Unit 2 Atoms and Radiation Revision Notes

Transcription

1 Unit 2 Atoms and Radiation Revision Notes Atoms are made of a small nucleus containing protons and neutrons, surrounded by electrons. Proton Neutron Electron Mass Charge Atoms are neutral overall because they have the same numbers of protons and electrons. They become ions by gaining or losing electrons. Prior to 1900 atoms were thought to consist of negative electrons embedded in a positive dough or pudding. Rutherford s experiment showed this could not be true because:- A most of the atom is empty space B nucleus is positive because it repels positive alpha particles C nucleus is tiny because so few approach head-on This evidence (Path C) contradicts the old model so scientists rejected it and used the new, nuclear model from then on. The nucleus is only about one ten-thousandth of the size of the whole atom. Isotopes are atoms with the same no. of protons but different no. of neutrons. Some isotopes are unstable and give out (emit) radiation to become stable. We call them radioactive. The original atom is called the parent nucleus which decays to form the stable product, called the daughter nucleus. An isotope symbol shows the mass number (total no. of protons & neutrons) at the top and the atomic number (no. of protons) at the bottom. The bottom number is often missing because the element symbol tells you the atomic number (if you look on the periodic table). So isotopes are also atoms of the same element with different mass numbers. You cannot stop radioactive isotopes from decaying e.g. by heating or reacting with chemicals. You cannot predict when an individual atom will decay it happens at random. Unstable isotopes emit one (or more) of three kinds of radiation which all come from their nucleus. Alpha (α) radiation is the same as a helium nucleus, made of 2 protons and 2 neutrons. Beta (β) radiation is made of electrons. Gamma (γ) radiation is high energy electromagnetic waves. Alpha emission: the atomic number decreasing by two (a new element) and the mass number decreasing by four. Beta emission: the atomic number increasing by one (a new element) and the mass number staying the same. This is because a neutron changes into a proton and electron; the electron is emitted from the nucleus as the beta particle.

2 Radiation is ionising it hits atoms and knocks electrons out of them, causing the radiation to stop. Radiation is penetrating it can pass through some materials. You can think of penetration and ionisation as somehow opposites as in this table. more less ionising ionising alpha beta gamma less more penetrating penetrating Radiation can also be affected by electric and magnetic fields. Compared to beta, alpha particles have double the charge but 8000x the mass so beta bends best in these fields. These properties make radiation both useful and dangerous. Alpha: used in smoke alarms: they cannot penetrate smoke if it enters the sensor; electronics detect this and set the alarm off. Beta: used in thickness monitoring: they only weakly penetrate thin sheets of e.g. paper or metal, so the count rate tells you the thickness of the material being monitored (low count rate = material is too thick). Electronics can adjust the machine s rollers to maintain a constant thickness. Gamma: used in tracers (industrial or medical): they are penetrating so the source can be tracked by detecting the gamma rays emitted through the pipes/building/body. Gamma: used to kill cells in sterilisation (of e.g. supermarket fruit, medical instruments) and in cancer treatment (radiotherapy/nuclear medicine). Radiation can be dangerous because it can ionise parts of human cells, killing them or causing DNA mutations that can lead to increased risk of future cancers. Being hit by radiation (irradiation) can be reduced by increasing distance, reduced exposure time or by using shielding. If radioactive materials get into/on your body you become contaminated. In this case the weaklypenetrating radiations will irradiate your internal organs causing severe damage. Radioactive isotopes are found in igneous rocks which get used for building and are eroded to make soil where food grows. One of the products of the decay of these rocks is radon gas which is found in the air. Areas built on large igneous rock formations include much of Cornwall. We are also exposed to cosmic rays from space which is a risk to people who fly a lot e.g. airline pilots. Finally, we are irradiated by man-made sources: medical procedures and living close to nuclear power stations or bomb tests. Together these are called background radiation. Our total irradiation is called our dose. Wearing a monitoring badge enables people who work with radiation to keep track of their dose compared to normal background

