Transcript P3 Further Physics - The Thomas Cowley High School

P3 Further Physics
1 Turning Forces
2 Light and Sound
Moments/stability
Circular motion
Reflection
Refraction
Sounds
3 Electromagnetism
4 Stars and Space
Motors
Generators
Transformers
Galaxies
Stars
Fusion
1.1 Turning forces - Moments
• The moment of a force about a pivot is equal to Force x
perpendicular distance from the pivot.
Perpendicular
distance
Fulcrum = pivot
E.g. A force of 50N is exerted on a claw hammer of length 0.3m. What is the
moment of the force?
Moment = Force x distance = 50N x 0.3m = 15Nm
(Units are multiplied together like algebra e.g. N x cm is written as Ncm.)
1.2 Centre of mass
• C.o.m. is the point where all an object’s mass may be thought to be.
• When an object is free to spin the centre of mass is directly
beneath the pivot point.
• The centre of mass of a symmetrical object lies along the axis of
symmetry.
Pivot point
Centre of mass
Pendulum to show
the vertical line
1.3 Moments in balance - Higher
• When an object is balanced:
Anticlockwise moments = clockwise moments
Anticlockwise moments = Clockwise moments
W1 x d1 = W2 x d2
E.g.
W x 0.4m = 2N x 0.3m
W=0.6Nm / 0.4m
W = 1.5N
1.4 Stability - Higher
• Stability is increased by;
– Making the base as wide as possible,
– Making the centre of mass as low as possible.
• An object will tend to topple over if the line of action of its
weight is outside of its base.
Tractor has a wide base
Engine at the bottom
top deck made of
aluminium.
High Chair has a
wide base.
1.5 Circular motion
• For an object moving in a circle at constant speed;
– The object accelerates constantly towards the centre
of the circle,
– The centripetal force increases;
• As the mass or the speed increases,
• As the radius of the circle decreases.
1.6 Gravitational attraction
• The force of gravity between two objects
– Is an attractive force,
– Increases with the mass of each object,
– Decreases the greater the distance between the objects.
Earth has a bigger mass and diameter
compared with Mars. Although we are
further from the centre the extra mass
makes the gravity stronger.
Distance between
centres
1.7 Planetary orbits
• To stay in orbit at a particular distance a small body (planet) must
move at a particular speed around a larger body (star).
• The larger an orbit is the longer the orbiting body takes to go
round the orbit.
Too fast
Just right!
Newton’s cannon. A cannon ball will follow a
curved path. The faster it is fired the less
curved the path becomes. At the right speed
the curve matches the curve of the planet.
Mercury’s orbit is 88days
long. Pluto’s is about 243
years!.
1.8 Satellites
• A geostationary satellite orbits once in 24hours in the same position
above the Earth’s equator.
• Geostationary orbits are usually used for communication satellites.
• Monitoring satellites use low polar orbits.
Plot of satellites
orbiting the Earth.
Polar and geostationary
orbits are crowded.
2.1 Light and Sound - Reflection
• The normal at a point on a mirror is perpendicular
to the mirror.
• The law of reflection is:
angle of incidence = angle of reflection
i
r
Normal
i = angle of incidence
r = angle of reflection
2.2 Curved mirrors
• The principal focus of a concave mirror is the point where
parallel rays are focused to by the mirror.
• A concave mirror forms:
– A real image if the object is beyond the principal focus of the mirror.
– A virtual image if the object is between the mirror and the principle
focus.
• A convex mirror always forms a virtual image of an object.
real
image
object
virtual
image
virtual
image
2.3 Refraction – bending waves
• Light changes direction when it crosses the boundary
between two substances.
– If the speed reduces refraction is towards the normal.
– If the speed increases refraction is away.
Fast
Fast
Slower
boundary
2.4 Lenses – curved glass
Convex virtual image
• A real image is formed by a converging
(convex) lens if the object is further away
than the principal focus.
• A virtual image is formed by a diverging
(concave) lens and by a convex lens if the
object is nearer than the principle focus.
Convex real image
Diverging (concave) lens = virtual image
Rays pass through image
2.5 Using Lenses
• A camera contains a
converging (convex) lens
that is used to form a real
image of an object.
