Physics Key Points

A force can do one of four things to an object: 1. Make it speed up – accelerate. 2. Make it slow down – decelerate. 3. Change its direction. 4. Change its shape. If something is doing one of these four things there must be net force acting upon it. Newton's First Law ‘Every body continues in a state of rest or uniform motion unless acted upon by an external force. ‘ Something without net force acting on it will either stay still or move at a constant speed in a straight line until you apply a force to it. F = ma Newton's Second Law: †¢ F is the force in Newton’s, N. m is the mass in kilograms, kg. †¢ a is the acceleration in m/s2. This shows that if you keep the mass constant and double the applied force the acceleration will double. Hooke's Law, elastic and plastic behaviour F = kx An elastic material is one that will return to its original shape when the force applied to it is taken away. A plastic (or inelastic) material is one that stay s deformed after you have taken the force away. If you apply too big a force a material will lose its elasticity. In solids If a force is applied over a smaller surface area you get a larger pressure. Pressure can be calculated using the following equation: Pressure = force/area Force will be in Newton’s, N. Area will be in either m2 or cm2. If the area is in m2 then the pressure will be measured in Pascal’s or N/m2. If the area is in cm2 then the pressure will be in N/cm2. In liquids 1. Pressure increases with depth. 2. Pressure acts equally in all directions. 3. Pressure is transmitted through liquids. Hydraulics All hydraulics systems work because the pressure is the same throughout the system. A really good example of this is a car brake system. You need to know all about this for your exams. In gases Although gases are compressible (squashy) they exert a pressure because of the gas particles bouncing off things. Boyle's Law For a fixed mass of gas the pressure x the volume of the gas stays the same. In other words, as you squeeze a gas its pressure will go up and its volume will get less. Important point: The temperature and mass of gas must stay the same for this to be true! We can write this as: Pressure x volume = constant or P1V1 = P2V2 Moments Moments make things turn or rotate. They are caused by forces but are not forces themselves. Like forces, moments have a direction. We say they are either clockwise or anti-clockwise, to show which way they will make something turn. The bigger the force causing the turning effect the bigger the moment will be. The further the force is from the pivot the bigger the moment will be. The size of a moment can be calculated using: Moment = Force x Distance Force is measured in Newton’s, N. Distance is measured in either m or cm. If the distance is in m then the moment will be measured in Nm. If the distance is in cm then the moment will be measured in Ncm. Distance As we all know, the distance between two points is how far apart they are. In science, we normally use metres as our unit. We often represent how the distance between two points changes using a distance: time graph. Speed Speed is how fast something is going. It is how quickly something covers a certain distance and can be worked out using the equation: Speed = distance/time Acceleration This is how quickly something gets faster. So if you were running and getting 1m/s faster every second you would have had an acceleration of 1 m per second per second. We normally write this 1 m/s2. We work out by the equation: Acceleration = change in speed/time taken Frequency, wavelength, amplitude and time period are used to describe waves. Waves can be transverse or longitudinal. Transverse waves – the vibration is at right angles to the wave motion, e. g. light, water waves and the electromagnetic spectrum waves. Longitudinal waves – the vibration is parallel to the wave motion, e. g. sound and some earthquake waves. Wave Speed (m/s) = Frequency (Hz) x wavelength (m) Reflection is the bouncing of waves off a surface. There are three rules of reflection that you need to know. 1. The angle of incidence always equals the angle or reflection. 2. The distance from the object to mirror is the same as the distance from the mirror to the image. 3. The image is always the same size as the object but is laterally inverted. Refraction is the bending of a wave when it goes from one substance into another. Refraction happens because the speed and wavelength of the wave changes as the wave goes into the other substance. The frequency of the wave stays the same. Total internal reflection happens when the angle of incidence, of a wave going from a substance into air, is greater than the critical angle. The wave bounces off the boundary, obeying the rules of reflection. Dispersion of white light produces a spectrum. This is caused by refraction. Light of different frequencies is refracted by different amounts. Red is refracted the least and violet the most. This causes white light to be split up into seperate colours. Diffraction is the spreading out of a wave as it