P2 Revision Learning Intentions: Basically all of P2.

P2.1 Forces and their effects

P2.1.1 Resultant forces a) Whenever two objects interact, the forces they exert on each other are equal and opposite. b) 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 single force is called the resultant force. c) A resultant force acting on an object may cause a change in its state of rest or motion.

P2.1.1 Resultant forces d) 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. e) If the resultant force acting on a moving object is: ■ zero, the object will continue to move at the same speed and in the same direction

Reaction Force Forced act in pairs. When 2 forces interact they are equal and opposite in direction e.g. a person exerts a force on the chair but the chair applies an equal force upwards on the person, a reaction force.

Weight(N)=Mass(kg)x Gravitational field strength (N/kg) Mass or Weight Weight is also a force measured in Newton's. Don’t confuse mass and weight as mass is actually the amount of ‘stuff’ that makes up an object measured in kilograms. Weight is the force calculated by Weight(N)=Mass(kg)x Gravitational field strength (N/kg) On Earth g=10N/kg Or g=10m/s2

Resultant Forces When more than 1 force acts upon an object, a resultant force can be calculated. This resultant force show how overall affect of the force. This shows us in which direction the force is accelerating in. If the resultant force is zero: A stationary object will not move. An object in motion will stay the same velocity.

Calculation force Force(N)=Mass(kg)x Acceleration(m/s2)

P2.1.2 Forces and motion a) The acceleration of an object is determined by the resultant force acting on the object and the mass of the object. F=ma AND a=F/m b) The gradient of a distance–time graph represents speed. c) Calculation of the speed of an object from the gradient of a distance–time graph. d) The velocity of an object is its speed in a given direction. e) The acceleration of an object is given by the equation: a=(v-u)/t f) The gradient of a velocity–time graph represents acceleration. g) Calculation of the acceleration of an object from the gradient of a velocity–time graph. h) Calculation of the distance travelled by an object from a velocity–time graph.

Distance-time graph Distance-time graphs tell you how an objects distance is changing over time. If there is a smooth slope (/) on your graph then the object is moving at a constant speed. If there is a flat line (-)then there is no movement. A steeper slope means a faster speed. If the slope is downwards the object is returning to the starting position. If there is an upwards curve ( )on a distance time graph then the object is accelerating, a downward curve ( ) means it is decelerating.

How do you work out speed? Where is it fastest? What is the average speed?

Velocity-time graphs Velocity-time graphs tell you how an objects velocity is changing over time. If there is a smooth slope (/) on your graph then the object is accelerating. If there is a flat line (-) then the object is moving at a constant speed. A steeper slope means a larger acceleration. If there is a downwards slope (\) then the object is decelerating. The area under the velocity time graphs tells you the distance travelled. To work out the acceleration from a section of the slope you use the same method as for the distance-time graph.

Where are the following: Acceleration Deceleration Constant Speed

Working out acceleration A velocity-time graph tells you how an objects velocity changes over a certain time. This is the acceleration.

P2.1.3 Forces and braking a) When a vehicle travels at a steady speed the resistive forces balance the driving force. b) The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance. c) The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance). d) A driver’s reaction time can be affected by tiredness, drugs and alcohol. e) 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 increase. f) A vehicle’s braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.

Stopping distance = thinking distance + braking distance How quickly a car can come to a stop depends on the car and the driver. The stopping distance is the thinking distance (which depends on the drivers reactions) and the braking distance (which depends on the car and road conditions). Stopping distance = thinking distance + braking distance

Thinking and braking distance The thinking distance will be increased if the driver is tired, been drinking alcohol, been on drugs etc. The braking distance will depend on the road surface, weather conditions and how well the car responds e.g. condition of brakes.

