Download P3 Revision Notes - Glan Afan School

P3 Notes
What you should know: Making electricity: Electricity is made to move in a wire
if there is movement between it and magnetic field lines. This can be because the
wire itself moves OR because a magnetic field moves through the wire:
Many separate coils of wire set at angles to each other
A radial magnetic field – so that coils are always spinning around and cutting
magnetic field lines – so making electricity
Output from an ac generator:
To increase the amount of electricity made:
 Use more coils of wire
 Move the coils of wire faster
 Use a stronger magnetic field.
*Reversing the direction of movement of the wire reverses the direction that
the electricity flows.
An ac generator consists of a coil of wire moving in a magnetic field. The direction of
the flow of the electricity made is given by Flemmings Right hand rule:
Point the first finger of your right hand out, put your thumb upwards at 900 to your
first finger and lastly put your middle finger at 900 to both of these.
In a commercial generator ( one used in power stations) – they will use:
At A: The spinning coil
of wire is moving
upwards at 900 to the
direction of the
magnetic field. This is
cutting magnetic field
lines at the fastest
rate – so producing
maximum voltage and
current.
At B and D: the wire is moving parallel to the magnetic field, so no lines are being
cut. This means no voltage or current is being made.
At D: The spinning coil of wire is moving downwards at 900 to the direction of the
magnetic field. This is cutting magnetic field lines in the opposite direction at the
fastest rate – so producing maximum negative voltage and current.
Output Voltage:
One spin of coil:
Either doubling coil
or
2x strength magnet
Spinning at
twice speed
(2x voltage; 2
frequency)
Radial magnetic field:
The magnetic field
lines radiate out
from centre North
Pole to Outer South
pole.
Np
Ns
= Vp
Vs
Np = number of coils on primary
Ns = number of coils on secondary
Vp= primary voltage
Vs = secondary voltage
This equation holds for 100% efficient transformers ( transformers
that lose no energy as heat or sound).
At 100% efficiency: Power in = Power out
Vp x Ip = Vs x Is
where I is the current flowing
Transformers: These are used to change the size of an AC voltage
WAVES
There are two types of waves:
1. Transverse, where the disturbance is at 900 to the direction in
which they travel.
Iron core
1.
2.
3.
4.
5.
The ac voltage in the primary circuit produces a magnetic field in the iron
core.
This produces a moving magnetic field in the iron core, which is transferred
to the secondary coil.
As the secondary coils are ‘cut’ by the moving magnetic field, a voltage is
induced in the secondary coil.
The size of the secondary voltage is determined by the number of primary
and secondary coils .
Step up transformers have more coils on the secondary than on the primary.
Step down transformers have less coils on the secondary than the primary.
On HIGHER tier: You have to calculate values using the equation:
Examples of transverse waves: water waves, electromagnetic waves
including light, secondary earthquake waves (S waves)
2. Longitudinal waves, where the disturbance is parallel to the
direction of motion.
Wave front diagrams:
As water waves and light waves are transverse waves, their direction of
motion is at 900 to their wavefronts.
Direction of motion.
You have to either complete, or interpret, wave front diagrams for
reflection and refraction
Reflection
Refraction
Examples of longitudinal waves:sound, ultrasound, Primary earthquake
waves (P waves)
Refraction of waves:
This occurs because the speed of the waves change as they travel
from one material to another eg fast in air, slow in transparent material.
Ultrasonic waves:
These are longitudinal waves of frequencies above human hearing i.e.
frequencies above 20 000Hz. They need particles to travel though, just
like sound – going fastest through solids, next fastest through liquids and
, with difficulty, slowly through air.
If used for foetal scanning or finding faults in solid materials, then a
layer of gel has to be used on the surface in order to transmit the waves.
At each density change some of the ultrasound is reflected to a
receiver. This reflected wave is transferred to a display eg computer
screen, so an image of the foetus is seen.
Faults: The ultrasound enters the block, some of the wave is reflected
(R1), the rest travels into the block. Some reflects off the top of the
crack (R2) the rest travels forward. When the ultrasound hits the
bottom the crack reflection R3 occurs, and the final refection comes
from the bottom of the block (R4). This is seen on the screen.
If the speed of the wave is known in the metal, and the time of reflection
is calculated from the display, then the equation;
Speed = distance / time
Can be used to find the depth of the crack on the metal block.
