Transcript Physics Unit 2 Revision Notes

PHYSICS UNIT 2
Distance-Time Graphs
• The line is straight - this means that the object is moving at a constant speed (40
seconds to go every 1000m)
• If the car had gone faster then the line would have been steeper. In other words
the gradient would have been greater (would have gone further than a 1000m
every 40 seconds)
• The gradient of a line on a Distance-Time graph represents speed
• Y and Z went at a constant speed because the lines are
straight
• X went the fastest because its gradient is greatest
• Part of Z’s line is straight – this means the object is stationary
(the car stopped)
Calculating Speed
• SPEED = Distance / Time
• Distance in metres
• Time in seconds
• Speed is m/s (metres per second)
• Distance – 1000m
• Time – 40 seconds
• Speed = 1000/40 = 25m/s
Calculating Speed
• Speed of Y = 30,000 / 1250 = 24m/s
• For Z you can only work out the average speed
• Speed of Z = 20,000 / 1500 = 13.33 m/s
What is Velocity?
• An object moving at a constant speed in a straight line
has a constant velocity
• An object moving in a circle at a constant speed does
not have a constant velocity
Velocity is speed in a given direction
• Cars travelling at the same speed in different
directions have different velocities
What is Acceleration?
• Acceleration is change of velocity per second
• Unit is metres per second squared
Velocity-Time Graphs
The greater the gradient – the greater the acceleration
Calculating change in velocity
Change of velocity = Final velocity (v) – initial velocity
(u)
Change of velocity = v - u
Calculating Velocity
Acceleration (A) = Change in velocity
Time taken for the change (T)
Acceleration (a) = v-u
t
The area under the line on a velocity-time graph
represents distance travelled
What is a force?
A force can change the shape of an object. It can
change its state of rest. It can change its motion
The unit is Newton (N)
Resultant Force
We can work out the effect of the forces acting on an
object by replacing them with a single force, the
resultant force
This is a single force that has the same effect as all the
forces acting on an object
Zero Resultant Force
• If an object remains stationary then the resultant
force is zero
• If the object continues to move at the same velocity
then the resultant force is zero
• In this case: If only two forces act on an object, they
must be equal to each other and act in opposite
directions
Calculating Resultant Force
• Resultant force is their difference if the forces act in
opposite directions
• Resultant force is the sum of the two forces if they act
in the same direction
Calculating Resultant Force
Resultant Force (F) = Mass (M) x Acceleration (A)
Resultant Force in Newtons
Mass in Kilograms
Acceleration in metres/second squared
The greater the mass of an object, the smaller
its acceleration is for a given force
Breaking Force
Breaking force to stop a vehicle = Mass * Acceleration
Acceleration when the breaks are first applied
Stopping Distances
Shortest distance a vehicle can safely stop
Stopping distance = Thinking Distance + Braking
distance
What increases thinking distance?
• Poor reaction time (Tired / Alcohol / Drugs)
• High speed
What increases braking distance?
• High speed
• Poor weather conditions
• Poor maintenance
We can solve this using speed cameras
• Fall in speed by 17%
• 55% reduction in deaths and serious injury
How to prevent skidding when breaking
• Anti skid surfaces
• Rougher
• This increases friction
• Reduces skidding when breaking hard
• Used at junctions
What is Weight?
• Weight of an object is the force of gravity on it
• Mass of an object is the quantity of matter in it
Calculating Weight
• Weight = Mass * Gravitational Field Strength
• Weight in Newtons (N)
• Mass in Kg
• Gravitational Field Strength in N/Kg
Where will you weigh less?
• The gravitational field strength is weaker at the
equator
• If you want to reduce your weight, go to the equator
Releasing an object
• When you release an object it falls because of its
weight
• Weight is the resultant force
Releasing an object into liquid
• Drag force acts in the opposite direction to the weight
of the object
• Resultant Force = Weight – Drag Force
• As it falls the acceleration will decrease
• It will reach a constant velocity when the Drag Force =
Weight. The resultant force will be zero
• This velocity is its TERMINAL VELOCITY
What is an Elastic Object?
