Download Physics 2

23/05/2017
Physics 2
Distance, Speed and Time
Speed = distance (in metres)
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D
time (in seconds)
1) Seb walks 200 metres in 40 seconds.
What is his speed?
S
T
50 m/s
2) Lucy covers 2km in 1,000 seconds. What is her speed?
2km = 2000 m  2 m/s
3) How long would it take Freddie to run 100 metres if he runs
at 10m/s?
10 m/s
4) Sue travels at 50m/s for 20s. How far does he go? 1000 m
5) Hannah drives her car at 85mph (about 40m/s). How long
does it take her to drive 20km?
500 s
Distance-time graphs
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4) Diagonal line
downwards =
Steady speed in
other direction
2) Horizontal line =
40
Stationary
30
Distance
(metres)
20
10
0
Time/s
20
1) Diagonal line =
Steady speed
40
60
80
100
3) Steeper diagonal line =
Faster steady speed
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40
Distance
(metres)
30
20
10
0
Time/s
20
40
60
80
1) What is the speed during the first 20 seconds?
100
0.5 m/s
2) How far is the object from the start after 60 seconds?40 m
3) What is the speed during the last 40 seconds?
4) When was the object travelling the fastest?
1 m/s
40 – 60 s
Speed vs. Velocity
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Speed is simply how fast you are travelling…
This car is travelling at a
speed of 20m/s
Velocity is “speed in a given direction”…
This car is travelling at a
velocity of 20m/s east
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Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
A
T
1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What
is her acceleration?
2 m/s2
2) A ball is dropped and accelerates downwards at a rate of
10m/s2 for 12 seconds. How much will the ball’s velocity
increase by?
120 m/s
3) A car accelerates from 10 to 20m/s with an acceleration of
2m/s2. How long did this take?
5s
4) A rocket accelerates from 1,000m/s to 5,000m/s in 2
2000 m/s2
seconds. What is its acceleration?
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Velocity-time graphs
1) Upwards line =
80
acceleration
Velocity
m/s
4) Downward line =
deceleration
60
40
20
0
10
2) Horizontal line =
Steady speed
20
30
40
50
3) Upwards line =
slower acceleration
T/s
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80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
1) How fast was the object going after 10 seconds?
2) What is the acceleration from 20 to 30 seconds?
3) What was the deceleration from 30 to 50s?
4) How far did the object travel altogether?
50
40 m/s
10 m/s2
1 m/s2
6700 m
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Balanced and unbalanced forces
Consider a camel standing on a road.
What forces are acting on it?
Reaction
These two forces would be equal –
we say that they are BALANCED.
The camel doesn’t move anywhere.
Weight
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Balanced and unbalanced forces
Reaction
What would happen if we took the
road away?
Weight
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Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Weight
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Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Air Resistance
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Air resistance is a force that opposes motion through air. The
quicker you travel, the bigger the air resistance:
The same applies to a body falling through a liquid (called
“drag” or “upthrust”).
Balanced and unbalanced forces
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Forward
movement
Acceleration
Constant
speed or
stopped
Backwards
movement
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Balanced and unbalanced forces
1) This animal is either
stopped or moving
________
speed
with steady
_____ _____…
2) This animal is getting
faster
_________…
3) This animal is getting
slower
_______….
4) This animal is… dead
Resultant Force
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Calculate the resultant force of the following:
500N
100N
100N to the
right
400N to the
right
200N up
700N
700N
200N
700N
600N
50N
50N up
800N
800N
100N
Force and acceleration
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If the forces acting on an object
are unbalanced then the object will
accelerate, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
M
A
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Force, mass and acceleration
1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
2 m/s2
F
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2. How
2000kg
heavy is the car?
3) A car accelerates at a rate of
5m/s2. If it weighs 500kg how
much driving force is the engine
applying?
2500N
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How
0.5m/s2
much does it accelerate by?
M
A
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
resistance is _______
small so he
accelerates downwards.