goes through a gap, or around an object. The smaller the gap or the larger the wavelength the greater the diffraction. Diffraction is most effective when the size of the gap is approximately the same as the wavelength of the wave. You will need to be able to draw diagrams showing how waves reflect, refract and diffract. Sound waves are caused by particles vibrating. The frequency of the vibration decides the pitch of the sound. The amplitude of the vibrations decides the loudness of the sound. Ultrasound waves are high frequency sound waves, which are beyond the human hearing range. Ultrasound is used for seeing babies in the womb, detecting cracks in metal and cleaning instruments. Waves can be represented on an oscilloscope screen, which can be used to measure the characteristics of the waves. You should be able to find the amplitude and time period of a wave from an oscilloscope screen. The electromagnetic spectrum is a series of waves that all travel at the same speed in a vacuum. They are all transverse. Each part of the spectrum has different uses and dangers. Each part of the spectrum has a different frequency and wavelength. Gamma waves are at the high frequency end of the spectrum. Radio waves are at the low frequency end. You will need to know the uses and dangers of each part of the spectrum. Different surfaces and materials absorb different frequencies of waves. White surfaces reflect most waves. Black surfaces absorb most waves. Information can be carried along copper cables as electrical signals, or long optical fibres as electromagnetic wave pulses. Optical fibres have advantages over copper cables. Optical fibres can carry more information; the signals can travel faster and lose less energy as they travel along the cable. There are two types of signals, analogue and digital. Analogue signals have a continuous range of values. Digital signals have only t wo values, on (1) and off (0). Digital signals have advantages over analogue signals. Digital signals are easier to transmit as they are less affected by noise; it is also possible to send more information, in a certain time, as a digital signal than as an anologue signal. Types of energy Energy can not be created or destroyed it can only change from one form into another. There are many types of energy including, †¢ sound †¢ heat †¢ light †¢ kinetic †¢ nuclear †¢ potential energy Kinetic energy is movement energy. Potential energy is stored energy. There are three main forms of potential energy including gravitational, chemical and elastic. Sankey diagrams can be used to represent energy changes. The size of the arrows represents the amount of that type of energy. Energy is measured in Joules, J or kilojoules, kJ. Conduction Heat energy always moves from hotter objects to colder objects. Heat energy is conducted through solids by particles vibrating and passing on the movement to neighbouring particles. Metals are best at conducting heat. As well as the vibrating particles, they move the heat energy by free electrons moving between their atoms. The poorest conductors are gases as their molecules are too far apart to affect each other much. Air is a very bad conductor. Most insulators work because of trapped air. Convection Convection is hot gases or liquids rising and cooler gases and liquids sinking to replace it. As substances heat up the density decreases, which is what makes them float. This movement of molecules is called a convection current. It can only happen in a gas or liquid where the molecules are free to move around. Radiation Radiated heat energy is infrared radiation. All hot objects radiate heat. Black, dull surfaces are the best emitters of heat radiation. Lighter, shinier surfaces are poor radiators of heat. Radiated heat can also be absorbed by cooler objects. Black is the best absorber. Surfaces coloured silver or white will reflect the radiated heat. Ways to save energy in the home Reducing heat losses from a home means less damage to the environment and lower heating bills. Installing insulation costs money. The payback time is how long it takes for the savings to cover the cost. Each strategy has to reduce conduction, convection, radiation or any combination of them. Common strategies are double-glazing, loft insulation, tank lagging, lined curtains, cavity wall insulation, blocking up disused fireplaces and putting foil behind radiators. Other conservation strategies include using of low-energy light bulbs, turning down heating thermostats, fitting draught excluders and switching off unattended appliances. Non-renewable fuels and power stations The fossil fuels are oil, gas and coal. They are non-renewable, which means that they can not be replaced. They will eventually run out. These fuels have many uses but the main ones are heating, transport and generating electricity. In power stations, the fuel is burnt and the heat turns water into steam. That steam pushes around a turbine that is connected to a generator. The generator produces electricity. This process is the same for all power stations. Nuclear power