3 The activity (rate of emission of radiation or no. of counts per minute) of a radioactive material decreases as the radioactive isotope emits radiation and its nuclei become stable. The average time it takes for half of the atoms to decay is called the half-life. In this time the activity will halve. The activity and the number of remaining radioactive nuclei are proportional so the y-axis on this graph could be either. This can be used for dating objects such as rocks or once-living materials like leather (cow skin) or textiles (tapestry, shroud). For rocks, the decay of 40 K is used as it decays into argon gas which is trapped in the rock and can be released by crushing. For living material, 14 C is used (carbon dating) because living things refresh their carbon continually when alive so the amount of 14 C only decreases after death. For example: if the activity has dropped to a quarter (half of a half) or if there are 3 daughter nuclei to each parent, then two half-lives have passed. Half-life helps to decide which isotope is used for which job. For example, medical tracers are injected into the body so an isotope is used with a halflife of a few hours. The half-life is long enough for the tracer to reach its destination and the radiation to be detected. The half-life is short enough to minimise the risk to the patient. The alpha source in a smoke alarm has a half-life of many years so it doesn t need to be replaced. A nuclear power station works a lot like a coal or gas power station. A fuel called uranium-235 or plutonium-239 is used in the reactor. The fuel undergoes a nuclear fission reaction this means the nuclei split after absorbing a neutron. Before fission After fission

4 When one nucleus of the fuel (blue) is hit with a neutron (red) it splits into two smaller nuclei (orange) and gives out a lot of heat plus two or three more neutrons. The spare neutrons go on to hit more fuel atoms causing a chain reaction. The used fuel and reactor parts all become radioactive thanks to absorbing neutrons; this radioactive waste is hard to dispose of as it typically has a long half-life. Nuclear fusion is the opposite of fission fusion means joining together rather than splitting apart. Nuclear fusion happens in stars. Hydrogen atoms are made to join together to make helium and other heavier elements. We don t know how to make practical fusion reactors here on Earth yet but several countries are working on it because it releases even more energy than fission and uses hydrogen-based fuel which we could get easily from the sea. It also produces less radioactive waste than fission. The universe originally contained almost entirely hydrogen; all the other elements we see today (up to iron) were formed by fusion in stars. Stars exploding as a supernova (see below) produce the elements heavier than iron and spread the elements across space. Stars form when enough dust and gas from space is pulled together by gravitational attraction. The dust includes elements spread by previous supernova explosions. Smaller masses form from the cloud and be attracted to each other (planets). A protostar forms when dust/gas from space is pulled together by gravitational attraction. Smaller masses form, around the protostar these are planets. In a protostar, fusion has not yet started. During the main sequence period of its lifecycle, a star is stable because the forces within it are balanced. This can last a long time (maybe 10 billion years) because the star s hydrogen is used up very slowly by the fusion process. A star leaves the main sequence when it runs out of hydrogen fuel for fusion. Its core collapses until hot enough to fuse helium into carbon and other elements up to iron. The outer layers swell and cool. This stage is called the red (super)giant phase. When the helium is used up either (a) the outer layers are lost into space forming a nebula and leaving a hot core behind (white dwarf) which cools and dies (black dwarf) or (b) the core collapses further producing a shock-wave which makes the star explode as a supernova spreading elements into space; the collapsing remnant becomes a neutron star or black hole depending on the mass (not the size).

5 Unit 2 Electricity Revision Notes In an electric circuit, charges (usually electrons) flow around the circuit. The unit of charge is the coulomb (C). 1 coulomb is a certain (very large) number of electrons. A current is a flow of charge. A current of 1 amp (A) means that 1 coulomb (C) of charges flows past a point in 1 second (s). Current is measured by an ammeter connected in series. The charges carry energy. Energy is collected from the battery and given up in the devices. The energy each charge collects/gives up in a device is called the potential difference or voltage. Potential difference (p.d.) is measured by a voltmeter connected in parallel. A potential difference of 1 volt (V) is measured between two points if 1 coulomb (C) of charges collects/gives up 1 joule (J) of energy. Current in A = Charge in C Time in s Potential difference in V = Energy transfer in J Charge in C Current is always said to be through a device; p.d. or voltage is always across a device. The energy transferred every second is called the power measured in joules per second or watts (W). Example a circuit consists of a 9V battery and a bulb; a current of 2A flows (this means 2C of charge flows through the bulb every second); each coulomb carries 9J, so the total energy supplied by the battery (and output from the bulb) every second is 2 x 9 = 18J. The power supplied by the battery (or power consumed by the bulb) is 18W. Mathematically Power in W = Potential difference in V x Current in A Devices in circuits (including the wires) are made of atoms the charges flowing in the circuit collide with the atoms. The collisions transfer energy from electrical to other forms e.g. a bulb takes electrical light and heat. The rate of energy transfer in the device depends on what it s made of, its size etc. The resistance of a device, in ohms (Ω), is a measure of the rate of collisions. The more the resistance, the less current flows. The more the resistance, the more energy is transferred as heat in the device. Resistance can be calculated by measuring the current through a device and the p.d. across it and using Ohm s Law:- Resistance in Ω = Potential difference in V Current in A A resistor A variable resistor has a fixed resistance the current is directly proportional to the p.d. can be adjusted by hand and used as a control knob.

