(Contact: 9855 9224) Electricity and Magnetism: Electromagnetic Induction (*) (#) Candidates should be able to: a) deduce from Faraday s experiments on electromagnetic induction or other appropriate experiments: i. that a changing magnetic field can induce an e.m.f. in a circuit ii. that the direction of the induced e.m.f. opposes the change producing it iii. the factors affecting the magnitude of the induced e.m.f. b) describe a simple form of A.C. generator (rotating coil or rotating magnet) and the use of slip rings (where needed) c) sketch a graph of voltage output against time for a simple a.c. generator d) describe the use of a cathode-ray oscilloscope (C.R.O.) to display waveforms and to measure potential differences and short intervals of time (detailed circuits, structure and operation of the C.R.O. are not required) e) interpret C.R.O. displays of waveforms, potential differences and time intervals to solve related problems f) describe the structure and principle of operation of a simple iron-cored transformer as used for voltage transformations g) recall and apply the equations Vp / Vs = Np / Ns and VpIp = VsIs to new situations or to solve related problems (for an ideal transformer) h) describe the energy loss in cables and deduce the advantages of high voltage transmission * not in combined Science syllabus # not in N level Science syllabus 1
(Contact: 9855 9224) Faraday s Law of Electromagnetic Induction Lenz s Law The e.m.f. induced in a conductor is proportional to the rate of change of magnetic lines of force linking the circuit. Define electromagnetic induction. Electromagnetic Induction is a process in which an electromotive force (e.m.f.) is induced in any conductor whenever there is a change in the magnetic field. Formula to calculate induced emf on a conductor moving perpendicular to a magnetic field emf = Blv The direction of the induced e.m.f. and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing the induced e.m.f. Factors affecting the magnitude of the induced emf: Number of turns of wire in the coil The greater the number of turns of wire in the coil, the stronger the induced emf The strength of the magnetic field The greater the strength, the stronger the induced emf The rate of magnetic field change The faster the change in magnetic field, the stronger the induced emf where: emf = induced electromotive force (V) B = magnetic flux density at the position of the charge l = length of the conductor v = velocity of the motion Using Fleming s Right Hand Rule to determine direction of induced current 2
Experiment to demonstrate Faraday s Law (Contact: 9855 9224) Experiment to demonstrate Lenz s Law 3
Describe how a rotating coil A.C. generator works (Contact: 9855 9224) Functions of Slip Rings The slip rings make electrical contact with the coil at all times and rotate together with it. It leads the induced current out of the coil to the external load. Functions of Carbon Brushes The carbon brushes provide a flexible, conducting surface that will be constantly in contact with the slip rings as the rings rotate, leading the induced current from the rings to the external load. 4
Graph of Voltage Output against Time (Contact: 9855 9224) 5
Describe how a fixed coil AC generator works (Contact: 9855 9224) A fixed coil a.c. generator is made by fixing the coil and rotating the magnet round it. In this case, the rotating magnetic field cuts the coil to produce an induced e.m.f. Slip rings and carbon brushes are not used as the output terminals of the coil are fixed. Advantages of Fixed Coil A.C. Generator over the rotating coil A.C. Generator Carbon brushes wear and tear easily and need to be replaced frequently, resulting in increased cost The connection with the slip ring becomes loose when the carbon brush is eroded. A loose connection in a circuit increases the resistance at the connecting point, which causing unnecessary heating. Besides wasting energy, the heat generated might cause the generator to break down. The fixed coil a.c. generator is more compact and space-saving (eg. bicycle dynamo) 6
