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SUMMARY Module 12: Electricity and Magnetism • Ferrous materials are attracted by magnets and are made from iron, steel, cobalt or nickel. • Like poles of magnets repel and unlike poles attract. • You can destroy a magnet by heating it or by continually dropping it. • Iron is easily magnetized but looses it magnetism quickly. • Steel is permanently magnetized. • Induced magnetism is the effect when a ferrous material can become a magnet when it is in the magnetic field of a magnet. • The magnetic field direction is from north to south. • An electromagnet can be made by putting an iron nail in a coil of wire which is connected to d.c. or a.c. supply. • Electromagnets can be used in scrap yards, electric bells and relay switches. • Permanent magnets can be used for loudspeakers, refrigerator doors and the removal of magnetic objects from people’s eyes. • Static electricity is produced by friction e.g. when rubbing a rod with a cloth. • Like charges will repel each other and unlike charges will attract each other. • Electric charge is measured in Coulomb (C). • The real charge on an electron is 1.6 x 10-19 C. • An electroscope is an instrument which can be used to detect small amounts of electric charge. • The direction of an electric field line shows the path that would be taken by a positive charge which was free to move in the field. • Electrostatics can be useful in photocopiers and electrostatic precipitators in chimneys. • A Van der Graaf generator is a device that generates static electricity continuously to the inside of a hollow metal dome by means of a rotating rubber belt. • Lightning is likely to hit the highest point of the ground, or it may strike a tall building or tree. • A building can be protected against lightning by a lightning conductor. • Electric current is the rate of flow of charges and can be given by the formula • The unit used for current is Ampere (A) and is measured with an ammeter. • Electromotive force refers to the energy supplied by a source in driving charge round a complete circuit. Emf is measured in volts. • Potential difference is the difference in electrical potential between two points through which a current flows and is also measured in volts with a voltmeter. • Resistance is the resistance to the flow of electric current and is measured in ohms (Ω). • Ohm’s Law states that the current flowing through a metallic conductor is directly proportional to the potential difference across its ends provided that the temperature remains constant. • The resistance of a wire depends on: 1. Length. 2. Cross section area. 3. Temperature. The Institute of Information Technology, 2008 SUMMARY Module 12: Electricity and Magnetism 4. Type of material. • In a parallel circuit the voltages across all the branches are the same. • The sum of the voltages in the external part of the circuit is equal to the voltage across the cell. • The current in a series circuit is the same everywhere. • The total resistance for the resistors in series is the sum of the resistance of each. • The current from the source in a series circuit is split up in the branches. 1 1 1 = + R R1 R2 • The total resistance for the resistors in parallel is calculated by • A diode can be used to change alternating current to direct current. • A capacitor is a device which is used to store electric charge. • The measure of the extent to which a capacitor can store charge is called capacitance. • Capacitance = • Capacitance is measured in farads (F). • Power = Current x Potential Difference • The unit of power is Watt (W). Ch arg e Potential.difference • P = I2R • Electrical energy • E = I2Rt P= C= Q V P = IxV V2 R E = IVt E= V 2t R • Electromagnetic induction is the process of inducing a voltage when a wire is moved through a magnetic field cutting the magnetic field at a right angle. • The size of the induced e.m.f. depends upon: a. The speed at which the wire cuts the magnetic field b. The number of wires or coils cutting the magnetic field c. The strength of the magnetic field • Lenz’s Law states that the direction of the induced e.m.f. or current is such as to oppose the change that is causing it. • An a.c. generator is a generator which produces electrical current in the form of alternating current. • Root mean square value (r.m.s) is the value of an alternating current which would give the same electrical power as a similar d.c. value. • A d.c. generator produces electrical current in the form of direct current. • A step-up transformer has more turns on the secondary coil and is used to increase the voltage. • A step-down transformer has less turns on the secondary coil and is used to lower the voltage. The Institute of Information Technology, 2008 SUMMARY Module 12: Electricity and Magnetism VS N s = Vp N p I pV p = I sVs • • The advantages of high voltage transmission are: a. A small current at high voltage delivers the same power as a large current at low voltage. b. At a smaller current, the heating effect is less and therefore less power is lost. For calculating power loss use P = I 2 R The d.c. motor changes electrical energy to kinetic energy. • The size of the force can be increased by: a. Increasing the strength of the magnetic field b. Increasing the size of the current c. Increase the number of turns on the coil • Thermionic emission happens when a metal is heated and it gives off electrons. • A beam of electrons coming from the cathode is called a cathode ray. • The X-plates are used to deflect the beam from side to side and the Y-plates deflect the beam up and down. • The beam is deflected whenever there is a potential difference across a set of plates. The Institute of Information Technology, 2008