P2.1.4 Forces and terminal velocity a) The faster an object moves through a fluid the greater the frictional force that acts on it. b) 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 (steady speed). c) Draw and interpret velocity-time graphs for objects that reach terminal velocity, including a consideration of the forces acting on the object. d) Calculate the weight of an object using the force exerted on it by a gravitational force: W=mg

Terminal velocity An object falling through a fluid or gas will initially accelerate due to the force of gravity. Eventually the force of gravity will be balanced by the up thrust of the liquid/gas; this makes the resultant force zero and the object will move at its terminal velocity (steady speed). The faster the object falls the greater the frictional force that acts.

Terminal velocity

P2.1.5 Forces and elasticity a) A force acting on an object may cause a change in shape of the object. b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy. c) For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape. d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded: F=ke

Force(N)=spring constant(N/m)x extension(m) Hooke’s Law When a weight (force) is applied to a spring it extends. The amount it extends is proportional to the force added. It is governed by the equation: Force(N)=spring constant(N/m)x extension(m)

Graph of Hooke’s law The spring constant can be determined from the gradient (slope of the line) on a force extension graph. Limit of proportionality

Graph of Hooke’s law Choose a section of the line and measure the amount of force and the extension. Then divide the force by the extension For example: In the sample graph the section of the line chosen if for a force of 6N and an extension of 3m. k=F/e k=6÷3=2N/m Also marked on the graph is the limit of proportionality. This is the point at which the spring can still return to its original length. Beyond this point the spring can never go back to its original length/shape.

P2.2 The kinetic energy of objects speeding up or slowing down

P2.2.1 Forces and energy a) When a force causes an object to move through a distance work is done. b) Work done, force and distance are related by the equation: W=Fd c) Energy is transferred when work is done. d) Work done against frictional forces. e) Power is the work done or energy transferred in a given time. P=E/t

P2.2.1 Forces and energy f) Gravitational potential energy is the energy that an object has by virtue of its position in a gravitational field. Ep=mgh g) The kinetic energy of an object depends on its mass and its speed. Ek=(1/2) mv2

Work done When a force acts upon an object causing it to move a through a distance energy is transferred and work is done. The amount of work done is equal to the amount of energy transferred. The amount of work done is calculated by: 2N 5m Work done = 2N x 5m = 10J Box moved from A to B A B Work done (Joules, J) = Force applied (N) x distance moved (m)

Power(W)=Energy transformed(J)/time(s) Power is the amount of work done (energy transferred) every second and is calculated using the following equation: Power(W)=Energy transformed(J)/time(s)

Elastic potential energy Work can also be done on other objects. If you change the shape of an object then the energy gets stored in the object, e.g. an elastic band. This is elastic potential energy. Remember, potential energy is stored energy that is ‘waiting’ to be used, kinetic energy is movement energy.

Gravitational potential energy Gravitational potential energy is the amount of energy an object has when it is held above the ground. It is calculated using the following equation: Ep(J)=m(kg)×g(N/kg)×h(m)

Kinetic energy To work out the kinetic energy a body has you need to know it’s mass and it’s velocity:

Total energy(J)=GPE(J)+KE(J) GPE and KE Gravitational potential energy and kinetic energy are interchangeable. If you get a question about a falling object the total energy is: Total energy(J)=GPE(J)+KE(J)

P2.2.2 Momentum a) Momentum is a property of moving objects. p=mv b) 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.

Momentum(kg m/s)=Mass(kg)xVelocity(m/s) Momentum (has the symbol p) describes how much motion an object has. It is measured in kilogram metre per second (kg m/s). Like velocity, momentum has magnitude acting in a certain direction. Momentum(kg m/s)=Mass(kg)xVelocity(m/s)

Momentum

Conservation of momentum In all situations, momentum is conserved, providing there are no external forces acting. For collisions, the momentum before the collision is equal to the momentum after the collision e.g. snooker balls.

Cannon momentum Another example is cannon before being fired and after being fired. Before the cannon is fired the momentum is zero, after it is fired the cannon ball moves forward and the cannon moves back. The momentum of the cannon ball is the same as the momentum of the cannon moving backwards. In this sort of example you should choose one direction to be positive and the other direction to be negative. The example below illustrates this point. I will choose the right to be positive and the left to be negative.

