[This is Unit 2 Physics, Additional Physics. This section comes after Core Physics in an AQA Course (Unit 1)]

Transcription

1 Contents Unit 1: Motion Unit 2: Speeding Up and Slowing Down Unit 3: Work and Energy Unit 4: Static Electricity Unit 5: Current Electricity Unit 6: Mains Electricity Unit 7: Nuclear Physics [This is Unit 2 Physics, Additional Physics. This section comes after Core Physics in an AQA Course (Unit 1)] P2-1 : Motion Speed and Velocity The table below shows the distances travelled by a car over a given amount of time: We can represent this as a graph: We call this type of graph a distance-time graph as it plots distance travelled against time taken. A slope on a distance-time graph represents speed. The steeper the slope is, the greater the speed. We can use chosen figures to calculate the speed from the graph.

2 This formula can be rearranged to show either of the following formulae which we use to work out distance or time: Generally, speed is measured in metres per second (m/s). Velocity is speed in a given direction. This means that if a moving object changes direction, its velocity changes even if its speed stays the same. When the velocity changes, we say it accelerates. Acceleration is calculated using the following equation: Acceleration is generally measured in metres per second squared (m/s²). If the value of acceleration is negative, the object is slowing down, or decelerating. A velocity-time graph plots the velocity of a moving body (y axis) against the time taken (x axis). the slope of a line on one of these graphs represents acceleration the steeper the slope the greater the acceleration if a slope has a negative gradient, it represents deceleration the area under the line of a velocity-time graph represents distance travelled the greater the area, the greater the distance travelled We can use the gradient of a distance-time graph to calculate the speed of an object. For example, if the graph shows that a body has moved 10 metres in 2 seconds, you can easily calculate the speed is 5m/s.

3 P2-2 : Speeding Up and Slowing Down Equal and Opposite Forces We measure forces in newtons, N. Objects always exert equal and opposite forces on each other. For example, if object A exerted a force upon object B, object B would exert an opposite force of the same power on object A. These are often referred to as action and reaction forces. Examples include: when a car hits a barrier it exerts a powerful force on the barrier, but the barrier exerts a force in the opposite direction of an equal amount on the car if you were to lay a book on a table, it exerts a force vertically down on the table, but the table exerts an equal and opposite force on the book Resultant Force Because most objects tend to have multiple forces acting on them, the resultant force is the single force that would have the same effect on the object as all the other forces together. When the resultant force is zero, it means the object will remain stationary if already stationary, or if moving it will carry on moving at a constant speed. When the resultant force is not equal to zero, it means that a stationary object will be accelerated in the direction of the resultant force; or if the object is moving in the same direction as the resultant force is will dramatically accelerate; or if the object is moving in the opposite direction to the resulatant force is will decelerate. Force and Acceleration A resultant force always causes acceleration, remembering that negative acceleration is deceleration. Without acceleration present, the resultant force must be zero. Resultant force, mass and acceleration are all related in the following equation: resultant force = mass x acceleration The greater the resultant force, the greater the acceleration. The larger the mass of an object, the bigger the force needed to give it a particular acceleration. Road Travel A vehicle travelling at a steady speed has a resultant force of zero. This means the driving forces are equal and opposite to the friction forces. The faster the speed of the vehicle, the bigger the deceleration needed to bring it to rest in a particular distance i.e. the bigger the breaking force needed. The stopping distance of a vehicle is the distance it travels during the driver s reaction time (thinking distance) plus the distance it travels under the breaking force (breaking distance). The thinking distance is increased when the driver is under the influence of drugs or alcohol. Falling in Air When an object falls freely, the resultant force acting on it is gravity. It will make the object accelerate around 10m/s² close to the earth. We call this force of gravity weight and the acceleration the acceleration due to gravity. Therefore the above equation becomes: weight (N) = mass (kg) x acceleration due to gravity (m/s²)