6 A diode has infinite resistance to a reverse p.d. but a constant resistance to a forward p.d. (above a certain minimum value, usually about 0.6V). In other words current can only flow in the forward direction. A filament bulb has a resistance that increases with increasing current. This is because a high current causes the filament to get hot and the hotter the filament gets, the more its resistance becomes (because the atoms of the wire are vibrating more so there are more collisions between charges and atoms). A fluorescent tube (or energy-saving bulb or CFL) wastes much less energy as heat than a filament bulb it is more efficient so costs less to run, but costs more to buy. A light-emitting diode (LED) is a diode that emits light when current passes through it in the forward direction. It is even more efficient and more expensive than a CFL. Another factor in lighting choice is how long a bulb lasts before blowing. A graph of p.d. against current has a particular shape for each device. The steeper the graph the lower the resistance. An LDR (light-dependent resistor) resistance is lower in bright light. conducts better as the light intensity increases its A thermistor conducts better as temperature increases its resistance is lower when it s hot. In a series circuit:- a) The current is the same everywhere in the circuit b) The p.d. of the supply is shared between the devices All ammeters read the same (rule a) Voltmeter readings add-up to battery p.d. (6V) (rule b) In a parallel circuit:- a) The p.d. across each device is the same as the supply p.d. b) The current from the supply is shared between each branch in the circuit c) Each device gets the same current as if it was connected directly to the supply. Voltmeter reads the same as battery p.d. (6V) (rule a) A 3 and A 2 add up to A 1 (rule b) A 1 and A 4 are the same If connected in series: add the p.d. s of cells (the right way round) and add the resistances of resistors.

7 Cells and batteries provide direct current (d.c. the charges flow one way round the circuit all the time). The current from the mains is alternating current (a.c. the charges flow repeatedly forwards then backwards). A graph of a d.c. supply never crosses zero. A graph of an a.c. supply goes repeatedly above and below zero. The highest point on an a.c. trace is called the peak voltage (shown on the left it is 6V). The time taken for an a.c. supply to complete one full cycle is called the period, T (as shown here it is 20ms or 20/1000 = 0.02s) The UK (& European) mains is 230V at 50Hz. The structure of a plug is shown opposite. The fuse protects the device from overheating and causing a fire. Inside the fuse is a thin wire; if a fault occurs that causes too high a current to flow, the wire melts, breaking the circuit. The frequency of the supply in hertz (Hz) is 1 T. e.g. the time period above is 0.02s, so the frequency is = 50 Hz fuse earth (symbol ) (green/yellow) neutral (blue) cord grip plastic case live (brown) plastic 3-core cable Common faults are: wires in wrong places, loose wires (in the pins or under the cord grip), wires too long getting squashed and exposing bare wire, wrong fuse (or bypassed with foil), too much insulation removed (so bare wire showing) and no earth wire (when one is needed). The fuse rating and cable thickness must be matched to the power of the appliance. To calculate the correct fuse rating, start with the power of the device (say 1200W). Divide by the supply p.d. (usually 230V mains) to find the normal current ( = 5.22A). Select the fuse with a rating just above this normal current (in thi s case 6A would be good). Select a cable with diameter suitable for the normal current e.g. upto 20A needs 2.5mm 2 cable. Too low a fuse rating the fuse will blow when the device is in normal use so it will not work. Too high a rating or too thin cable there may be a fire (the fuse will not blow when there is a fault). In normal use there is no difference in the current in the live and neutral wires. A circuit breaker (RCCB) detects any difference (due to a fault) and breaks the circuit. So it does a similar job to a fuse/earth but it acts faster and can be easily reset. Appliances with exposed metal parts (e.g. washers, irons) must have an earth wire. The earth wire (when acting with the fuse) protects the user from being electrocuted. Other devices do not have an earth wire and can use two-core cable (cable with no earth wire; the earth pin is not connected in the plug). They are called double-insulated and may have this symbol. When selecting a suitable mains device, consider both the power and efficiency as these affect the cost and impact on the environment. Most devices now have an efficiency rating sticker on them.

