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Transcript of P2-Physics Revision
P2- Physics Revision
Work, Energy and Momentum
More about Velocity-Time graphs
Velocity & Acceleration
Distance - Time Graphs
The distance-time graph for any object that is
stationary is a horizontal line
moving at a constant speed is a straight line that slopes upwards.
The gradient of a distance-time graph for an object represents the object's speed.
Speed in meters per second, m/s
(distance travelled in metres)/(time taken in seconds )
Velocity is speed in a given direction.
Acceleration is change of velocity per second. The unit of acceleration is meter per second squared m/s 2
change of velocity/time taken
Deceleration is the change of velocity per second when an object slows down
If a velocity-time graph is a horizontal line, the acceleration is zero.
The gradient of the line on a velocity-time graph represents acceleration.
The area under the line on a velocity-time graph represents distance travelled.
The speed of an object is given by the gradient of the line on its distance-time graph.
The acceleration of an object is given by the gradient of the line on its velocity-time graph.
The distance travelled by an object is given by the area under the line of its velocity-time graph.
Forces between Objects
Forces and Acceleration
Stretching and Squashing
Force and Speed Issues
A force can change the shape of an object or change its motion or its state of rest.
The unit of force is the newton (N).
When two objects interact, they always exert equal and opposite forces on each other.
The resultant force is a single force that has the same effect as all the forces acting on an object.
If the resultant force on an object is zero, the object stays at rest or at a constant velocity. If the resultant force on an object is not zero, the velocity of the object will change.
If two forces act on an object along the same line, the resultant force is:
1. Their sum if the forces act in the same direction.
2. Their difference if the forces act in opposite directions.
The bigger the resultant force on an object is, the greater its acceleration is.
The greater the mass of an object is, the smaller its acceleration is for a given force.
Resultant force (newtons, N)= mass (kilograms) x acceleration (m/s 2)
On the Road
Friction and air resistance oppose the driving force of a car.
The stopping distance of a car depends on the thinking and braking distances.
High speed, poor weather conditions and poor maintenance all increase the braking distance. Poor reaction time and high speed both increase the thinking distance.
The weight of an object is the force of gravity on it, its mass is the quantity of matter in it.
An object acted on only by gravity accelerates at about 10 m/s2
The terminal velocity of a falling object is the velocity it reaches when it is falling in a fluid. The weight is then equal to the drag force on the object.
The extension is the difference between the length of the spring and its original length.
The extension of a spring is directly proportional to the force applied to it, provided the limit of proportionality is not exceeded.
The spring constant of a spring is the force per unit extension needed to stretch it.
Fuel economy of road vehicles can be improved by reducing the speed or fitting a wind deflector.
Average speed cameras are linked in pairs and they measure the average speed of a vehicle.
Anti-skid surfaces increase the friction between a car tyre and the road surface. This reduces skids, or even prevents skids altogether.
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)
Work done to overcome friction is transferred as energy that heats the objects that rub together and the surroundings.
Gravitational potential energy.
The gravitational potential energy of an object depends on its weight and how far it moves vertically.
The gravitational potential energy of an object increases when the object goes up and decreases when the object goes down.
The change of gravitational potential energy of an object is equal to its mass x the gravitational field strength x its change in height.
The kinetic energy of a moving object depends on its mass and speed.
Kinetic energy (J) = 1/2 x mass (KG) x speed (m/s)
Elastic potential energy is the energy stored in an elastic object when work is done on the object.
Momentum = mass x velocity
The unit of momentum is kg m/s.
Momentum is conserved whenever objects interact, provided the objects are in a closed system so that no external forces act on them
Momentum is mass x velocity and velocity is speed in a certain direction.
When two objects push each other apart, the move apart with:
-different speeds if they have unequal masses.
-equal and opposite momentum so their total momentum is zero.
When vehicles collide, the force of the impact depends on mass, change in velocity, and the duration of the impact.
The longer the impact time is, the more the impact force is reduced.
When two vehicles collide:
-they exert equal and opposite forces on each other.
-their total momentum is unchanged.
Seat belts and air bags spread the force across the chest and also increase the impact time.
Side impact bars and crumple zones give way in an impact to help increase the reaction time
we can use the conversion of momentum to find the speed of a car before an impact
Electrons are transferred when objects are charged:
- Insulating materials that become positively
charged when rubbed they lose electrons.
- Insulating materials that become negatively
charged when rubbed gain electrons.
Certain insulating materials become charged when they are rubbed together.
R= Resistance (Ohms)
V= Potential difference, volts.
I= Current, amperes.
Ohm's law states that the current through the resistor at constant temperature is directly proportional to the potential difference across the resistor.
Reversing the current through a component reverses the PD across it.