• It is an object that regains its original shape
• For example rubber
Extension
• Increase in length of an elastic object is known as
extension
• Extension = New length – Original length
Hooke’s Law
• Extension of a spring is directly proportional to the
force applied
• As long as the limit of proportionality is not exceeded
• It obeys Hooke’s law
Energy and Work
• Work is done on an object when a force makes the object move
• Energy transferred = work done
• Work done (joules) = force (newtons) x distance moved in the direction of
the force (metres)
Gravitational Potential Energy
• This increases when the object goes up and decreases when it goes down
• Change of gravitational potential energy = mass * gravitational field
strength* change in height
Kinetic Energy and Momentum
• Kinetic energy (J) = ½ * Mass (Kg)* Speed (m/s)²
• Momentum (Kg m/s) = Mass * Velocity
• In a closed system, the total momentum before an event is equal to the
total momentum after an event
Inside an atom
• In the nucleus there are protons and neutrons
• Electrons move around the nucleus
• PROTONS – Positive charge
• ELECTRONS – Negative charge
• NEUTRONS – Uncharged
Equal number of electrons and protons means that the object
is UNCHARGED
Objects become charged if electrons or added or lost
STATIC ELECTRICITY
• Adding electrons makes it negative – more electrons than
protons
• Removing electrons makes it positive – more protons than
electrons
Electric Current
• This is a flow of electrons around a circuit (flow of charge)
• Move from negative end of battery (repelled from it) to positive end
of battery
• They transfer energy from the battery to the bulb
• Battery is two or more cells connected together
• In a circuit every component has an agreed symbol
• A circuit diagram shows how they are linked together
Calculating Electric Current
• Electric current is the flow of charge (flow of electrons) per second
• More electrons = More current
• Current = Charge flow / Time
• I = Q/T
• I is current in amperes (A)
• Q is charge in coulombs (c)
• T is time in seconds (s)
Ammeter
• Ammeter measures current
• In this case it measures the current going through the bulb (flow of
electrons which carry the energy)
Voltmeter
• Voltmeter measures the potential difference across the torch bulb
• This is the amount of energy transferred to the bulb by each coulomb
of charge
• It is called voltage
• The unit is volts (V)
Calculating Voltage
• Voltage = Work done /charge
• V = W/Q
• V is Voltage in volts (V)
• W is work done in Joules (J)
• Q is charge in Coulombs (C)
What is Resistance?
• Electrical components RESIST the passage of electrons
• Resistance = Voltage /Current
• R = V/I
• R is Resistance in Ohms
• V is Voltage in Volts
• I is current in amperes
OHMS Law
• The current through a resistor at constant temperature is directly
proportional to the potential difference across the resistor
• Double the voltage – double the current
• This means that the resistance is constant
• Changing the current does not change the resistance
• If you change the temperature – you change the resistance
• For example the resistance of a metal filament in a bulb increases
because the temperature causes the electrons to move faster and
thus they collide more frequently
Series:
Parallel:
Current
Same current passes through
components in series with each other
Current splits. Thus total current is sum
of the currents
Potential
Difference
This is shared between the components.
Thus you add the potential differences
to get the total potential difference
This is the same for each component in
parallel
Cells
Add the potential difference from each
of the cells together to get the total
potential difference
Resistance
Add resistances to get total resistance
The bigger the resistance the smaller the
current
• The battery in a torch makes the current go round the circuit in one
direction only. This is called DIRECT CURRENT (dc)
• At home you use ALTERNATING CURRENT (ac) which means the current
repeatedly reverses direction. It flows one way and then the opposite way.
• Its FREQUENCY is the number of cycles it passes through each second
• An OSCILLOSCOPE measures this
• Frequency = 1 / Time taken for one cycle
• Frequency = 1/0.02 = 50Hz
• The frequency of the a.c. mains electricity in the UK is 50Hz
Cables
• Every mains circuit has a LIVE wire and a NEUTRAL wire
• The live wire alternates between + and – volts
• The neutral wire is always at 0V
Safety
• Live wire touches the metal case
• Big current will flow from the live into the metal case
• This will give you an electric shock
• The metal case is attached to the Earth wire
• The earth wire takes the current away and blows the fuse.