stations don't burn the fuel. Uranium fuel generates heat that turns water into steam just like in other power stations. Nuclear accidents are rare, but can be serious. The waste from the reactors can be radioactive. It is easy to store it safely for now but it will stay radioactive for years. Environmental impacts of burning fuels Carbon dioxide is the most common of several gases that contribute to the greenhouse effect. The result is global warming. This would result in the weather being more extreme and the ice caps melting raising the sea levels. Sulphur dioxide is the most common cause of acid rain. It dissolves in rainwater to form an acid. The acid rain harms plants, animals and stonework. Alternative energy sources Most of the alternative energy sources are renewable. This means there is either an endless supply of them so that they will not run out, or they can be easily replaced. Hydroelectric power is only possible where the geology is right, such as Scotland. Water runs fast down an incline and turns a turbine. Some developing countries get all their energy from HEP schemes on large dams. The large lake made behind the dam drastically alters the surroundings. Waves and tides have a lot of energy. Few schemes exist because of technological problems and environmental objections. Solar power converts the suns energy into electricity using solar panels. These panels are expensive to make. Wind farms are groups of wind turbines that generate electricity from wind. Some people don't like wind farms because they spoil the view or make a noise. Geothermal energy uses the natural heat in volcanic rock to generate electricity. Gas called methane is produced when matter rots. This gas can be used to generate heat to produce electricity. Burning rubbish is not a way to avoid pollution but it does preserve fossil fuels as well as avoid rubbish having to be put in landfill sites. Crops can be grown to be burnt in a power station. Another version of this is to process the crops into alcohol and use it instead of petrol in cars. Work is done whenever a force acts over a distance, e. g. a car motor produces a forward force to move the car a certain distance. Energy is measured in Joules, J. The work done or energy transferred can be calculated using: Work done or energy = force x distance When working out the work done the force must be in the same direction as the movement. If more than one force is acting in that direction then the resultant force must be used. Kinetic energy is the amount of movement energy an object has. Kinetic energy can be calculated using: Kinetic energy = ? x mass x velocity2 Gravitational potential energy is the extra amount of stored energy an object has because it is higher up. GPE can be calculated using: Change in gravitational potential energy = mass x gravity x change in height This is the same thing as GPE = weight x height Power is the rate at which work is done, or in other words, the amount of energy transferred per second. Power is measured in Watts, W or J/s. Power can be calculated using: Power = energy transferred / time taken Or Power = work done / time taken Energy is often lost to the surroundings as heat energy. This is wasted energy as it cannot be easily used again. Efficiency tells us how much energy is wasted when an energy transfer has happened. The more efficient something is the less energy that is wasted. Efficiency can be calculated using: Power out/power in*100 =efficiency in % Energy out/energy in*100=efficiency in % Static Electricity Static Charge Static charge is a charge that can't move. There are two kinds positive (+) and negative (-). All atoms contain positive particles (protons) and negative particles (electrons) but because they contain the same number of protons and electrons they have no overall charge. Static electricity is caused by an atom having too many or too few electrons (e-). A Van de Graff Generator is a machine that generates huge amounts of static charge, by rubbing electrons off a roller and depositing them on the metal dome. Induction and Earthing The basic rule you need to know is that like charges repel and opposite charges attract. Induction – This is the effect caused when a charged object causes electrons in another object to move. This causes the uncharged object to become attracted to the charged object. Earthing – If enough charge builds up on an insulator, the charge can leap the gap, causing a spark. This can be prevented by discharging the object, gradually. This is called earthing. Useful Static Static electricity is used in many useful machines like photocopiers and smoke stacks (to remove pollution from the smoke). Nasty Static If clouds get charged up enough, you get lightning, the biggest spark of all. Static can also be dangerous when refuelling aircraft. The fuel rubs against the side of the hose and lots of charge builds up. If the plane isn't earthed, the spark can blow the plane up. Basic Circuits Current, Voltage and Resistances Current – This is a measure of the flow of electrons around a circuit (measured