_____________
2) As his speed increases his air
resistance will increase
_______
3) Eventually the air resistance will be
big enough to _______
balance the
skydiver’s weight. At this point
the forces are balanced so his
constant - this is
speed becomes ________
called TERMINAL VELOCITY
Words – increase, small,
constant, balance, accelerates
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Terminal Velocity
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenlyincreases
________,
causing him to start _____
slowing____.
down
5) Because he is slowing down his air
resistance will decrease
_______ again
weight
until it balances his _________.
The skydiver has now reached a
terminal velocity
new, lower ________
_______.
Words – slowing down, decrease,
increases, terminal velocity, weight
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Velocity-time graph for terminal velocity…
Parachute opens –
diver slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
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Weight vs. Mass
Earth’s Gravitational Field Strength is 10N/kg. In other
words, a 1kg mass is pulled downwards by a force of 10N.
W
Weight = Mass x Gravitational Field Strength
(in N)
(in kg)
(in N/kg)
M
1) What is the weight on Earth of a book with mass 2kg?
2) What is the weight on Earth of an apple with mass 100g?
3) Dave weighs 700N. What is his mass?
70kg
g
20N
1N
4) On the moon the gravitational field strength is 1.6N/kg. What will
Dave weigh if he stands on the moon?
112N
Stopping a car…
Tiredness
Thinking
distance
Too many
drugs
(reaction time)
Too much
alcohol
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
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Braking
distance
Driving too
fast
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Work done
When any object is moved around work will need to be
done on it to get it to move (obviously).
We can work out the amount of work done in moving an
object using the formula:
Work done = Force x distance moved
in J
in N
W
in m
F
D
Example questions
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1. Bori pushes a book 5m along the table with a force of 5N.
He gets tired and decides to call it a day. How much work
did he do?
25J
2. Alicia lifts a laptop 2m into the air with a force of 10N.
How much work does she do?
20J
3. Martin does 200J of work by pushing a wheelbarrow with a
force of 50N. How far did he push it?
4m
4. Chris cuddles his cat and lifts it 3m in the air. If he did
75J of work how much force did he use?
25N
5. Carl drives his car 1000m. If the engine was producing a
driving force of 2000N how much work did the car do?
200000J
Elastic Potential Energy
Elastic potential energy is the
energy stored in a system
when work is done to change
its shape, e.g: a spring, a
bungee cord
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Kinetic energy
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Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found using
the formula:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
KE =
½
in m/s
mv2
Example questions
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1) Nicole drives her car at a speed of 30m/s. If the
combined mass of her and the car is 1000kg what is her
kinetic energy?
450000J
2) Shanie rides her bike at a speed of 10m/s. If the
combined mass of Shanie and her bike is 80kg what is her
kinetic energy?
4000J
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Momentum
Any object that has both mass and
velocity has MOMENTUM. Momentum
(symbol “p”) is simply given by the formula:
P
Momentum = Mass x Velocity
(in kg m/s)
(in kg)
(in m/s)
M
V
What is the momentum of the following?
1) A 1kg football travelling at 10m/s
0.1 kg m/s
2) A 1000kg Ford Capri travelling at 30m/s
33.3 m/s
3) A 20g pen being thrown across the room at 5m/s
4) A 70kg bungi-jumper falling at 40m/s
0.1 m/s
2800 m/s
Conservation of Momentum
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In any collision or explosion momentum is conserved (provided that there
are no external forces have an effect). Example question:
Two cars are racing around the M25. Car A collides with the back of car B
and the cars stick together. What speed do they move at after the
collision?
Speed = 50m/s
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Mass = 1800kg
Speed = ??m/s
Momentum before = momentum after…
…so 1000 x 50 + 800 x 20 = 1800 x V…
…V = 36.7m/s
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Momentum in different directions
What happens if the bodies are moving in opposite directions?
