P2 Physics

Topics 1)Motion 2)Forces 3)Work, Energy and Momentum 4) Current Electricity 5) Mains Electricity 6) Radioactivity 7) Energy from the Nucleus

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Motion

Distance-time Graphs

The gradient of the line on a distance-time graph represents an objects speed.

The steeper the line on a distance-time graph the greater the speed it represents.

Speed(m/s)=distance(m) divided by time(s).

Velocity-time Graphs

Velocity is speed in a given direction.

Acceleration is the change of velocity per second.

Acceleration(m/s2)= final velocity(m/s)-initial velocity(m/s) divided by time taken for change(s).

If the line in a velocity-time graph is horizontal, the acceleration is zero.

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Motion

The gradient of a velocity-time graph represents acceleration.

The area under the line in a velocity-time graph is the distance travelled in a given time. The bigger the area, the greater the distance travelled.

Using Graphs

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.


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Forces

Forces Between Objects

A force can change the shape/motion/state of rest of an object.

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

The resultant force is a single force that has the same effect as all the forces acting on an object.

If an object is accelerating there must be a resultant force acting upon it.

If an object is accelerating, it can be speeding up, slowing down or changing direction.


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Current Electricity

Electrical Charges

Certain insulating materials become charged when rubbed together.

Electrons are transferred when objects become charged.

Like charges repel: unlike charges attract.

Electric Circuits

Every component has its own agreed symbol.

Current(A)=Charge (C) divided Time(S)


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Current Electricity

Resistance

Potential Difference(V)=Work Done(J) divided Charge(C)

Potential Difference(V)=Energy Transferred(J) divided Charge(C)

Resistance(ohms)= Potential Difference(V) divided Current(A)

Ohms law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.

Reversing the current through a component reverses the potential difference across it.

Ammeters are always connected in series.

Votmeters are always connected in parallel


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Current Electricity

Current-potential Difference Graphs

Filament bulb: resistance increases with increase of filament temperature.

Diode: "forward" resistance low; "reversed" resistance high.

Thermistor: resistance decreases if temperature increases.

LDR: resistance decreases if the light intensity on it increases.

Current-potential difference can be plotted on the x or y axis.


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Forces

Force and Acceleration

The bigger the resultant force on an object, the greater its acceleration.    

   The greater the mass of an object, the smaller its acceleration for a given force.

Friction and air resistance oppose the driving force of a car.

The stopping distance of a car depends on the thinking distance and the braking distance

The reaction time depends on the driver.

The braking distance depends on the road, weather conditions and the condition of the vehicle.


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Forces

Falling Objects

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 10m/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 of the object.

Mass is the amount of matter in an object, and weight is the force of gravity acting on it.

Drag force may also be called air resistance/ fluid resistance.


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Forces

Stretching and Squashing

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.

Two quantities are directly proportional to each other only if plotting them on a graph gives a straight line through the origin.


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Forces

Force and Speed Issues                                                                              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 friction between a car tyre and the road surface. This reduces skids, or even prevents them all together. 

Energy and Work                                                                                          Work is done on an object when a force makes the object move.

Energy transferred=Work Done.

Work Done(J)=Force(N) times Distance (m).

Work done to overcome friction is transferred as energy that heats the objects that rub together and the surroundings.


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Work, Energy and Momentum

Gravitational Potential Energy

The gravitational potential energy of an object depends on its weight and how far it moves vertically.

Change in gravitational potential(J)=mass(kg) times gravitational field strength(N/Kg) times height (m).

Kinetic Energy

The kinetic energy of a moving object depends on its mass and speed.

Kinetic Energy(J)= 1/2  times mass(Kg) times speed(m/s).

Elastic potential energy is the energy stored in an elastic object when work is done on the object.

If an elastic band is stretched and released, elastic potential energy is transferred to kinetic energy.


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Work, Energy and Momentum

Momentum

Momentum(Kgm/s)= Mass(Kg) times Velocity(m/s).

The unit of momentum is Kg m/s.

Momentum is conserved whenever objects interact, provided no external forces act on them.

Momentum has a size and direction.

Momentum is speed in a certain direction.

Explosions

When two objects push each other apart, they move apart:

1) with different speeds if they have different masses                                           2) with equal and opposite momentum so their total momentum is zero.


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Work, Energy and Momentum

Impact Forces                                                                                                When vehicles collide, the force of the impact depends on mass, change of velocity and the duration of the impact.

When two vehicles collide:                                                                                    1) They exert equal and opposite forces on each other                                        2) Their total momentum is unchanged

Crumple zones in cars reduce the forces acting by increasing the time taken to change the momentum of a car

Car Safety                                                                                                              Seat belts and air bags spread the force across the chest and they also increase the impact time.