in Amperes or Amps). Voltage – This is a measure of how much energy the electrons are carrying around the circuit (measured in Volts). Resistance – This is a measure of how hard it is for the purple to travel through a part of the circuit (measured in Ohms). Direction Problem! Current flows from the positive (+ve) terminal of the battery to the negative (-ve). This is called conventional current flow. The problem is, electrons are negatively charged, so they want to get away from the -ve and go to the +ve. So if electrons are going left to right, you say that the current is going right to left. Circuits An ammeter needs to measure the flow of charge, so it is in series. This means that all the charge has to flow through it and can be counted. It also means that an ammeter needs to have a very low resistance. A voltmeter measures voltage across a component, which you may have heard as potential difference. This means it is in parallel and it also needs a high resistance (otherwise all the current would flow through the meter instead f the component). Series Circuits Current in series: same all the way round (all the current has to flow through everything). Voltage in series: voltages across each component add up to the total voltage supplied by the battery, as they have to share the voltage between them [(A) = (B) + (C) in the diagram]. Higher resistances will need more of the volt age. Final point – resistors in series: To work out the total resistance of two resistors, just add them together. This is because the current has to go through both of them. Parallel Circuits Voltage in parallel: all voltages the same. Current in parallel: the current is shared out between the branches, but recombines near the battery. In the diagram (A) = (B) + (C) = (D). How much current each branch gets depends on the individual resistors – bigger resistance = lower current. Resistance in parallel: you don't normally have to work out numbers, but the rule of thumb is that the total resistance of two resistors in parallel is less than the lowest individual resistor. Circuit Symbols Cells and Batteries: strictly speaking one cell represents 1. 5V, but of you write the voltage above it (e. g. 6V'), most people will understand the cell has 6 volts. Power Supplies: come in all shapes and sizes; just label them as you want. Switches: several types, I've shown the main two that you will come across Lamps/Bulbs: either symbol could be used – it doesn't matter. Resistors: a few types – Fixed, Variable (you can change the resistance), Thermistor (as it gets hotter, its resistance decreases) and Light Dependent Resistor or LDR (the more light that shines on it, the lower its resistance gets). Diode: A diode is like an electrical valve, it only lets current flow one way. If it is connected with the arrow pointing to the negative terminal, current can easily flow, if it is the other way round, it will block the current. A LED or Light Emitting Diode is just the same except it gives off light†¦ Ohmmeter: is connected directly to a resistor, of any kind, to find its resistance (no other circuit is used with it) Check in your syllabus to see if there are anymore you need to know! Know Your Formulae Ohm's Law The law actually says that the resistance of a metal conductor is the same whatever the current – unless it's getting hotter. However most people think of these equations when the law gets mentioned: V=IR and so on, Voltage (V) in Volts, Current (I) in Amps and Resistance (R) in Ohms. Charge (Q) in Coulombs, Time (t) in seconds and Power (P) in Watts. Always remember to show all your working out, including writing the formula properly (not just the triangle! ) and checking your units (e. g. check for mV or kW instead of V or W) Prefixes: These are little letters added to units to make them a different size, but always use the base unit if unsure. Base units are given in the topics, the ones to watch for are time (seconds) and mass (kilograms not grams). |Prefixes: |Name |Value: |Example: | |M |Mega |x 1,000,000 | 1MW = 1,000,000W | |k |kilo |x 1,000 |1kg = 1,000g | |c |centi |? 100 |1cm = 0. 01m | |m |milli |? 1,000 |1ms = 0. 001s | Mains Supply (AC and DC) We use two main sorts of electrical supplies, DC and AC. DC – This is Direct Current. The current flows in one direction only and has a consistent value. Provided by batteries or DC adaptors/transformers that plug into the mains supply. AC – This is Alternating Current. The current flows first one way then the other at a frequency of 50Hz. AC is what comes out of the mains sockets, usually at around 240V. The Ring Main This is the name given to the circuit in your home. You only need to know that it is a parallel circuit and that the lighting circuit is separate from the circuit for sockets. The National Grid This is the circuit that carries electricity all around the country, from the power stations to homes and businesses. Producing the Power Energy is produced by burning fuel which turns water to steam, this drives a turbine, which make electricity via a generator. This electricity