Eg If ultrasound travels at 600m/s in the metal and R2 is received after
0.0002 seconds.
Distance travelled by ultrasound = speed x time
= 600 x 0.0002
= 0.12m
This is NOT the depth of the crack – it is the time for the ultrasound to
travel to the crack and back!!!
So the depth of the crack is 0.12/2 = 0.06m
Cleaning: ultrasonic waves through a liquid vibrate dirt particles trapped
in delicate jewellery etc until it is dislodged, so leaving the dirt in the
liquid and the jewellery clean.
Wave equation: (Higher tier only)
You will have to use: wave speed = frequency x wavelength
In the context of ultrasound.
Example: ultrasound of frequency 25 000 Hz travels at 750m/s. What is
the wavelength of the wave?
Speed = frequency x wavelength
750 = 25 000 x wavelength
750 / 25000 = wavelength
0.03 m = wavelength
Seismic waves:
When an earthquake occurs some of its energy travel as waves around the
world. There are three waves you need to know about:
P waves: these are longitudinal, travel very fast and can travel through
solids and liquids
S waves: these are transverse, travel more slowly and can only travel
through solids.
Surface waves: cause of damage and destruction.
The path of P waves through
the mantle and core,
together with the fact that S
Waves only travel through
the mantle – suggests the
outer core as being liquid.
Refraction of waves is shown
in drawing.
The time lag between a P wave and an S wave arriving at a seismology
station indicates the distance between the station and the earthquake.
b) Velocity time graphs
Speed
C
A
O
If only the P wave arrives then the earthquake was on the opposite side
of the earth and unable to travel through the liquid of the outer core.
Motion
Speed is the distance travelled each second in ANY DIRECTION.
Velocity is also the distance travelled each second IN A GIVEN
DIRECTION. (In collision questions velocity is a positive value in one
direction and a negative value in the opposite direction)
Graphs of motion:
a) Distance time graphs
Distance
C
A
O
B
Time
The graph shows:
O-A steady fast speed
A-B stopped
B-C slower steady speed
**the greater the gradient of
the line the faster the speed
(Gradient means steepness of
line).
OR Mean Speed = distance
time
B
D
Time
The distance travelled
Is equal to the area
under the graph.
O-A shows slow rate of acceleration
(Because the gradient is small)
A-B shows a steady speed
B-C shows a fast rate of
Acceleration (steeper line)
C-D shows a deceleration
** the gradient of the line gives the
acceleration OR
Acceleration = change in speed
Time taken
Equations of motion:
These can be used for any object moving at a steady rate of
acceleration. If it’s not accelerating just use equation for mean
speed!
Symbol
What it represents
U
Starting speed (m/s)
V
Final speed (m/s)
A
Acceleration (m/s2)
T
Time (Seconds)
x
Distance travelled (m)
Foundation paper: you will need to be able to use
V= u + at
x= u+v
T 2
Also remember that a mean speed can be calculated as any
other mean – add two speeds together and divide by 2!!
Higher paper: you will also be required to use
x= ut +1/2 at2
V2 = u2 + 2 ax
Momentum
This is another way to measure motion. It is useful when explaining
collisions between objects.
The bigger the mass of object and the faster it is moving, the
more momentum it has. You will have to be able to use the
equation:
Momentum = mass x velocity
(kgm/s)
(kg)
(m/s)
Because it is velocity in the equation, momentum also has direction.
So it is positive in one direction and negative in the opposite
direction.
When something moving is brought to a stop, it has to have its
momentum taken away from it.
The more momentum it has, the bigger the force needs to be OR
its momentum could be taken away by a smaller force acting for a
longer length of time.
The latter is achieved by crumple zones in vehicles OR by air bags
for passengers.
HIGHER Tier only: Force, energy loss in collisions, circular
motion
In any collision or explosion, momentum is conserved, This means:
Momentum before = momentum after
Collision
collision
Explosions: momentum before = 0 kgm/s as nothing is moving.
After the explosion fragments move in all different directions,
with equal sized momentums (which then all added together total
zero).
This means large masses have to have small velocities in order to
have the same momentum as lighter masses with high velocities .
This explains why smaller particles travel further after an
explosion.