Side impact bars+crumple zones give way in an impact so increase impact time.

We can use conservation of momentum to find the speed of a car before an impact.


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Current Electricity

Series Circuits

For components in series:                                                                                   1) The current is the same in each component.                                                     2) Adding the potential differences gives the total potential difference.                 3) Adding the resistances gives the total resistance.

Parallel Circuits

For components in parallel:    

1) The total current is the sum of the currents through separate components.     2) The bigger the resistance of a component the smaller the current is.

In a parallel circuit the potential difference is the same across each component.

To calculate the current through a resistor in a parallel circuit:

Current(A)=Potential Difference(V) divided by Resistance(ohms).

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Mains Electricity

Alternating Current

Direct current is 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.

Frequency(Hz)=1 divided by Times (s).

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Mains Electricity

Cables and Plugs                                                                                              Sockets and plugs are made of stiff plastic materials, which enclose the electrical connections.

Cables consist of two or three insulated copper wires surrounded by an outer layer of flexible plastic material.

In a three-pin plug or a three-core cable:                                                               1) The live wire is brown.                                                                                       2) The neutral wire is blue.                                                                                     3) The earth wire is green and yellow.                                                                The earth wire is used to earth the metal case of a mains appliance.

Fuses                                                                                                                    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 electromagnet that opens(trips)and cuts the current off if too much current passes through it.

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Mains Electricity

Electrical Power and Potential-Difference                                                 The power supplied to a device is the energy transferred to it each second.

Power(W)=Current(A) times Potential-Difference(V).

Correct rating for a fuse(A)=Electrical Power(W) divided Potential-Difference(V).

Electrical Energy and Charge                                                                        An electrical current is the rate of flow of charge.

Charge(C)=Current(A) times Time(s).

When the charge flows through a resistor, energy transferred to the resistor make it hot.

Energy(J)= Potential-Difference(V) times Charge(C).

When a charge flows in a circuit the component will heat up. This means that most electrical appliances have vents to keep them cool.

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Mains Electricity

Electrical Issues

Electrical faults are dangerous because they can cause electric shocks and fires.

Never touch a mains appliance, plug or socket with wet hands. 

Never touch a bare wire or a terminal at a potential of more than 30V.

Check cables, plugs and sockets for damage regularly.

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Radioactivity

Observing Nuclear Radiation

A radioactive substance contains unstable nuclei that become stable by emmiting radiation.

There are three main types of radiation from radioactive substances-alpha, beta and gamma radiation.

Radioactive decay is a random event-we cannot predict or influence when it

happens.

Background radiation is from radioactive substances in the environment, or space, or devices such as X-ray machines.

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Radioactivity

The Discovery of the Nucleus

Rutherford uses 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.

The alpha particle has a positive charge. Because some of the alpha particles rebound, they must be repelled by another positive charge.

Nuclear Reactions

Isotopes of an element are atoms with the same number of protons but different number of neutrons. Therefore they have the same atomic number but different mass numbers.

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Radioactivity

More about Alpha, Beta and Gamma Radiation                                      Radiation is stopped by paper of a few centimeters of air.

1) Beta Radiation is stopped by thin metal or about a meter of air.             2)Gamma Radiation is stopped by thick lead as it has an unlimited range in air.       3)An magnetic or electric field can be used to separate a beam of alpha, beta and gamma radiation.

Alpha, beta and gamma radiation ionise substances they pass through.

Half Life                                                                                                               

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 radioactive source is the number of nuclei that decay per second.

The number of atoms of a radioactive isotope+activity decrease by half every half-life

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Radioactivity

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 sample.

Nuclear Fission                                                                                              Nuclear fissions 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 nucleus or a 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 rods absorb fission neutrons to ensure that, on average, only one neutron per fission goes on to produce further fission.

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Energy from the Nucleus

Nuclear Fission

Nuclear fission 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.

Nuclear Issues

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 of 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 (to be used in the future) is removed from it.

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Energy from the Nucleus

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 distance between neighboring stars is usually millions of times greater that the distance between planets in our solar system. The distance between neighboring galaxies is usually millions of times greater than the distance between stars within a galaxy.

So the universe is mostly empty space.

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Energy from the Nucleus

The Life History of a Star

A protostar is a gas and dust cloud in space that can go on to 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 once it collapses.

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Energy from the Nucleus

How the Chemical Elements Formed

Elements as heavy as iron are formed inside stars as a result of nuclear fission.

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.

In the process of fusion, light nuclei fuse to form heavier nuclei and energy is released.

For elements heavier that iron to be formed there must be an input of energy.

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