4 If the object is on the Earth, not falling, we use: weight (N) = mass (kg) x gravitational field strength (N/kg) When an object falls through a fluid (i.e. a liquid or a gas, e.g. air), the fluid exerts opposite forces on the falling object reducing its motion, for example air resistance. The faster the object falls, the greater the frictional force. Eventually, this would be equal to the weight of the object this resultant force is now zero, so the object will stop accelerating and begin moving at a steady velocity called the terminal velocity. P2-3 : Work and Energy Energy & Work When a force moves an object, energy is transferred and work is done. When an object starts to move a force must have been applied to it. This force needs a supply of energy from somewhere, e.g. from electricity or fuel. When work is done moving the object, the supplied energy is transferred to the object so the work done is equal to the energy transferred. Both work and energy have the unit joule, J. The work done on an object is calculated using this equation: work done = force x distance moved in the direction of the force Therefore when the distance moved is nothing, the work done is zero. An elastic object will go back into its original shape when it has been stretched or squashed. When work is done on an elastic object to stretch or squash it, the energy transferred is stored as elastic potential energy. When the object returns to its original shape this energy is released. Kinetic energy is the energy of movement. The kinetic energy depends on the mass and speed of a moving object. The greater the mass, and the faster the speed, the more kinetic energy it has. We calculate kinetic energy using the following equation: kinetic energy = ½ (mass x speed²) Momentum Every moving object has momentum. Momentum is measured in kilogram-metres per second, kg m/s and is calculated using the equation here: momentum = mass x velocity If two objects were to collide, the total momentum before the collision is equal to the total momentum afterwards (provided no external force acts on them) this is known as the conservation of momentum. In other words, the total change in momentum before and after collisions is zero. The same is for explosions. After a collision, the two objects may move off together in the same direction, or the may separate apart. As with velocity, momentum has both size and direction. A positive value for the momentum in a calculation means in the opposite direction to the negative value. Two objects at rest have a momentum of zero. In an explosion, two objects will move apart with equal and opposite momentum. One of these momentums will be positive, the other negative and as they share the same value, the total momentum after the explosion will be zero. An example of an explosion is

5 firing a gun: as you fire, the bullet moves out with a momentum in one direction, and the gun recoils in the opposite direction with equal movement. When a force acts on a moving object (or an object which is able to move), its momentum changes. The equation below describes this: force = change in momentum time taken for change N.B. Look at car safety features, especially air bags and crumple zones, to investigate how we make use of momentum changes. P2-4 : Static Electricity Charge If two insulating materials rub against each other, electrons are rubbed off one material and deposited on the other. Which one gains and which one loses electrons depends on the materials used. Because electrons (e ) are negative, the material gaining electrons becomes negatively charged, and the one that loses them becomes positively charged. An example would be rubbing a dry cloth on a polythene rod, in which case the electrons are transferred from the dry cloth onto the polythene rod. If the same cloth is then rubbed against a Perspex rod, the electrons from the Perspex rod would move onto the dry cloth. As you can see from the diagram, the dry cloth is rubbed against the polythene rod this causes the rod to gain electrons from the dry cloth, and because the cloth therefore loses electrons, it becomes positively charged. When the same dry cloth is rubbed on the Perspex rod, the electrons are transferred from the rod onto the cloth. Like charges repel and opposite charges attract, the bigger the distance between the forces, the weaker the force. When a charge flows through a conductor, there is a current in it. Electric current is the rate of flow of charge. In a solid conductor, e.g. metal wire, the charge carriers are electrons. The reason metals are good conductors of electricity is because they have free conduction electrons that are not confined to one single atom. This is why insulators cannot conduct electricity all the electrons are held within atoms. A conductor can only hold charge when it is isolated from the ground otherwise electrons will flow to or from the earth and discharge it.