Cannon momentum

http://www.youtube.com/watch?v=sLoWQ-I6zzw

P2.3 Currents in electrical circuits

P2.3.1 Static electricity a) When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material and onto the other. b) The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge. c) When two electrically charged objects are brought together they exert a force on each other. d) Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract. e) Electrical charges can move easily through some substances, eg metals.

P2.3.2 Electrical circuits a) Electric current is a flow of electric charge. The size of the electric current is the rate of flow of electric charge. The size of the current is given by the equation: I=Q/t b) The potential difference (voltage) between two points in an electric circuit is the work done (energy transferred) per coulomb of charge that passes between the points. V=W/Q

P2.3.2 Electrical circuits I=Q/t V=W/Q

P2.3.2 Electrical circuits c) Circuit diagrams using standard symbols. The following standard symbols should be known:

P2.3.2 Electrical circuits d) Current–potential difference graphs are used to show how the current through a component varies with the potential difference across it. e) The current–potential difference graphs for a resistor at constant temperature. f) The resistance of a component can be found by measuring the current through, and potential difference across, the component. g) The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor. h) Calculate current, potential difference or resistance using the equation: V=IR

P2.3.2 Electrical circuits i) 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. j) The potential difference provided by cells connected in series is the sum of the potential difference of each cell (depending on the direction in which they are connected). k) For components connected in series: ■ the total resistance is the sum of the resistance of each component ■ there is the same current through each component ■ the total potential difference of the supply is shared between the components. I) For components connected in parallel: ■ the potential difference across each component is the same ■ the total current through the whole circuit is the sum of the currents through the separate components.

P2.3.2 Electrical circuits m) The resistance of a filament bulb increases as the temperature of the filament increases. n) The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction. o) An LED emits light when a current flows through it in the forward direction. p) The resistance of a light-dependent resistor (LDR) decreases as light intensity increases. q) The resistance of a thermistor decreases as the temperature increases.

Static electricity In static electricity when two objects are rubbed together the electrons move from one object to another. This causes one object to have an overall positive charge and the other object to have an overall negative charge

Static electricity Like charges repel Unlike charges attract Neutral objects are attracted to both positively and negatively charged objects. If you wanted to test if an object was charged then you could check if it attracted bits of paper, hair etc. It could attract or repel another charged object.  If an object becomes highly charged then the potential difference between then object and the ground increases and the objects will discharge. When a charged object discharges (goes to ground) then a spark might occur. This is the electrons jumping from the object to the earthed conductor.

Current (Ampere,A)=Charge (Coulombs, C)÷Time(s) Current (symbol I, measured in amperes, A) is the rate of flow of electrical charges (symbol Q) or electrons i.e. The number of charges per second. Current is the amount of charges (measured in Coulombs) that flow every second, it is represented by the equation: Current (Ampere,A)=Charge (Coulombs, C)÷Time(s)

Voltage Voltage or potential difference (symbol V, measured in volts, v) is the amount of energy transferred by the charges i.e. the amount of energy per charge. If there is a 2V cell or battery in a circuit then it gives 2 joules of energy to every coulomb of charge. When these charges get to the device in the circuit e.g. a bulb, then the energy gets transferred to the device. To calculated potential difference/voltage you use the following equation

Resistance Resistance (symbol R, measured in ohms, Ω) is something that apposes the flow of current. Voltage, current and resistance related by the equation: V = I x R

Current-volt graphs Current- potential difference graphs tell you how the current through a component varies with voltage.

