build a model as shown.
film
light proof
material
3 mm
aluminium
paper
Few cm of
lead
If the film badge is exposed to the radiation (first column), under which parts would the film darken?
Very thin light
Sheet of paper
3 mm Aluminium Few cm Lead
proof material
α only
β only
α and β
β and γ
γ only
Background radiation
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If a G-M tube is connected to a ratemeter and switched on, it will register a count rate of about 15
counts/min, even though there is no radioactive source close by. This is due to background radiation.
What is background radiation?
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Background radiation is the radiation constantly present in the natural environment of the Earth. It is
caused by ___________, _____________, ________________________________________________.
The background count rate is the count rate given by the ratemeter because of the background radiation.
(e.g. 15 counts/min).
Mr. N. Briffa B.Ed (Hons.)
Radioactivity – Theme 7 – Radiation and its uses
6
What is a corrected count rate?
This is the count rate of the source ONLY without background count rate.
Example: A ratemeter gives a count rate of 540 counts/min and the background count rate is 20
counts/min. Find the corrected count rate. _________________________________________________.
Penetrating power of α, β and γ
A _________________________ is enough to stop α radiation. β radiation passes through paper, but
is stopped by a ______________________________. A __________________________ must be used
to stop γ radiation .
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Describe an experiment to test what radiation is
emitted by an unknown radioactive source.
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The background count rate is first noted without
the source. A sheet of paper is then placed in
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front of the source. If the count rate falls, then
____________ radiation is present. A 3 mm sheet
of aluminium is then used instead, and if the
count rate falls further then ______________
radiation is also present. If the count rate falls to
background when the 5 cm lead block is used then ___________ radiation is present as well.
Mr. N. Briffa B.Ed (Hons.)
Radioactivity – Theme 7 – Radiation and its uses
7
Describe an experiment including a diagram of the apparatus, to show that a radioactive source
emits only α and γ radiation.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
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____________________________________________________________________________________
____________________________________________________________________________________
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____________________________________________________________________________________
Find the radiation emitted in each case by 3 different radioactive sources.
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Air
Paper
3mm
Aluminium
5cm
Lead
Count rate source 1
200
202
23
20
Count rate source 2
510
21
20
21
Count rate source 3
425
424
220
26
Mr. N. Briffa B.Ed (Hons.)
Radiation emitted
Radioactivity – Theme 7 – Radiation and its uses
8
The decay process by which a radioactive element changes into another is a continuous process. For
some elements this decay process is very fast, while for others it is very slow.
Half-lives vary greatly.
For example, the half life of radium is 1620 years. This means that every 1620 years the number of
radium atoms decrease by half. So in this case the decay process is very slow. On the other hand, radon
has a half life of only 4 days.
Every 4 days, the number of atoms of radon decrease by half.
Radioactive decay is not affected by _____________________, __________________, or
____________________________. This means that you cannot slow or speed up the decay process.
Definition of half life:
HALF-LIFE is the _______________ for half the atoms to decay.
Example !:
A radioactive sample has a mass of 16 g and a half-life of 10 days. What mass of the original
sample remains after a) 10 days b) 20 days c) 40 days ?
_______________________________________________________________________________
_______________________________________________________________________________
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_______________________________________________________________________________
_______________________________________________________________________________
Example2:
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A radioactive source has a count rate of 200 counts/min. If its half-life is 5 minutes, what is the
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count rate after 15 minutes?
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Radioactivity – Theme 7 – Radiation and its uses
9
Example 3:
A radioactive sample has a mass of 600 g. If after 1 hour, the mass remaining is 75 g, what is the
half–life of the source?
_______________________________________________________________________________
_______________________________________________________________________________
Half life questions with fractions or percentages
After 1 half life the fraction which remains is ( ½ ) or _______ %
After 2 half lives the fraction which remains is (½ x ½) = _________ or _______ %
After 3 half lives the fraction which remains is (½ x ½ x ½) = _________ or _______ %
After 4 half lives the fraction which remains is (½ x ½ x ½ x ½) = ________ or _______ %
Example 4:
If the half-life of a radioactive gas is 2 minutes, then after 8 minutes, the activity will have fallen to
a fraction of its initial value. What is this fraction?
_______________________________________________________________________________
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_______________________________________________________________________________
Example 5:
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A radioactive sample gives a count rate of 15 counts/min after 10 minutes.
minutes:
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If the half-life is 2
a) what was the initial countrate?
b) what percentage of the original sample remains after i) 6minutes ii) 8 minutes.
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
Mr. N. Briffa B.Ed (Hons.)
Radioactivity – Theme 7 – Radiation and its uses 10
Experiment: Finding the half-life of a radioactive source
(note the source should have a half-life of a few minutes)
Method: The background count rate is first noted without the
source. The source is placed in front of the G-M tube and the
count rate on the ratemeter is noted every minute.
Table of results:
Time (min)
Corrected count rate(counts/min)
Graph: A graph of ___________________________________ against ____________________.
Conclusion: The half-life will be the time for the count rate to decrease by ____________.
Example:
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In an experiment to find the half-life of a radioactive source, the following graph was obtained.
Corrected count rate (counts/min)
From the graph find the half life of the source. _________________________________.
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time (minutes)
Mr. N. Briffa B.Ed (Hons.)
Radioactivity – Theme 7 – Radiation and its uses 11
USES of Radioactivity
•
Measuring and controlling thickness of materials
If the radiation reaching the detector changes, the detector
makes the rollers move further apart or closer.
•
Archeology (carbon 14 dating)
Dating of archeological findings. Since carbon 14 has a
half life of about 5700 years, it is only accurate when
dating remains which are thousands of years old.
•
Treatment of cancer
Gamma radiation is used to kill cancerous cells.
•
Tracers in industrial work or medicine.
An underground leakage could be traced.
Detect whether the thyroid gland is working properly.
•
Sterlise medical equipment.
Gamma radiation is used to sterilize medical equipment
for operations.
•
Smoke alarms.
When smoke enters an alarm unit, it stops radiation
causing the alarm to go off.
•
Irradiation of food.
Food can be safely exposed to radiation to make it last longer.
.
Safety
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Radioactive radiation can be dangerous as it might cause radiation burns, or delayed effects like
leukaemia or cancer.
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Safety in the laboratory
Mr. N. Briffa B.Ed (Hons.)
Safety in industry
Radioactivity – Theme 7 – Radiation and its uses 12
Question:
In many countries it is against the law to throw radioactive waste in rivers or the sea.
a)
Name the 3 types of radiation which can be emitted by radioactive substances.
_______________________________________________________________________________
b)
Radioactive waste with a long half-life is stored in galleries about 200m below the ground.
Explain why this is a safe method of storage.
_______________________________________________________________________________
_______________________________________________________________________________
c)
Radioactive materials are very useful. One use is to detect leaks in underground water pipes.
A radioactive substance is introduced into the water pipes. Explain how workmen can find the
exact position of the leak.
_______________________________________________________________________________
______________________________________________________________________________
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_______________________________________________________________________________
______________________________________________________________________________
d)
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What type of radiation should the radioactive substance give off? Why?
_______________________________________________________________________________
_______________________________________________________________________________
e)
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Why is it important for the radioactive substance used to have a short half-life?
_______________________________________________________________________________
_______________________________________________________________________________
(10 marks)
Mr. N. Briffa B.Ed (Hons.)
Radioactivity – Theme 7 – Radiation and its uses 13
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Mr. N. Briffa B.Ed (Hons.)
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Radioactivity – Theme 7 – Radiation and its uses 14
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Mr. N. Briffa B.Ed (Hons.)
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