6 The bigger the charge on an isolated object, the higher the potential difference between the object and the earth. If the potential difference becomes high enough, a spark may jump across the gap between the object and any earthed conductor brought near it [a metal object is earthed by connecting it to the ground]. Using Electrostatics In a photocopier, a copying plate is given a charge. An image of the page to be copied is projected onto the charged plate. When light hits the image, the charge leaks away leaving behind a pattern of the image. Black ink powder is attracted to the charged parts of the plate. This powder is then transferred onto a piece of paper. The paper is heated so that the powder melts and sticks to it, producing a copy of the original document. Electrostatic smoke precipitators are used in chimneys to attract dust and smoke particles so that they aren t released into the open air. The particles pass over a charged grid and pick up a charge, they are then attracted to plates on the chimney walls with the opposite charge. The particles stick to these plates, and are then shaken off and collected. Items like cars are usually painted using an electrostatic paint sprayer. The spray nozzle is connected to a positive terminal. This means that as the paint droplets pass through it, they pick up a positive charge. This makes the paint droplets repel each other, so they spread out to form a fine cloud. The car will be connected to a negative terminal, giving it a negative charge so that the positively charged paint droplets are attracted to it. Dangers of Electrostatics Static electricity has its dangers as well as uses. The filler pipes on road tankers that are used to pump fuel into storage tanks are earthed to prevent them becoming charged because a spark could cause an explosion of the fuel vapour. Apart from earthing, another way in which we can deal with the dangers of electrostatics is by using antistatic materials. P2-5 : Current Electricity Electrical Circuits Every circuit component has its own symbol. These are some of the main ones.

7 The symbols shown relate to their descriptions below. We use these symbols to make circuit diagrams to show how components are connected to make a circuit. A cell is necessary to push electrons around a completed circuit A battery consists of two or more cells joined together, increasing the power supply of the circuit A bulb is used as an indicator to show when current passes through An ammeter is used to measure electric current A voltmeter is used to measure voltage A switch enables the current to be switched on or off A fixed resistor limits the current in a circuit A variable resistor allows the current to be varied A diode allows current through one direction only A fuse will melt and break the circuit if the current gets over a certain amount A heater transforms electrical energy into heat Resistance We use current-potential difference graphs to show how the current through a component varies with potential difference across it. The current is measured with an ammeter which is always placed in series with the component. The unit of current is called the ampere, or amps (A). The potential difference, or voltage, is measured using a voltmeter, which is always placed in parallel with the component. The unit is a volt, V. The graph shows a current-p.d. graph for a wire at a constant temperature. If the resistor is kept at a constant temperature, the graph will always show a straight line passing through the origin. This shows us that the current is directly proportional to the potential difference across the resistor. Potential difference and current are related by an equation from Ohms law: potential difference = current x resistance Resistance is measured in ohms, Ω, and is the opposition to charge flowing through the resistor. When the resistor remains at a constant temperature, the resistance stays constant; if the temperature increases, the resistance increases so the line on the current-p.d. graph is no longer straight. The current-p.d. graph for a filament lamp curves, so the current is not directly proportional to the potential difference. The resistance of the filament increases as current increases, this is because the resistance increases with temperature.