is a very high voltage and is passed over the National grid to a step down station then passed straight to your home. Why the High Voltage? High voltage is used over the National grid, to keep current low. This stops energy being wasted. Energy and the Cost Kilowatt-hours (kWh) The kilowatt-hour is the common unit used by energy companies to measure electricity. This is a unit of energy not power or time. It is the amount of energy if a 1kW appliance was left on for 1 hour. The Cost 1kWh of electrical energy costs around 6p, though it may change depending on your supplier. So multiplying the number of Kilowatt-hours you use by the unit cost (approx 6p), give you the total cost of the electricity you use. Safety A common question is to give you a picture of domestic bliss and get you to identify the hazards, such as the person sticking their fingers in the toaster. Things to look for are: . bad wiring, 2. water near appliances, 3. too many double plugs/adaptors, 4. Frayed wires. Just use your common sense and you should get some easy marks! Wiring a Plug One big problem used to be wiring plugs. By law now, all new appliances are fitted with one already, which helps, but you do need to know what's going on inside there Fuses Fuses help protect the circuit aga inst faults. The key thing is to get the wire just thick enough to carry the current you want, but thin enough to melt if there is a current surge. Fuse Ratings Common sizes are 3, 5 and 13Amp fuses, but there are many others. Always choose one slightly higher than the current rating of the appliance, so that it doesn't blow under normal conditions. Circuit Breakers Fuses are not always effective at protecting you, so circuit breakers are also used. They automatically compare the current entering and leaving the circuit and even if there is the tiniest difference they ‘trip' off. Earth The Earth (yes, I do mean our planet) is very good at soaking up loose charge. The earth in your house is probably connected to the plumbing (goes to ground) or a large metal spike in the ground somewhere. Double Insulation If something is completely cased in an insulator, like plastic, it is said to be double insulated, and does not need earthing. You can't get a shock from the case! Atoms are made up of: †¢ protons; †¢ electrons; †¢ Neutrons. Protons and neutrons are in the nucleus and the electrons orbit the nucleus. Protons have a positive charge, electrons have a negative charge and neutrons have no charge. The shape of the atom was discovered using the alpha-scattering experiment. This showed the original plum-pudding model to be wrong! Atomic notation is used to describe atoms. The top number is the mass or nucleon number. It tells us how many protons and neutrons there are in the nucleus. The bottom number is the proton or atomic number, which tells us how many protons are in the nucleus. During reactions the total number of protons and neutrons must stay the same. Isotopes of an element have the same number of protons but a different number of neutrons in the nucleus. It is this different number of neutrons that makes some isotopes unstable and radioactive. These isotopes are called radioisotopes. Ionisation is where an electron is removed from a neutral atom, leaving the atom with a positive charge. Radiation causes ionisation. This can be used to detect radiation, as the amount of ionisation can be measured with a Geiger-Muller tube. Ionisation can damage or kill living cells, this can cause cancer to develop. Alpha particles, beta particles and gamma waves are the three main types of radiation emitted during radioactive decay. All three types of radiation are emitted from the nucleus of the atom. When radiation is emitted the unstable atom loses energy to become more stable. If alpha or beta particles are emitted, new elements are formed because of the change in the number of protons in the nucleus. Alpha, beta and gamma radiation all behave slightly differently due to the way they are made up. Alpha ionises the most over a small distance but is not very penetrating. Gamma is the most penetrating but ionises less over the same distance. Decay equations can be used to work out what new daughter element will be produced when radioactive decay takes place. Safety precautions must be taken when handling radioactive substances. These include, using long handled tongs, pointing sources away from people, wearing lead lined clothing, not inhaling or eating sources. The half-life of a substance is the time it takes for half of the original parent atoms to decay. It is also the time it takes for the count rate of a substance to fall to half the original value. Radiation is used in medicine to cure cancer, in industry to detect the thickness of materials and in dating. Background radiation is radiation that is produced around us all of the time. Sources include certain rocks, cosmic radiation, radon gas in the air, nuclear waste and experiments, medical uses and some foods. The background radiation needs to be subtracted from experiment results on radioactivity.