Speed = 50m/s
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Momentum is a VECTOR quantity, so the momentum of the
second car is negative…
Total momentum = 1000 x 50 – 800 x 20 = 34000 kg m/s
Speed after collision = 34000 kg m/s / 1800 = 18.9m/s
Random questions…
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1) Sophie tries to run 100m in 12 seconds and succeeds. How fast did she
run?
8.3 m/s
2) Tommy accelerates at a rate of 2m/s2 for 3 seconds. If he started at
10m/s what was his final speed?
16 m/s
3) Charlie decides to lift his book up into the air. His book has a mass of
100g and he lifts it 50cm. Calculate the work done. 1g = 0.01N 0.5J
4) Lewis accelerates from 0 to 10m/s in 5 seconds. If his mass is 70kg
how much force did his legs apply?
140N
5) Rachel rides 1km at a speed of 20m/s. How long did the journey take?
50s
6) Claire thinks it’s funny to push James with a force of 120N. If James
has a mass of 60kg calculate his acceleration.
2 m/s2
7) Lauren brakes do 20,000J of work. If the combined mass is 100kg
what speed was she travelling at?
8) Tom has a mass of 75kg. If he accelerates from 10 to 20m/s in 2s how
much force did he apply?
375N
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9) Georgina amuses herself by throwing things at Sarah. If she throws a
ball with a speed of 20m/s and the distance between her and Sarah is
5m how long will it take to reach her?
0.25s
10) Mr Richards throws calculators around the room with a force of 20N.
If each calculator has a mass of 200g calculate the acceleration.
100 m/s2
11) Sam has a mass of 70kg. What is his weight on Earth, where the
gravitational field strength is 10N/kg?
700N
12) Zak pushes a box around with a force of 1N. He does 5J of work and
decides to call it a day. How far did he push it?
5m
13) On the moon Matt might weigh 112N. If the gravitational field
strength on the moon is 1.6N/kg what is his mass? What will he weigh
on Earth?
70kg
700N
14) Dan likes bird watching. He sees a bird fly 100m in 20s. How fast was
it flying?
5 m/s
15) How much kinetic energy would Richard have if he travelled at a speed
of 5m/s and has a mass of 70kg?
875J
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Static Electricity
Static electricity is when charge “builds up” on an object and
then stays “static”. How the charge builds up depends on what
materials are used:
-
+
-
+
-
+
+
-
-
+
-
+
-
+
-
+
+
-
-
+
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Static Electricity
+
+
-
-
+
-
-
-
-
-
-
-
Van de Graaf generators
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Uses of Static – Smoke Precipitators
Chimney
Negatively
charged
plates
Positively
charged
grid
-
-
-
-
-
+
+
+
Circuit Symbols
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Variable
resistor
Diode
Switch
Bulb
A
V
Ammeter
Voltmeter
LDR
Resistor
Cell
Fuse
Thermistor
Battery
Electric Current
Electric current is a flow
of negatively charged
particles (i.e. electrons).
+
-
e-
Note that
electrons go
from negative
to positive
By definition, current is “the
rate of flow of charge”
e-
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Basic ideas…
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Electric current is when electrons start to flow around a
ammeter to measure it and it is
circuit. We use an _________
measured in ____.
amps
Potential difference (also called _______)
voltage is
how big the push on the electrons is. We use a
voltmeter to measure it and it is measured in
________
______,
volts a unit named after Volta.
Resistance is anything that resists an electric current. It is
volts
measured in _____.