8 The current through a diode can only flow in one direction in the reverse direction, the resistance is so high that the current is zero. The resistance on an LDR (light-dependent resistor) decreases as the light falling on it gets brighter. As the temperature of a thermistor, however, goes up the resistance goes down. The two graphs above show the current-potential difference graphs for a filament lamp (left) and a diode (right). Series Circuits In a series circuit, the components are connected one after the other, so if there is a break anywhere, the current stops flowing. Because there is no choice about the route of the charge as it flows around the circuit, the current flowing through each component is the same. The current depends on the potential difference (p.d.) of the supply and the total resistance of the circuit: current = potential difference of supply total resistance The potential difference of the supply is shared between all the components in a series circuit, so the potential differences of each component added together is the potential difference of the supply. Likewise, the resistance of each component added together is the same as the total resistance. The bigger the resistance of a component, the larger its share of the total supply potential difference. Parallel Circuits In a parallel circuit, the components are connected across the supply so that if there is a break in one part of the circuit, current may continue to flow in the other parts. Because each component is connected across the supply p.d. the potential difference across each component is the same. Because there are junctions in the circuit, different amounts of charge can flow through different components this means that the current can change between components. The current of a component depends on the resistance the bigger the resistance of a component, the smaller the charge flowing through it. The total current running through the whole circuit is equal to the sum of the currents through each separate component. Remember that this means: in a series circuit, the current is the same across the whole circuit in a series circuit, the p.d. varies between components, and is dependent on resistance of each component in a parallel circuit, the p.d. is constant across the whole circuit in a parallel circuit, the current varies with resistance between each component

9 Series circuit: the current of the bulbs will have the exact same current flowing through them, e.g. 8 amps. If they all have the same resistance, they will all share the same potential difference (e.g. all at 6V) but if one bulb had a significantly higher resistance than the others, the potential difference of that bulb might be 4V whilst the other two have 1V Parallel circuit: the potential difference is the same throughout each component in the circuit because it is equal across the supply. However, the current which flows through each component varies: it may be the same (i.e. 2A each) if each bulb shares the same resistance; or if one bulb has a higher resistance than the other two, and the second bulb has a slightly higher resistance than the third bulb they may have amp readings of 1A, 2A and 3A P2-6 : Mains Electricity AC & DC Cells (and batteries) supply a current which only flows in one direction this is called direct current (or DC). However, the current from the mains supply passes in one direction, then reverses and passes in the other direction this is called alternating current (or AC). The frequency of the UK mains supply is 50 Hertz (Hz) this means it alternates direction 50 times a second. The voltage of the mains is 230V. Plugs The live wire from the mains supply alternates between a positive and a negative potential with respect to the neutral wire. The neutral wire remains at zero volts. The live wire alternates between +325 V and -325 V. Most electrical appliances are connected to the mains supply using a cable and a three pin plug. The outer cover of the three pin plug is made from either plastic or rubber, as these are both good insulators, and the pins themselves are made from brass (because it is naturally a good conductor, and will not oxidise or rust). Inside the three pin plug: there is a blue wire connected to the neutral pin there is a brown wire connnected to the live pin there is a green-yellow wire found in three-core cables which is connected to the earth pin - but a two-core cable does not have the earth wire

10 Appliances with a metal exterior must be earthed, appliances with plastic cases do not they are said to be double insulated and are connected to the mains supply using only a neutral wire and a live wire. There are also fuses fitted to the plugs. If a fault develops in an earthed appliance, a large current will flow to earth, melting the fuse and disconnecting the supply. A fuse is put in the live wire so that if it melts it cuts off the current. The rating at which the fuse is set to melt should be slightly higher than the average working rating if it is set to be too high it will not melt soon enough; and obviously if it is too low, it will melt and disconnect the power supply as soon as the appliance is switched on. An alternative to using a fuse is to put in a circuit breaker. This is an electromagnetic switch that opens and cuts off the supply when the current increases above a certain value. Electrical Power An electrical device transforms energy from one form into another and transfers energy from one place to another. The rate at which it does this is called the power. Power can be calculated using this equation: power = energy transformed time Power is measured in watts (W). Energy is measured in joules, J and time in seconds, s. In an electric circuit, it is more common to measure the current and potential difference of a device. We can also use current and p.d. to calculate the wattage: power = current x potential difference Power again is measured in watts. Current in amps, A and p.d. in volts, V. Electrical appliances must have their power rating shown on them. The potential difference of the mains supply is 230 volts. So the equation above can be used to calculate the size of the fuse to use when we work out the current of the circuit. In a resistor, the electrical energy is transformed to heat. The amount of energy that s transformed can be worked out using the below equation: energy (joules, J) = potential difference (volts, V) x charge (coulombs, C) A coulomb is the amount of electric charge transported in one second through a current of 1 amp. The equation linking charge, time and current is: charge (coulombs, C) = current (amps, A) x time (seconds, s) Because every circuit component has some resistance, including connecting wires, when a charge flows through a circuit, the components will heat up. This means that the majority of electrical appliances have vents to keep them cool.