• A magnifying glass is a
converging lens that is
used to form a virtual
image of an object.
Virtual image
Real image
2.6 Sound
• Sound waves;
– Travel through solids,
liquids and gases,
– Cannot travel through a
vacuum,
– Are longitudinal waves,
– Can be reflected and
refracted.
Reflection
Longitudinal wave
2.7 Musical sounds
• The loudness of a note depends on the amplitude of the
sound waves.
• The pitch of a note depends on the frequency of the sound
waves.
2.8 Ultrasound
• Ultrasonic waves are:
– Sound waves above 18 000 Hz
– Partly reflected at boundaries between substances
– Non-ionising.
• Uses of ultrasonic waves include cleaning devices, flaw detectors and
medical scanners.
Edges of the material and
cracks are shown as spikes
on the screen.
Electromagnetism – The motor effect
• A current carrying wire in a magnetic field experiences a force.
• The force:
– Increases with more current and a stronger field.
– Is at right angles to the magnetic field and current.
– Is reversed if the current or field is reversed.
Motor turns as the current
passes in opposite
directions around the coil.
Split ring commutator makes
sure current always passes the
N and S in the same direction
each revolution.
Electromagnetic induction - generators
• When a wire cuts through magnetic field lines a
potential difference is induced in the wire.
• If the wire is part of a complete circuit a current
will flow.
• The current is increased by moving the wire faster
or using stronger magnets.
Alternating current
generators use slip
rings to transfer the
current from the coil
Maximum voltage when the coil is
horizontal.
No voltage when coil is vertical.
Field
(N to S)
Movement of
wire
Current
(+ to -)
Transformers
• Transformers contain a primary (input) coil and a secondary
(output) coil wrapped around an iron core.
• Transformers only work constantly with an alternating current
(changing current)
More primary coils = step
down
More secondary coils = step
up.
‘Soft’ iron laminated core gives a very
strong magnetic connection between
the coils and reduces wasteful currents
in the core.
Practical
transformer with
both coils
wrapped around
each other.
a.c. Gives a moving magnetic field inducing a voltage in the secondary coil.
The National Grid
• Transformers are used to step up or step down the voltage.
p.d. Across primary, Vp
p.d across secondary, Vs
=
number of turns on primary, Np
number of turns on secondary, Ns
Higher
E.g. A transformer is used to step a p.d. of 230V down to 10V. The secondary coil has 60
turns. How many turns are there on the secondary coil?
Vp = 230V, Vs = 10V, Ns = 60, Np = ?
Np = Vp/Vs x Ns = 230/10 x 60 = 1380 turns
Transformer efficiency
Electrical power = p.d (voltage) x current
At 100% efficiency
Primary power = Secondary power
Primary p.d. x primary current = secondary p.d. X secondary current
VpIp = VsIs
Higher
Stars and Space - Galaxies
• Galaxies are groups of billions of stars attracted to each other by
gravity.
• Gravity causes matter to become attracted together to form stars and
galaxies.
Quarks and electrons form from radiation
Big Bang
13 billion
years ago
Neutrons and protons form
Hydrogen and Helium atoms form
0.1s
100s
100 000 years
A few billion years
First galaxies and stars form
Organised Universe
The life history of a star
• Stars radiate energy from nuclear fusion reactions in their cores.
• Lighter elements fuse to form heavier ones.
• The life of a star depends on its mass.
Protostar
Low mass star e.g. Yellow
dwarf - The Sun
Main stage
Red Giant
High mass star
e.g. Blue
Supergiant
Rigel
White Dwarf
Super Red
Giant
Supernova
Black hole
Very massive star
Neutron star
Black Dwarf
How the chemical elements formed (Higher)
• Light elements up to iron are formed by fusion in stars.
• The heavier elements are formed during the collapse and
supernova of a massive star.
Main sequence
Hydrogen fuses to helium and other light elements.
Red Giant
Helium and other small nuclei fuse into larger nuclei.
Iron is the largest that can be made.
Collapse
Enormous forces form larger nuclei. Lots of energy
released.
Supernova
Heaviest elements formed. Explosion scatters debris
through space. Planets and new stars are formed from
this.