Words: volts, amps, ohms, voltage, ammeter, voltmeter
More basic ideas…
If a battery is
added the current
increase
will ________
because there is a
push on
greater _____
the electrons
If a bulb is added
the current will
decrease
_______ because
there is greater
resistance in the
________
circuit
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Current in a series circuit
If the current
here is 2
amps…
The current
here will
be… 2A
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The
current
here will
2A
be…
And the
current
here will
be… 2A
In other words, the current in a series
circuit is THE SAME at any point
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Current in a parallel circuit
A PARALLEL circuit is one where the current has a “choice
of routes”
Here comes the current…
Half of the current
will go down here
(assuming the bulbs
are the same)…
And the rest will
go down here…
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Current in a parallel circuit
If the
current
here is 6
amps
And the
current here
will be…
6A
The current
here will be…
2A
The current
here will be…
2A
The current
here will be…
2A
Some example questions…
3A
6A
1A
1A
1A
3A
1.5A
1.5A
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Voltage in a series circuit
If the voltage
across the
battery is 6V…
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V
…and these
bulbs are all
identical…
…what will the
voltage across
each bulb be?
V
V
2V
Voltage in a series circuit
If the voltage
across the
battery is 6V…
…what will the
voltage across
two bulbs be?
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V
V
4V
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Voltage in a parallel circuit
If the voltage across
the batteries is 4V…
What is the
voltage here?
4V
V
And here?
V
4V
Summary
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In a SERIES circuit:
Current is THE SAME at any point
Voltage SPLITS UP over each component
In a PARALLEL circuit:
Current SPLITS UP down each “strand”
Voltage is THE SAME across each”strand”
An example question:
6V
3A
A1
V1
6V
A3
1.5A
1.5A
A2
V2
3V
V3
3V
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£A
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Resistance
Resistance is anything that will
RESIST a current. It is measured
in Ohms, a unit named after me.
Georg Simon Ohm
1789-1854
The resistance of a component can be
calculated using Ohm’s Law:
Resistance
(in )
=
V
Voltage (in V)
Current (in A)
I
R
An example question:
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Ammeter
reads 2A
A
V
Voltmeter
reads 10V
1) What is the resistance across
this bulb?
5Ω
2) Assuming all the bulbs are the
same what is the total resistance
in this circuit? 15Ω
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More examples…
3A
6V
12V
4Ω
2Ω
3A
2A
4V
2Ω
2V
1A
What is the
resistance of
these bulbs?
Resistance
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Resistance is anything that opposes an electric current.
Resistance (Ohms, ) =
Potential Difference (volts, V)
Current (amps, A)
What is the resistance of the following:
1) A bulb with a voltage of 3V and a current of 1A.
3Ω
2) A resistor with a voltage of 12V and a current of 3A 4Ω
3) A diode with a voltage of 240V and a current of 40A 6Ω
4) A thermistor with a current of 0.5A and a voltage of
10V
20Ω
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Resistors, bulbs and diodes
Current-voltage graphs
I
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I
I
V
V
V
1. Resistor
Current
increases in
proportion
to voltage
2. Bulb
As voltage increases
the bulb gets hotter
and resistance
increases
3. Diode
A diode only lets
current go in one
direction – it has
very high
resistance in the
other direction
LDRs and Thermistors
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Two simple components:
1) Light dependant
resistor – resistance
DECREASES when light
intensity INCREASES
Resistance
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2) Thermistor –
resistance DECREASES
when temperature
INCREASES
Resistance
Amount of light
Temperature
Wiring a plug
1. Earth
wire
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4. Live
wire
5. Fuse
2. Neutral
wire
3. Insulation
The neutral wire of a plug stays at
a potential close to zero relative
to the Earth
6. Cable
grip
The live wire of a plug alternates
between positive and negative
potential relative to the Earth
DC and AC
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V
DC stands for “Direct
Current” – the current only
flows in one direction:
Time
1/50th s
AC stands for “Alternating
Current” – the current
changes direction 50 times
every second (frequency =
50Hz)
230V
T
V
Fuses
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safety devices. If
Fuses are _______
there is a fault in an appliance
which causes the ____
live and neutral
(or earth) wire to cross then a
large current will flow through
______
fuse and cause it to _____.
melt
the _____
circuit and
This will break the _______
protect the appliance and user
harm
from further _____.