Series Circuits The total resistance is the sum of the resistance of each component in the circuit. Total resistance (Rtotal) = R1 + R2 The current is the same at every point in the circuit. The voltage is shared between each component in the circuit. Total voltage (Vtotal) = V1 + V2

Parallel circuit The voltage is the same across each branch Vtotal = V1 = V2 The total current through the circuit is the sum of the current through each component Total current (Itotal)= I1 + I2

P2.4 Using mains electricity safely and the power of electrical appliances

P2.4.1 Household electricity a) Cells and batteries supply current that always passes in the same direction. This is called direct current (d.c.). b) An alternating current (a.c.) is one that is constantly changing direction. c) Mains electricity is an a.c. supply. In the UK it has a frequency of 50 cycles per second (50 hertz) and is about 230V. d) Most electrical appliances are connected to the mains using cable and a three-pin plug. e) The structure of electrical cable. f) The structure and wiring of a three-pin plug.

P2.4.1 Household electricity g) If an electrical fault causes too great a current, the circuit is disconnected by a fuse or a circuit breaker in the live wire. h) When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit. i) Some circuits are protected by Residual Current Circuit Breakers (RCCBs). j) Appliances with metal cases are usually earthed. k) The earth wire and fuse together protect the wiring of the circuit.

Direct current In circuits which are powered by cells/batteries the current only flows in one direction, this is called direct current (d.c.).

Alternating current Alternating current (a.c.) is what we receive from power station and what comes out of plug sockets. This current changes direction i.e. the current move back and forth in the circuit. In the UK we use 230V at a frequency of 50Hz.

Mains Plug Green/Yellow BROWN BLUE

Structure of an Electrical Cable Electrical cabling has 3 main sections. Stranded copper wire. Coloured inner insulation. Harden outer insulation.

Fuses A fuse is a small safety device that contains a length of wire that is designed to melt if the current in the circuit gets too high. Describe how fuses and circuit breakers are used in electrical safety. Be able to correctly wire a plug.

Earth The Earth is a low resistance path for the current to flow through. This means if the electricity has a choice of going through us or the Earth wire, it will flow through the earth wire.

Circuit breakers Much more sensitive. Much faster. Can be reset. There are also Residual Current Circuit Breaker (RCCB) which, like circuit breakers, but work much faster than circuit breakers and fuses.

P2.4.2 Current, charge and power a) When an electrical charge flows through a resistor, the resistor gets hot. b) The rate at which energy is transferred by an appliance is called the power. P=E/t c) Power, potential difference and current are related by the equation: P=IV d) Energy transferred, potential difference and charge are related by the equation: E=VQ

Power Power is the amount of energy that is transferred in 1 second. Power is measured in Watts. Power(W) = Energy(J)/Time(s)

Calculating Power from current and voltage Power (W) = Current (A) x Voltage (V) Power (W) Voltage(V) Current(A)

Energy transfer Electrons are ‘pushed’ through an electrical circuit by the battery or other electrical supply. Potential difference (voltage) is a measure of the electrical ‘push’. The amount of energy transferred by 1 coulomb of charge (lots of electrons) depends on the p.d. that pushes it. E=VQ

P2.5 What happens when radioactive substances decay, and the uses and dangers of their emissions

P2.5.1 Atomic structure a) The basic structure of an atom is a small central nucleus composed of protons and neutrons surrounded by electrons. b) The relative masses and relative electric charges of protons, neutrons and electrons. c) In an atom the number of electrons is equal to the number of protons in the nucleus. The atom has no overall electrical charge. d) Atoms may lose or gain electrons to form charged particles called ions. e) The atoms of an element always have the same number of protons, but have a different number of neutrons for each isotope. The total number of protons in an atom is called its atomic number. The total number of protons and neutrons in an atom is called its mass number.

Proton Charge = +1 Neutron Charge = 0 Electron Charge = -1

The discovery of the nucleus Dalton’s Atomic Theory: Atoms are indestructible and indivisible (cannot be divided into smaller particles). John Dalton (1766-1804) Thomson’s Plum Pudding Model of the Atom: Thompson (1856-1940) Thompson discovered the ‘electron’ . So Dalton’s model of the atom was no longer acceptable because now there is something inside the atom. Thomson believed the atom was made of positively charged matter with negatively charged electrons scattered throughout like plums in a plum pudding (or chocolate chips in chocolate chip cookie).

