Zusammenfassung der Ressource
Physics Unit 2
- FORCES AND THEIR EFFECTS
- Resultant Forces
- Whenever two objects interact,
the forces they exert on each
other are opposite and equal.
- A number of forces acting at a point may
be replaced by a single force that has the
same effect on the motion as the original
forces all acting together. This singe force
is called the resultant force.
- A resultant force acting
on an object may cause
a change in its state of
rest or motion
- If the resultant force
acting on a stationary
object is: >zero, the object
will remain stationary.
>not zero, the object will
accelerate in the direction
of the resultant force.
- If the resultant force
acting on a moving object
is: >zero, the object will
continue to move at the
same speed in the same
direction. >not zero, the
object will accelerate in
the direction in the
resultant force.
- Forces and Motion
- Newton's Second Law:
F = m x a
- The gradient of a
distance-time graph
represents speed.
- The velocity of an object
is its speed in a given
direction.
- a = (v - u) / t
- The gradient of a velocity-time
graph represents acceleration.
- Forces and Braking
- When a vehicle travels
at a steady speed the
resistive forces balance
the driving force.
- The greater the speed of a
vehicle the greater the braking
distance needed to stop it in a
certain distance.
- Stopping distance = thinking distance +
braking distance
- When the brakes of a vehicle are applied, work done
by the friction force between the brakes and the
wheel reduces the kinetic energy of the vehicle and
the temperature of the brakes increases.
- Forces and Terminal
Velocity
- An object falling through a fluid will initially
accelerate due to the force of gravity. Eventually
the resultant force will be zero and the object will
move at its terminal velocity.
- W = m x g
- (parachutist
example)
- Forces and Elasticity
- A force acting on an object
may cause a change in the
shape of an object.
- A force applied to an elastic
object such as a spring will
result in the object stretching
and storing elastic potential
energy.
- F = k x e
- THE KINETIC ENERGY OF OBJECTS
ACCELERATING OR DECELERATING
- Forces and Energy
- When a force causes an
object to move through
a distance, work is
done.
- W = F x d
- Energy is transferred when
work is done.
- P = E / t
- Kinetic Energy = 1/2 x m x v^2
- Force is proportional to extension.
- Momentum
- p = m x v
- In a closed system, the total
momentum before an event is
equal to the total momentum
after the event. This is called
conservation of momentum.
- CURRENTS IN
ELECTRICAL CIRCUITS
- Static Electricity
- When certain
insulating materials
are rubbed together
they become
electrically charged.
Negatively charged
electrons are rubbed
off one material and
onto the other.
- The material that
gains electrons
becomes positively
charged. The
material that loses
electrons is left
with an equal
positive charge.
- When two
electrically
charged objects
are brought
together, they
exert a force on
each other.
- Two objects
carrying the
same type of
charge repel.
Two objects
carrying
different types
of charges
attract.
- Electrical
charges can
move easily
through some
substances,
e.g. metals.
- Electric Circuits
- Current:
I = Q / t
- Potential
Difference:
V = W / Q
- Current-potential
difference graphs are
used to show how the
current through a
component varies with
the potential difference
across it.
- The resistance of a
component can be
found by measuring
the current through
and potential
difference across a
component.
- V = I x R
- The current
through a
component
depends on its
resistance. The
greater the
resistance the
smaller the
current for a
given potential
difference across
the component.
- The potential
difference
provided by
cells
connected in
series is the
sum of the
potential
difference of
each cell.
- The resistance
of a filament
bulb increases
as the
temperature
of the filament
increases.
- The current
through a
diode flows
in one
direction
only.
- The
resistance
of a LDR
decreases
as light
intensity
increases.
- The
resistance of
a thermistor
decreases as
the
temperature
increases.
- MAINS ELECTRICITY
- Household Electricity
- Cells and batteries supply
current that always passes in
the same direction - direct
current
- Alternating current always
changes direction.
- Mains electricity is an a.c. supply. In
the UK it has a frequency of 50Hz
and is about 230v.
- Most electrical appliances are connected
to the mains using a cable and a three-pin
plug.
- If an electrical fault
causes too great a
current, the circuit is
disconnected by a fuse or
circuit breaker in the live
wire.
- When the current in a
fuse wire exceeds the
rating of the fuse it
will melt, breaking
the circuit.
- Some circuits are protected
by RCCBs (Residual Current
Circuit Breakers)
- Appliances with metal
cases are usually earthed.
- The earth wire and fuse together
protect the wiring of the circuit.
- Current, Charge and Power
- When an electrical charge
flows through a resistor, the
resistor gets hot.
- P = E / t
- P = I x V
- E = V x Q
- RADIOACTIVE SUBSTANCES
- Atoms and Radiation
- Some substances give out radiation
from the nuclei of their atoms all the
time, whatever is done to them. These
substances are said to be radioactive.
- Ionising Radiation -
when a charged
particle comes near
another atom, it can
pull electrons off the
atom. This will leave
behind ions.
- Alpha particle -
helium nuclei, 2
neutrons and 2
protons, travels
cm in air,
stopped by paper
- Beta Particle - high
speed electron, given
out by nucleus, neutron
changes into proton and
electron, same number
of particles in nucleus,
mass number doesn't
change, travels m in air,
stopped by aluminium.
- Gamma - unstable nucleus --> new nucleus +
gamma radiation, high energy wave, travels lots
of m in air, stopped by thick lead and concrete
- Gamma has NO IONISING POWER but is still
the most powerful form of radiation.
- Alpha and beta radiations are deflected by both
electric and magnetic fields but gamma radiation
is not.
- (uses and dangers)
- The half-life of a radioactive isotope is the average time it
takes for the number of nuclei of the isotope in a sample
to halve, or the time it takes for the count rate from a
sample containing the isotope to fall to half its initial level.
- Alpha Decay - 2 protons and 2 neutrons are
lost from the mass of an atom in alpha decay.
- Beta Decay - a neutron changes into a proton and an
electron so the atomic number increases by one.
- NUCLEAR FISSION AND FUSION
- Nuclear Fission
- The splitting of an
atomic nucleus.
- There are 2 fissionable
substances in common use in
nuclear reactors - Uranium-235
and Plutonium-239.
- For fission to occur, the
uranium-235 or plutonium-239
nucleus must first absorb a neutron.
- The nucleus undergoing fission splits
into two smaller nuclei and two or
three neutrons and energy is released.
- The neutrons may go on to start a chain
reaction.
- Nuclear bombs:
>not controlled so
the nuclear
fission is much
more dangerous.
>more neutrons
are absorbed by
nuclei so more
nuclear fission
means more
energy is emitted.
- Nuclear Power
Stations:
>Boron control
rods absorb
neutrons and
so slows chain
reactions down
as less
neutrons are
being absorbed
by the nuclei.
>less energy
emitted = less
dangerous.
- Nuclear Fusion
- The joining of two
atomic nuclei to form
a larger one.
- Nuclear fusion is the
process by which
energy is released in
stars.
- Stars form when enough dust and gas from
space is pulled together by gravitational
attraction. Smaller masses may also form
and be attracted by a larger mass to become
planets.
- During the 'main sequence' period of its life
cycle a star is stable because the forces within
it are balanced.
- A star goes through a life cycle. This life cycle is
determined by the size of the star.