In explosions, momentum is conserved ( same before and after),
but energy can be lost (or changed into other non-useful forms).
You have to use:
Kinetic energy = 1/2mv2
To calculate the energy before the collision and then again
calculate it after collision. Subtract values to find the change or
loss in Kinetic energy.
To calculate the force of impact in a collision, or the time taken
for a collision, you will be expected to use
Force x time = change in momentum during collision
(N)
(s)
Kgm/s
A force is needed to make an object change direction. If that
force stays at right angles to the direction of motion of the
object, the object will move in a circle.
Direction of
Motion
Force force acts toward centre
of circle
Higher tier: Sling shots
This led to the atomic structure we now accept.
If the speed of the probe is u m/s, and the speed of the planet is
v m/s
The speed of the probe leaving the planet =
Original speed + 2xspeed of planet
U + 2v
Foundation tier& higher:
Atomic Structure:
The original model of what was in the atom was put forward by
Thomson. A solid sphere with electrons embedded in it.
This became known as the Plum Pudding model
of the atom.
Rutherford tested this model in his alpha scattering experiment:
This atom has: 3 electrons (negative charges)
3 proton (positives – atomic number)
4 neutrons (zero charge)
Symbol: 7 Li
Atomic mass = 7
3
Proton/Atomic Number = 3
(Atomic mass = 4 neutrons + 3 protons)
Isotopes: these are atoms of the same element that have the
same number of protons but different number of neutrons
Eg 12 C
14 C
6
6
Both have 6 protons
The first has 12 particles in the nucleus
The second has 14 particles in the nucleus – due to 2 more
neutrons.
These are isotopes of carbon.
Higher tier: atomic spectra
When energy is added to an
atom, the electrons in the
outer shell become ‘excited’ –
this extra energy allows them
to move into s higher energy
electron shell.
When they return to their
original energy shell, the atom
emits the extra energy as light.
As every atom has a different
configuration of electrons,
each atom emits its own
specific spectrum of light.
Foundation & higher:
The chemical symbols below need to be known;
Alpha:
4α
Beta: 0β
2
-1
Electron: 0 e
Helium: 4 He
-1
2
Higher tier: Balanced decay equations; using radioactive decay to age the
earth/solar system.
You will be asked to complete equations like the one here:
226 Ra
..... Rn + 4 α
88
.....
2
Top line: 226 particles in Radium = ? + 4 removed by alpha
So Radon has 226-4 = 222 articles in its nucleus
Bottom line: Radium has 88 protons, 2 of which are removed by the
alpha emission, so radon has 88-2=86 protons.
So the answer is:
226 Ra
.222. Rn + 4 α
88
...86
2
Nuclear fission:
235 U + 1 n
144 Ba + ....... Kr +1 n +1 n +energy
92
0
56
......
0
0
Top line: 235 +1 = 144 + ? +1+1 so ? = 90
Bottom line: 92 +0 = 56 +? =0: so ? = 36
235 U + 1 n
144 Ba + .90.... Kr +1 n +1 n +energy
92
0
56
..36.
0
0
Nuclear fusion:
2H + 3 H
4 He + 1 n + energy
1
1
2
0
These are isotopes of hydrogen fusing together at high temperatures.
Age of earth: Some radioactive elements that have been present in the
earth since it was formed, have very long halflives.
For example Uranium 235 has a half life of 700 000 000years (7x108)
By examination of how much of the original material remains, and using its
half life, we can work out how many half lives it has undergone and so find
the age of the earth. (Just like carbon dating in P2)
Foundation and Higher:
Nuclear fission:
When the uranium or plutonium atoms split and release energy, they leave
behind other radioactive elements, some of which have a long half life.
Making disposal a problem.
Nuclear fusion:
This diagram shows a chain reaction of neutrons splitting uranium atoms
releasing huge quantities of energy.
If the process is controlled you have a nuclear reactor.
An uncontrolled chain reaction leads to a bomb.
As we have so much hydrogen on earth – most of which is combined with
oxygen in water. We have a potential source of energy.
Unfortunately we have to raise its temperature so high that we, at
present, put more energy into fusing hydrogen than we get out of the
reaction.
The Sun however has the high temperatures needed.
Reaction:
H+ 3H→ 4He+ 1n
2
1
1
2
0