Potential difference(volts)=Work done, energy transferred(j)/ charge(coulombs)
Current-potential difference graphs
Filament bulb: resistance increases with the increase of temperature
Diode: forward resistance low, reverse resistance high.
Thermistor: resistance decreases if its temperature increases
LDR: resistance decreases if thelight intensity on it increases
For components in series:
-the current is the same in each component
-adding the potential differences gives the total potential difference
-adding the resistances gives the total resistance.
for components in parallel:
-the total current is the sum of the currents through the separate components
-the bigger the resistance of a component, the smaller the current.
in a parallel circuit the potential difference is the same across each component
Direct current is in one direction only. Alternating current repeatedly reverses its direction.
The peak voltage of an alternating potential difference is the maximum voltage measured from zero volts.
A mains circuit has a live wire that is alternately positive and negative every cycle and a neutral wire at zero volts.
Cables and Plugs
Sockets and plugs are made of stiff plastic materials, which encloses the electrical connections.
Cables consist of 2 or 3 insulated copper wires surrounded by an outer layer of flexible plastic
A three-pin plug consists of:
-the brown live wire
-the neutral blue wire
-the green and yellow earth wire
The earth wire is used to earth the metal case of a mains appliance.
A fuse contains a thin wire that heats up and melts if too much current passes through it. This cuts off the current.
A circuit breaker is an electromagnetic switch that trips and cuts the current off if too much current passes through it.
Electrical energy,charge and issues
An electrical current is the rate of flow of charge.
When charge flows through a resistor, energy transferred makes it hot.
Electrical faults are dangerous as they can cause electric shocks and fires.
Never touch a mains appliance(or plug or socket) with wet hands. Never touch a bare wire or a terminal at a potential of over 30v. Check cables, plugs and sockets for damage regularly.
Observing nuclear radiation
Radioactive substance contains unstable nuclei that become stable by emitting radiation.
There are three main types of radiation from radioactive substances-alpha,beta and gamma.
Radioactive decay is a random event-we can't predict or influence when it happens
Background radiation is from radioactive substances in the environment, space or other devices like x-ray machines.
Discovery of the Nucleus
Rutherford used the measurements from alpha particle scattering experiments as evidence that an atom has a small, positively charged, central nucleus where most of the mass of the atom is located.
The nuclear model of the atom correctly explained why the alpha particles are scattered and why some are scattered through large angles.
Isotopes of an element are atoms with the same number of protons but different numbers of neutrons. Therefore they have the same atomic numbers but different mass numbers.
More about alpha,beta and gamma radiation
Alpha radiation is stopped by paper of a few centimeters of air.
Beta radiation is stopped by thin metal or about a meter of air.
Gamma radiation is stopped by thick lead.
A magnetic or electric field can be used to separate a beam of alpha, beta and gamma radiation.
All types of radiation ionise substances they pass through.
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.
The activity of a radioactive source is the number of nuclei that decay per second.
The number of atoms of a radioactive isotope and the activity both decrease by half every half-life.
Radioactivity at work
The use we can make of a radioactive isotope depends on its half-life, and the type of radiation it gives out.
For radioactive dating of a sample, we need a radioactive isotope that is present in the sample which has a half-life about the same as the age of the sample.
Energy from the Nucleus
Nuclear fission is the splitting of a nucleus into two approximately equal fragments and the release of two or three neutrons.
Nuclear fission occurs when a neutron hits a uranium- 235 or plutonium-239 nucleus and the nucleus splits.
A chain reaction occurs when neutrons from the fission go on to cause other fission events.
In a nuclear reactor control rods absorb fission neutrons to ensure that, on average, only one neutron per fission goes on to produce further fission.
Nuclear fusion is the process of forcing two nuclei close enough together so they form a single larger nucleus.
Energy is released when two light nuclei are fused together.
Radon gas is an alpha emitting isotope that seeps into houses in certain areas through the ground.
There are thousands of fission reactors safely in use throughout the world. None of them are the same type as the Chernobyl reactors that exploded.
Nuclear waste is stored in safe and secure conditions for many years after unused uranium and plutonium is remover from it.
The Early Universe
A galaxy is a collection of billions of stars held together by their own gravity.
Before galaxies and stars formed, the universe was formed of hydrogen and helium.
The force of gravity pulled matter into galaxies and stars.
The life history of a star
A protostar is a gas and dust cloud in space that can go on the form a star.
low mass star:
protostar-main sequence star-red giant-white dwarf-black dwarf
high mass star:
protostar-main sequence star-red supergiant-supernova-black hole if sufficient mass
The sun will eventually become a black dwarf.
A supernova is the explosion of a supergiant after it collapses.
How the chemical elements formed
Elements as heavy as iron are formed inside stars as a result of nuclear fusion.
Elements heavier than iron are formed in supernovas, along with lighter elements.
The sun and the rest of the solar system were formed from the debris of a supernova.