Words – large, harm, safety, melt, live, circuit, fuse
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Power and fuses
Power is “the rate of doing work”.
The amount of power being used in
an electrical circuit is given by:
Power = voltage x current
in W
in V
in A
P
V
I
Using this equation we can work out the fuse rating for any
appliance. For example, a 3000W fire plugged into a 240V
12.5
13
supply would need a current of _______
A, so a _______
amp fuse would be used (fuse values are usually 3, 5 or
13A).
Power and fuses
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Copy and complete the following table:
Appliance
Power rating
(W)
Voltage (V)
Current
needed (A)
Fuse needed
(3, 5 or 13A)
Toaster
960
240
4
5
Fire
2000
240
8.3
13
Hairdryer
300
240
1.25
3
Hoover
1000
240
4.2
5
Computer
100
240
Stereo
80
240
0.42
3
0.33
3
Energy and Power
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The POWER RATING of an appliance is simply how much
energy it uses every second.
In other words, 1 Watt = 1 Joule per second
E
E = Energy (in joules)
P = Power (in watts)
T = Time (in seconds)
P
T
Earth wires
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metal
Earth wires are always used if an appliance has a _____
fault in the appliance, causing the live
case. If there is a _____
touch the case, the current “_______”
surges
wire to ______
down the
earth wire and the ______
fuse blows.
Words – fuse, fault, metal, surges, touch
Structure of the atom
A hundred years ago people thought
that the atom looked like a “plum
pudding” – a sphere of positive
charge with negatively charged
electrons spread through it…
Ernest Rutherford, British scientist:
I did an experiment (with my colleagues
Geiger and Marsden) that proved this
idea was wrong. I called it the
“Scattering Experiment”
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The Rutherford Scattering Experiment
Alpha
particles
(positive
charge, part
of helium
atom)
Most particles passed
through, 1/8000 were
deflected by more than
900
Conclusion – atom is made up of a small, positively
charged nucleus surrounded by electrons orbiting
in a “cloud”.
Thin gold
foil
The structure of the atom
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ELECTRON –
negative, mass
nearly nothing
NEUTRON –
neutral, same
mass as
proton (“1”)
PROTON –
positive, same
mass as
neutron (“1”)
The structure of the atom
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Particle
Proton
Relative Mass
1
Relative Charge
+1
Neutron
Electron
1
0
0
-1
MASS NUMBER = number of
protons + number of neutrons
SYMBOL
PROTON NUMBER = number of
protons (obviously)
Isotopes
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An isotope is an atom with a different number of neutrons:
Notice that the mass number is different. How many
neutrons does each isotope have?
Each isotope has 8 protons – if it didn’t then it just
wouldn’t be oxygen any more.
A “radioisotope” is simply an isotope that is radioactive –
e.g. carbon 14, which is used in carbon dating.
Background Radiation
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13% are
man-made
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power
Types of radiation
Unstable
nucleus
New
nucleus
Alpha
particle
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1) Alpha () – an atom decays into a new
atom and emits an alpha particle (2
neutrons – the nucleus
protons and 2 _________
helium atom)
of a ______
2) Beta () – an atom decays into a new
atom by changing a neutron into a
proton and electron. The fast moving,
_______
_____
Beta high energy electron is called a beta
particle particle.
Unstable
nucleus
New
nucleus
Unstable
nucleus
New
nucleus
3) Gamma – after  or  decay surplus
______
energy is sometimes emitted. This is
called gamma radiation and has a very
frequency with short
high ___________
wavelength. The atom is not changed.
Gamma
radiation
Words – frequency, proton,
energy, neutrons, helium, beta
Nuclear fission
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More
neutrons
Neutron
Uranium or
plutonium
nucleus
Unstable
nucleus
New nuclei
(e.g. barium
and krypton)
Chain reactions
Each fission reaction releases
neutrons that are used in
further reactions.
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Nuclear Fusion in stars
Proton
Neutron
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