11 P2-7 : Nuclear Physics Nuclear Reactions The atom consists of three sub-atomic particles, the proton, neutron and electron. 1. protons have a relative mass of 1 and a charge of neutrons have a relative mass of 1 and a charge of zero 3. electrons have a negligible mass and a charge of -1 An atom has the same number of protons as electrons so overall has no charge. It becomes an ion (charged particle) when it gains or loses electrons. Atoms of the same element can also have a different number of neutrons, in which case it is an isotope. Note that only neutron numbers and electron numbers differ, all atoms of a particular element have the same number of protons. This number of protons in an atoms is called the proton number or atomic number. The total number of protons and neutrons in an atom is called the mass number. An alpha particle consists of two protons and two neutrons. This means that when a nucleus emits an alpha particle, the atomic number decreases by 2 and the mass number decreases by 4. For example, the element radium emits an alpha particle and becomes radon: A beta particle is a high speed electron from the nucleus. It is emitted when a neutron in the nucleus changes to a proton and an electron. The proton stays inside the nucleus and so the atomic number goes up by one and the mass number is unchanged. The electron is instantly emitted. For example, carbon-14 emits a beta particle when it becomes nitrogen: Background radiation is the radiation present around us all the time. It comes from a variety of sources, including cosmic rays, from rocks and from nuclear power stations. When a nucleus emits gamma radiation - there is NO change to the atomic mass or mass number because gamma radiation is an electromagnetic wave which has no charge nor mass. Changes in our View on the Atom Not so long ago, it was believed that atoms consisted of spheres of positive charge with electrons stuck into them, like plums in a pudding, so this was called the plum pudding model of the atom. Three scientists (Rutherford, Geiger and Marsden) devised an alpha particle scattering experiment in which they fired alpha particles at an incredibly thin sheet of gold foil, which was meant to be an atom thick, but obviously could not be done so they attempted the best they could. They found that are the fired the alpha particles at the gold foil, most of them passed straight through this means that most of the atom is just empty space. A minority of the alpha particles were deflected through small angles, which suggested the nucleus had a positive charge. Also, a minority of alpha particles were deflected through large angles which lead us to believe the nucleus had a large mass, and a very large positive charge.

12 Nuclear Fission The process of an atomic nucleus splitting is called nuclear fission. There are two fissionable isotopes in common use in nuclear reactors: uranium-235 and plutonium-239. Naturally occurring uranium is mostly uranium-238, which is non-fissionable. This is why most nuclear reactors use enriched uranium that contains around 2% 3% uranium-235. For fission to occur, the uranium-235 or plutonium-239 nucleus must absorb a neutron. The nucleus then splits in to two smaller nuclei and two or three neutrons and energy is released. The energy released in such a nuclear process is significantly more than that of a chemical process, e.g. burning. The neutrons go on to produce further fissions, creating a chain reaction. In a nuclear reactor, the process of fission is controlled, so one fission neutron per fission on average goes on to produce further fission. Nuclear Fusion Nuclear fusion is the process of two atomic nuclei joining to form a single, larger nucleus. During this process, energy is released fusion is the process in which energy is released in stars. There are enormous problems with producing energy from nuclear fusion in reactors. Nuclei approaching each other will repel one another due to their positive charge. To overcome this issue, the nuclei must be heated to extremely high temperatures to give them enough energy to overcome the repulsion and fuse together. Because of these enormously-high temperatures, the reaction cannot take place in a normal container, but has to be contained by a magnetic field.