8 Friction can cause electrons to be transferred between insulating materials e.g. when rubbing a balloon on your jumper. The objects become static (we say they have gained a static electric charge). Objects that lose electrons become positively charged. Objects that gain electrons become negatively charged. Unlike charges attract; like charges repel. Note: below here may not be in the syllabus but there have been too few past papers so far to be sure. Charged objects can be discharged by connecting them to earth with a conductor. Charges flow through the conductor and are lost amongst the charges distributed across the Earth. If the object is negatively charged, the spare electrons it has gained flow down to earth. If the object is positively charged, the missing electrons it has lost flow up from the earth. Because like charges repel, the more charge on an insulator the more energy each charge has. (Remember: the energy of each charge is called the p.d. or voltage.) If the p.d. becomes high enough even an insulator like the air will conduct we see a spark e.g. lightning. Sparks can start fires and damage equipment. To prevent this we must connect objects that might get charged to earth with a wire so charge doesn t build up. Examples: When fuel is pumped rapidly into an aircraft the fuel rubs on the inside of the rubber pipe the charge build-up might cause a spark and ignite the fuel. For safety we connect a wire (called a bonding line) between the plane and the refuelling tanker. Computer chips are damaged by static charges. When working inside a computer, an engineer wears a conducting wristband connected to earth. Lightning is caused when clouds become charged. To protect buildings from the lightning spark, a thick copper rod (lightning conductor) is connected along the sides of tall buildings to carry the charge safely to earth. Static electricity is useful in photocopying and smoke precipitators (see BBC Bitesize for details).

9 Unit 2 Forces Revision Notes The forces on an object can be combined into a resultant force by adding or subtracting. This is the single force which has the same effect on the motion as all the other forces acting together. If the forces are balanced there is no resultant force. Forces cause a change in speed, direction or shape. Velocity is speed in a given direction e.g. 30mph North, 20m/s backwards. Acceleration is the change of velocity in a given time. So a resultant force causes acceleration (or a change of shape). Mathematically Speed or velocity in m/s = Distance moved in m Time taken in s Acceleration in m/s 2 = Change in speed in m/s Time taken in s The bigger the resultant force, the more the acceleration. Objects with a bigger mass are harder to accelerate (this is called inertia). Always say large acceleration and never fast acceleration etc. Force in N = Mass in kg x Acceleration in m/s 2 Force diagrams tell you nothing about how an object is moving right now; only about how its movement is about to change. When an object isn t moving, a resultant force makes it accelerate (start moving) in that direction When an object is already moving a resultant force:- makes it accelerate in the direction it is already going (speed up) or makes it accelerate in the opposite direction (i.e. slow down) You can use distance-time and velocity-time graphs to show the movement of an object. Graph Shape Distance-time Speed-time Steady forward speed Steady acceleration Stopped Steady speed Steady backward speed Steady deceleration Stopped at the starting point Going behind the starting point Accelerating Stopped Slowing to a stop then speeding-up in reverse Changing acceleration (in this case, increasing acceleration)

10 On a distance-time graph the steeper the line the higher the speed On a speed-time graph the steeper the line the higher the acceleration As shown below: calculate the speed (or accel.) by finding the gradient of the line; on a speed-time graph the area under the line is the total distance travelled. (Break complex graphs into triangles and rectangles to find the area.) When an object moves through a liquid or gas it feels a drag force (air resistance or liquid friction) opposing its weight or thrust. Remember: weight is a force in N (the pull of Earth s gravity on an object) equal to 10x the mass in kg. The drag force is caused by collisions between the object and the particles of the liquid or gas, so as you go faster you hit more particles more often so the drag force increases with speed Initially the drag force is small and the object rapidly accelerates As the speed increases the drag force increases, so the resultant force decreases the acceleration decreases The object is still speeding-up, just not as rapidly as before. Eventually the drag force equals the weight/thrust the resultant force is zero the object stays at a steady speed this is called its terminal velocity (its maximum speed) For a skydiver when the parachute is opened the air resistance suddenly increases there is a resultant force upwards the skydiver is moving downwards so this means he slows down the air resistance gets smaller the resultant force decreases the deceleration decreases Eventually the skydiver is going so slowly that the air resistance equals the weight again there is no resultant force the skydiver falls at a steady speed again; a new (slower) terminal velocity because the parachute means there is more drag for any given speed (it is un-streamlined ) For the racing car, the terminal velocity (top speed) can be increased by having less drag force for any given speed (streamlining the shape) and/or by having more forward force (from more engine power).