(Contact: 9855 9224) Describe the use of a cathode-ray oscilloscope (c.r.o.) to display waveforms and to measure potential differences and short intervals of time The cathode-ray oscilloscope (C.R.O.) is commonly used to show how a voltage varies with time. The C.R.O. works by deflecting a beam of electrons in an electric field. It consists of: - a vacuum glass tube containing an electron gun - a system of deflecting plates (Y-plates and X-plates) and - a fluorescent screen The electron gun in the C.R.O. emits a beam of electrons (known as thermonic emission) which is also referred to as a cathode ray because the beam comes from the cathode. In the C.R.O., when a beam of electrons strikes the fluorescent screen that is coated with zinc sulfide, it creates a bright dot. The number of electrons reaching the screen will determine the brightness of the dot. By varying and controlling the voltage across the X-plates, the electron beam can be made to sweep horizontally across the screen at different speeds. The voltage to be studied is usually applied across the Y- plates to vary the vertical position of the electron beam. By adjusting the controls at the front panel of the C.R.O., the trace of how a voltage varies with time is displayed. The trace is similar to a voltage-time graph where the y-axis gives the voltage and the x-axis the time. The C.R.O. is turned on before connecting it to the voltage to be studied. The X-shift and Y-shift knobs at the front panel are used to position the trace at the centre of the screen. Besides the X- and Y-shift controls, there are two other parameters to consider in order to get a proper waveform on the C.R.O. screen. Y-gain: This amplifies the Y-deflection. Amplifying circuits are built into the C.R.O. so that small input voltages are amplified before they are applied to the Y-plates. Time-base: This controls the speed at which the electron beam sweeps across the screen horizontally from the left to right. This is done by altering the frequency of the time base an internal circuit that applies a changing voltage to the X-plates. 7
(Contact: 9855 9224) Uses of C.R.O. Displaying waveforms of an input voltage. Measuring short intervals of time. Measuring voltage. Displaying waveforms of an input voltage f = 1 T Measuring Short Intervals of Time 8
(Contact: 9855 9224) Da ny a l Ed u ca tio n Measuring Voltage 9
(Contact: 9855 9224) A closed-core transformer consists of two coils of wires, the primary and secondary coils, each with an appropriate number of turns. These coils are wound round a laminated soft iron core which consists of thin sheets of iron insulated from each other by a coat of lacquer. The lamination of the soft iron core reduces heat loss due to induced eddy currents. Ed u ca tio n Describe the structure of a simple iron-cored transformer as used for voltage transformations Describe the principle of operation of a simple iron-cored transformer as used for voltage transformations l The transformer transfers electrical energy supplied from the primary coil to the secondary coil by electromagnetic induction. At the primary coil, the applied alternating voltage sets up a changing magnetic field which induces an e.m.f. in the secondary coil. ny a Applications of a Transformer A transformer is a device that changes a high alternating voltage (at low current) to a low alternating voltage (at high current), and vice versa. It is an electrical device used for: o Electrical power transmission from power stations to households and factories, and o Regulating voltages for proper operation of electrical appliances, eg. the television and CD player. Da 10
(Contact: 9855 9224) Formulas involving Step-up/Step-down Transformer V V = N N ca Transformers can only change the voltage, they cannot change the power of the supply If a transformer is 100% efficient, the total power of the supply in both coils is conserved. For a 100% efficient transformer, we can determine the voltage and current in both coils: Power in primary coil = Power in secondary coil Ed u VpIp = VsIs ny a l where: Vp: primary voltage Vs: secondary voltage Ip: current in primary coil Is: current in secondary coil Da n There are two types of transformers: o Step-down transformer: N > N or primary voltage > secondary voltage o Step-up transformer: N < N or primary voltage < secondary voltage The magnitude of the voltage transformation between the primary coil and the secondary coil can be calculated by using: where: Vp: primary voltage Vs: secondary voltage p p Np: turns in primary coil s s Ns: turns in secondary coil tio 11
Describe the energy loss in cables. Ed u ca tio n (Contact: 9855 9224) l When electricity is delivered over long distances, some of the electrical energy will be lost as heat energy due to the resistance in cable lines. ny a Advantages of High Voltage Transmission Transmitting electricity at high voltage, low current, means energy loss due to resistance in the cables is reduced. Output Power = V x I, hence I = Power / V Power lost as thermal energy: o Power loss = I2R = (Pout/V)2R Hence, the greater the value of V, the smaller the power loss. Electrical power can be transmitted more efficiently at higher voltages and lower currents. Da 12