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Physics 2b Summary Sheet Items in Itallic are HIGHER TIER only P2.3.1 Static electricity Static electricity can be explained in terms of electrical charges. When electrical charges move we get an electric current. a) When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material 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 bodies are brought together they exert a force on each other. d) Two bodies that carry the same type of charge repel. e) Two bodies that carry different types of charge attract. f) 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 charge. The size of the current is given by: I = Q / t I is current in amps (A); Q is the charge in coulombs (C) b) The potential difference (or voltage, it doesn’t matter which term you use) between two points in a circuit is the work done (energy transferred) per coulomb of charge that passes between the points. V = W / Q V is potential Difference in volts (V); W is work done in joules (J) c) The size of the current in a circuit depends on how hard the supply tries to push charge through the circuit and how hard the circuit resists having charge pushed through it. d) Circuit symbols which you have to learn You should understand the uses of LDR’s e.g. switching on lights when it gets dark and the uses of thermistors in thermostats. Current-potential difference graphs are used to show how the current through a component varies with the potential difference across it. Resistor at constant temperature Filament lamp Diode The resistance of the filament lamp increases as the lamp gets hot. Be able to explain resistance change in term s of ions and electrons A diode only lets current pass one way. It has a very high resistance in the opposite direction e) The resistance of a component can be found by measuring the current passing through, and potential difference across, the component. f) The current through a resistor at a constant temperature is directly proportional to the potential difference across the resistor. g) You should be able to calculate current, potential difference or resistance using: potential difference = current × (volt, V) (ampere, A) resistance (ohm, Ω) V = I x R h) The current through a component depends on its resistance. The greater the resistance the smaller the current for any given potential difference. i) The potential difference provided by cells connected in series is the sum of the potential difference of each cell (as long as they are connected the correct way.) Series Rules (OK) 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. Parallel Rules (OK) • 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. j) An LED emits light when a current flows in the forward direction. LEDs are being used more and more, they are small, don’t get as hot and are cheaper to run (but more expensive to buy) than traditional filament bulbs k) The resistance of an LDR (light dependent resistor) decreases as light intensity increases. l) The resistance of a thermistor decreases as temperature increases. P2.4.1 Household electricity a) Cells and batteries supply current which always passes in the same direction. This is called direct current (d.c.). b) An alternating current (a.c.) is one which 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 about 230 volts. d) You should be able to calculate the potential difference for Ac and DC supplies form an oscilloscope trace. e) The period of the ac supply can be seen on the oscilloscope, it is the distance between each of the crests or troughs. The frequency of the ac supply is 1/(the period) f) Most electrical appliances are connected to the mains using cable and a three-pin plug. You should be familiar with two and three core cable, the materials used to make a plug and the colour coding of the wires. Structure of a cable Wiring of a 3 pin plug g) If an electrical fault causes too great a current the circuit should be switched off by a fuse or a circuit breaker RCCB. h) When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit. i) Residual Current Circuit Breaker (RCCB) works by detecting a difference in the current in the live and neutral wire. It works much faster than a fuse. j) Appliances with metal cases are usually earthed. Some appliances are double insulated and so do not need an earth wire. k) The earth wire and fuse together protect the appliance and the user. The size of cable and fuse need to be matched to the amount of electricity an appliance uses. Thicker cables can take more electricity. The fuse should be the next value higher than the current the appliance uses. P 2.4.2 Current, Charge and Power a) Electric current is the rate of flow of charge. b) When an electrical charge flows through a resistor, electrical energy is transformed into heat energy. Modern energy saving bulbs waste less energy as heat. New appliances are now rated to show how much energy they use. The rate at which energy is transformed in a device is called the power. power = energy transformed (joule, J) (watt, W) time (second, s) Power, potential difference and current are related by the equation: power = current × potential difference (watt, W) (ampere, A) (volt, V) HIGHER TIER ONLY Energy transformed, potential difference and charge are related by the equation: Energy Transformed= Potential Difference x Charge (Joule) (Volt) (Coulomb) E = V x Q P2.5.1 Structure of the atom (You may have covered this in Chemistry as well) a) An atom has a small nucleus containing protons and neutrons surrounded by electrons. The nucleus is very small in relation to the whole atom. Particle name Relative Mass Relative (how their masses compare charge with each other) Neutron 1 0 Proton 1 +1 Electron Very small -1 b) In an atom the number of electrons is equal to the number of protons in the nucleus. The atom has no net electrical charge. c) Atoms may lose or gain electrons to form charged particles called ions. d) All atoms of a particular element have the same number of protons. Atoms of the same element which have different numbers of neutrons are called isotopes. e) The total number of protons in an atom is called its atomic number. f) The total number of protons and neutrons in an atom is called its mass number. g) We used to think that atoms were like a plumb pudding where the negative charges are trapped in a ‘pudding mixture of positive charge. h) The Rutherford and Marsden scattering experiment led to the plum pudding model of the atom being replaced by the nuclear model. Electrons were fired at a leaf of gold, and every so often one would be reflected back, most would go straight through. This led to the idea of most of the atom being empty space. P2.5.2 Atoms and Radiation a) Some substances give out radiation from their nucleus all the time. This is because the nucleus is unstable and has to release energy. These substances are radioactive.The process is random. (It is all related to the nucleus and so is not “Chemistry”) b) There is always background radiation present. This comes from some rocks and cosmic rays as well as man-made sources. c) There are three types of radiation. d) An alpha particle is 2 neutrons and 2 protons (same as a helium nucleus) e) A beta particle is an electron from the nucleus f) Gamma radiation is part of the electromagnetic spectrum. g) You should know what alpha, beta and gamma are made up from, know what their range is in air, and their penetrating powers. h) Alpha and Beta particles are deflected in opposite directions by electric and magnetic fields because they have opposite charges. Beta particles are deflected most because they are smallest. Gamma are not deflected because they have no charge. i) You should be able to understand alpha and beta decay equations and balance them in terms of atomic and mass number j) The half-life of a radioactive material is the time it takes for half of the nuclei to decay, or the time it takes for the count rate to go down to half its original value. It is a constant. You should be able to select an appropriate radioactive source for a job, from a list, by choosing a suitable type, strength and half-life. Be able to give uses and dangers associated with each kind of radiation. Nuclear fission is the splitting of an atomic nucleus. There are two fissionable substances in common use in nuclear reactors, uranium 235 and plutonium 239. P2.6.1 Nuclear Fission For fission to occur the uranium 235 or plutonium 239 nucleus must first absorb a neutron The nucleus undergoing fission splits into two smaller nuclei and 2 or 3 neutrons and energy is released. 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. The early universe only contained hydrogen but fusion has made more elements. 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 a star is stable because the forces within it (gravity and the explosion of the reactions) are balanced. e) A star goes through a life cycle. This is determined by the size of the star. Fusion processes in stars produce all the naturally occurring elements throughout the universe by the explosion of supernova stars at the end of their life.