11 Remember: energy is conserved (never created or destroyed, only transferred between places/forms) There are 8 kinds of energy:- Stored (potential) energy: gravitational (GPE), chemical and elastic potential energy; Energy on the move : heat, light, sound, electrical and kinetic energy (KE). When a force moves its point of action it transfers energy; the amount of energy transferred is called the work done by the force Work done in J = Force in N x Distance moved in m Examples I lift books onto a table. My lifting force transfers chemical energy from muscles into GPE of books. The books fall off a table. Force of gravity transfers GPE of books into KE. The books land on the floor. Push force of floor transfers KE of books into sound and heat energy. Meteorite falls through the atmosphere. Gravity transfers GPE into both KE and heat. Box of books pushed up a ramp. My push force transfers chemical energy into both GPE and heat. In the last two examples the heat arises because the forces have to do work against friction. Gravitational PE in J = Mass in kg x Height in m x Earth s gravity (10 N/kg) Kinetic energy in J = ½ x Mass in kg x (Speed in m/s) 2 What is the KE of a 500kg car going at 40m/s? KE = ½ x 500 x 40 2 = ½ x 500 x 1600 = 500 x 800 = = 400 kj How fast does the high-jumper hit the mat? Call his speed just before landing v. So KE = ½ x 70 x v 2 This KE came from his GPE at the top (assuming none dissipated), so equals 1260J A 70kg man clears a high jump of 1.8m. How much GPE does he have at the top? GPE = 70 x 1.8 x 10 = 1260 J ½ x 70 x v 2 = x v 2 = 1260 v 2 = v = 36 v = 6 m/s GPE=KE Write me. Forces can make objects change shape instead of accelerate. Some objects return to their original shape after being stretched (or compressed) e.g. springs, rubber bands. They are called elastic. The work done by the force that stretches/compresses them transfers energy into elastic potential energy (not just elastic) which is released when the object goes back to its normal shape. For an elastic object like a spring, the increase in length when it is stretched is directly proportional to the force applied (this is called Hooke s Law) up to a point called the elastic limit after which the object is permanently stretched. Force in N = Spring constant in N/m x Extension in m If a force causes an object to change shape which isn t elastic (or has gone past the elastic limit) the work done by the force dissipates energy as heat in the object.

12 In an emergency a car takes some time to stop. So it travels a certain distance before stopping this is the stopping distance which is made up of thinking distance + braking distance The thinking distance depends on the driver s reaction time but it is a distance not a time. It is affected by being under the influence of drink, drugs or being tired or distracted by e.g. using a phone. The braking distance depends on the braking force and the car s KE. It is the distance it takes for the brakes to transfer all of the car s KE into heat in the brake discs. This is affected by the amount of KE the car has (which depends on its mass and speed) and the condition of tyres/brakes and road surface (tarmac/concrete and any rain/ice). In regenerative braking, pressing the brakes connects electric generators to the drivetrain which transfer the vehicle s KE into electrical energy which is stored in a battery. This reduces the amount of fuel consumed on average which reduces cost and is good for the environment. Momentum can be thought of as how hard it is to stop something moving. Momentum in kgm/s = Mass in kg x Velocity in m/s A resultant force changes the momentum of an object. If no forces are acting from outside a situation then the momentum in an event is the same before and after the event (momentum is conserved if no external forces are acting). Remember: momentum backwards is a minus. Example: Total momentum = 0 Total momentum = (200 x v) (40 x 5) 0 = 200v 200 so v = 1 In an emergency situation e.g. crash or fall, to stop you need to lose a lot of kinetic energy. Something has to do work on you to transfer that energy into another form e.g. heat or elastic PE. Safety features reduce the force needed by increasing the distance of the impact. Model wording for e.g. an airbag:- In a car crash you lose a lot of kinetic energy. An airbag increases the distance over which that KE is transferred. For a given amount of initial KE, if the impact distance is longer the average force will be smaller (because work done = force x distance), so you are less likely to be injured. Using the idea in reverse, to get a golf, tennis or cricket ball up to a high speed, apply a big force for a long distance (follow-through) Examples Windscreen applies a large force in a short distance: you die. Seat-belt a bit stretchy; smaller force, longer distance: safer. Airbags and crumple zones increase the distance even more. Crash-mats in PE, the surface of children s playgrounds and the soles of trainers are made squashy to increase impact distance. Climbing ropes: stretchy to increase distance for less yank.