Ernest Rutherford (1871-1937) wanted to test Thompson’s plum pudding model of the atom using his newly discovered alpha particles. He carried out the ‘gold foil experiment in 1910: He bombarded thin gold foil with a beam of ‘alpha’ particles. He expected it to be just like firing bullets at a tissue paper. “If the positive charge was evenly spread out like Thompson says, the beam should have easily passed through”. Expected Found Conclusion: All of the positive charge, and most of the mass of an atom are concentrated in a small core, called the nucleus.

Proton Number and Mass Number The number of protons in an atom tells you what element it is. The number of neutrons tells you if it is an isotope of an element. Mass number = Number of protons + Number of neutrons. Number of Protons and neutrons Number of Protons (Also the number of electrons)

What happens when an atom loses or gain an electron? They become ions. Atoms with a charge that is not zero. If an electron is lost the charge is positive. If an electron is gained the charge is negative.

P2.5.2 Atoms and radiation a) Some substances give out radiation from the nuclei of their atoms all the time, whatever happens to them. These substances are said to be radioactive. b) The origins of background radiation. c) Identification of an alpha particle as two neutrons and two protons, the same as a helium nucleus, a beta particle as an electron from the nucleus and gamma radiation as electromagnetic radiation. d) Nuclear equations to show single alpha and beta decay. (HT)

P2.5.2 Atoms and radiation e) Properties of the alpha, beta and gamma radiations limited to their relative ionising power, their penetration through materials and their range in air. f) Alpha and beta radiations are deflected by both electric and magnetic fields but gamma radiation is not. g) The uses of and the dangers associated with each type of nuclear radiation. h) 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.

Types of radiation

Half-Life The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation. It is not possible to predict when an individual atom might decay. But it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay. This is called the half-life of the radioactive isotope. If the number of counts recorded on a Geiger counter starts at 100 and 3 hours later it counts 50; then the half life of that substance will be 3 hours.

Only Alpha and beta radiation are affected by electric and magnetic fields.

P2.6 Nuclear fission and nuclear fusion

 P2.6.1 Nuclear fission a) There are two fissionable substances in common use in nuclear reactors: uranium-235 and plutonium-239. b) Nuclear fission is the splitting of an atomic nucleus. c) For fission to occur, the uranium-235 or plutonium-239 nucleus must first absorb a neutron. d) The nucleus undergoing fission splits into two smaller nuclei and two or three neutrons and energy is released. e) The neutrons may go on to start a chain reaction.

 P2.6.2 Nuclear fusion a) Nuclear fusion is the joining of two atomic nuclei to form a larger one. b) Nuclear fusion is the process by which energy is released in stars. c) 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. d) During the ‘main sequence’ period of its life cycle a star is stable because the forces within it are balanced.

 P2.6.2 Nuclear fusion e) A star goes through a life cycle. This life cycle is determined by the size of the star. f) Fusion processes in stars produce all of the naturally occurring elements. These elements may be distributed throughout the Universe by the explosion of a massive star (supernova) at the end of its life.

Nuclear Fission Nuclear fission occurs when a Uranium-235 nucleus or a Plutonium-239 nucleus splits. When a nucleus undergoes fission, it releases two or three neutrons which go on to cause further fission resulting in a chain reaction. The energy released could be used to generate electricity. the waste product is highly radioactive substances (Barium and Krypton) which need to be disposed of safely.

Nuclear Fusion Nuclear fusion occurs when two small nuclei are forced close enough together so they join to make large nucleus. Nuclear fusion is the process by which energy is released in the Sun. Energy is released when two nuclei are fused together. The energy released could be used to generate electricity. The waste product is Helium which is a harmless gas. On the other hand it has technical difficulties as a very high temperatures are needed to start the fusion of nuclei.

Black Dwarf Main Sequence Stars