• The fuse in the plug must be in series with the appliance. The fuse
melts.
• Alternatively there is a circuit breaker which pulls a switch open
(RCCB) This works by detecting the difference between the current in
the live and neutral wire
Plugs
• Remember the colours
• Cable grip and case and
made of plastic because
they are good insulators
• Wall sockets are
connected in parallel
1. POWER = Energy transferred / Time – UNIT IS WATTS (W)
2. POWER SUPPLIED = Current * Potential Difference – UNIT IS VOLTS (V)
3. CORRECT RATING FOR A FUSE = Electrical power / Potential Difference
4. CHARGE = Current * Time – UNIT IS COULOMBS (C)
5. ENERGY TRANSFERRED = Potential difference * Charge – UNIT IS JOULES
• Atoms have a nucleus that are unstable.
• They become stable by emitting particles
Radioactivity
Radioactivity is
a random
process
Alpha
2 protons and 2
neutrons stuck
together
Positively charged
Beta
Fast moving electron
Negatively charged
Gamma
Electromagnetic
radiation
Uncharged
Nuclear Reactions
• Atomic number: Number of protons in a nucleus
• Mass number: Number of protons and neutrons
• Number of neutrons = Mass number – Atomic number
• Alpha emission reduces the atomic number by 2 and the mass number by 4
• Beta emission adds 1 to the atomic number and does not change the mass
number
Background radiation
• Radon gas
• Rocks and buildings
• Cosmic rays
• Food and drink
• X-rays
• The LNT model states that risk of cancer increases as the
radiation dose increases
• The Radiation hormesis model states that at low radiation
levels the risk of cancer drops
Half life is the time it takes for the number of
nuclei of an isotope (mass of atoms to half)
What are the uses of radioactivity?
1. Measure thickness
2. Trace the flow of a substance through a system
3. Find the age of an ancient material
The discovery of the nucleus
• Scientists used to use the ‘plum pudding’ model to explain the
structure of the atom
• The pudding was a mass of + charge with electrons dotted around
• This can be disproved by firing alpha particles at a thin gold sheet
• Most of the alpha particles go straight through – showing that most
of the atom is empty space
• Some alpha particles are deflected – showing that the nucleus has a
large positive charge
• Only a small number of alpha particles rebound backwards – showing
that the nucleus of the atom is very small
Nuclear
Fission
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•
•
•
•
•
Nucleus releases 2 or 3 neutrons and energy (radiation)
This causes other nuclei to split – CHAIN REACTION
This happens in a nuclear reactor
The energy is used to generate electricity
Uses Urannium-235 or Plutonium-239
https://www.youtube.com/watch?v=EsuRgxsd6bk
Nuclear
Fusion
1. Small nuclei are fused into large nuclei (for example hydrogen)
2. This releases energy
3. It happens in stars and the sun
4. Could do this with water – lots of hydrogen atoms
5. The nuclei need to move fast or they will repel
The Birth of the Universe
1. 13 billion years ago- THE BIG BANG
2. A hot glowing ball of radiation and matter were created
3. The university expanded and the temperature fell – it became cold and
dark – THE DARK AGES
4. Uncharged matter was attracted to one another and formed gigantic
clumps
5. This became galaxies and stars
The Life Cycle of a Star
1.
2.
3.
4.
5.
6.
7.
5.
6.
7.
Gravity pulls dust and gas together. This forms a PROTOSTAR
As it gets denser –it gets hotter
Nuclear fusion changes hydrogen to helium. This is the MAIN SEQUENCE. The star is stable
because the force of gravity is balanced by the outward force of radiation. The bigger the mass,
the shorter the main sequence because the hydrogen is used up more quickly
Star runs out of hydrogen
Those the same size of the sun swell out, cool down and turn red. This is called a RED GIANT
Gravity then causes it to collapse in on itself. It heats up and turns white. This is called a WHITE
DWARF
It then goes cold and becomes a BLACK DWARF
Those bigger than the sun swell out and become a RED SUPERGIANT
It starts to collapse but then explodes in a massive explosion called a SUPERNOVA
Most then become NEUTRON STARS. Very heavy stars create a BLACK HOLE