Beatty 6091 Electromagnetic induction teacher

1 A) Faraday’s experiment Experiment to show that a changing magnetic field can induce an e.m.f. in a circuit 1. Set up the apparatus as shown in the diagram. 2. A permanent magnet is inserted into a solenoid connected to a sensitive galvanometer. 3. The galvanometer needle deflects as the permanent magnet move within the solenoid. 4. This shows that e.m.f. can be induced when there is a changing magnetic field. 5. This process is called electromagnetic induction. Note: Both e.m.f. and current are induced in a conducting wire when it is put in a changing magnetic field. So when e.m.f. is said to be induced in a coil, it should be understood that current is also induced at the same time. Definitions 1. Electromagnetic induction is the process through which an induced e.m.f is produced in a conductor due to a changing magnetic field. 2. Induced current is a current produced in a conducting wire due to changing magnetic field. Beatty Secondary School Secondary 4E Pure Physics Electromagnetic Induction Name: __________________________ ( ) Class: __________ Date: _____________ Learning Outcomes (a) deduce from Faraday’s experiments on electromagnetic induction or other appropriate experime nts: (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 co il 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 an d 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 operati on 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

2 Factors affecting the magnitude and direction of the induced e.m.f. Procedure Galvanometer response Insert North pole of 1 bar magnet into solenoid Insert North pole of 1 bar magnet into solenoid at twice the speed Insert North pole of 2 bar magnets into the solenoid Insert North pole of 1 bar magnet into a solenoid with twice the number of turns From the above process, we deduce that the strength or magnitude of the induced e.m.f. are affected by: 1. The speed with which the magnet is inserted into or withdrawn from the solenoid. 2. The magnetic field strength of the magnet. 3. The number of turns in the solenoid. Determine the direction of induced current during electromagnetic induction Procedure Galvanometer response Insert North pole of 1 bar magnet into solenoid Withdraw the same bar magnet out of solenoid Insert South pole of 1 bar magnet into solenoid Withdraw the same bar magnet out of solenoid Solenoid move towards North pole of magnet Solenoid withdraw from the same magnet Laws of electromagnetic induction 1. Faraday’s Law of electromagnetic induction states that the magnitude of the induced e.m.f in a circuit is directly proportional to the rate of change of magnetic flux in the circuit.

3 2. Lenz’s Law states that the direction of the induced e.m.f and hence the direction of induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it. Practice 1. The diagrams below show the set-up for which a short bar magnet is dropped through a coil of wire. Which row correctly indicates the direction of the induced current via the load R in the circuit? Magnet entering the coil Magnet leaving the coil A X to Y X to Y B X to Y Y to X C Y to X X to Y D Y to X Y to X

4 2. A small coil is connected to a galvanometer as shown. When the magnet is allowed to fall towards the coil, the galvanometer pointer gives a momentary deflection to the right of the zero position. The magnet moves through the coil and, as it falls away from the coil, the galvanometer pointer ………… . A gives a continuous reading to the left. B gives a continuous reading to the right. C gives a momentary deflection to the left. D gives a momentary deflection to the right. 3. The S pole of a bar magnet is pushed into a solenoid, as shown in the diagram. An electromotive force is induced which causes the induced current to flow from X to Y through the galvanometer. Which action, using the same end P of the solenoid, would produce an induced current flowing in the same direction? A Pulling a S pole out of the solenoid B Pushing a N pole into the solenoid C Pulling the solenoid away from a N pole D Pulling the solenoid away from a S pole

5 4. The diagram shows a metal bar swinging like a pendulum across a uniform magnetic field. The motion induces an e.m.f between the ends of the bar. Which graph represents this e.m.f during one complete oscillation of the bar, starting and finishing at P? 5. The figure below shows a solenoid and a permanent magnet. a) On the figure, draw the induced current flowing in the solenoid owing to the presence of the induced e.m.f when the magnet dropped into the solenoid. b) Explain why there is an induced e.m.f in the coil. There is an induced e.m.f because there is a change in the magnetic flux inside the coil as the magnet enters and exits the coil.

6 c) The magnet is dropped through a solenoid. Sketch a graph of the current reading against time on the axes below. B) Alternating Current Generator A generator is an electromagnetic device which tra nsforms mechanical energy into electrical energy . (Output: Alternating Current). How it works? 1. By turning the axle, the coil is made to rotate between the poles of a permanent magnet. 2. As the coil rotates, the magnetic field through the coil changes. 3. E.m.f or current is induced between the ends of the coil. 4. The induced current does not flow until the ends of the coil are connected to an external circuit with an electrical load (resistor). What is the use of the slip ring? The slip ri ngs ensure that the direction of the induced current flowing in the external circuit changes every half-revolution. The output current becomes alternating (AC). What is the use of carbon brushes? They provide electrical contacts between the rotating slip rings and the external circuit. current t Magnet completely inside solenoid (no change in magnetic flux) Magnet moving towards solenoid (change in magnetic flux increasing) Magnet moving away from solenoid at faster speed (larger change in magnetic flux lead to larger e.m.f)

7 Fleming’s Right-hand rule Given the direction of the magnetic field and force, t he direction of the induced current can be found using Fleming’s right-hand rule. Practice Use Fleming’s right- hand rule to determine and indicate the direction of the induced current on the diagrams below. a) b) c) d) e) f) Wire moving down S N B A Wire moving down N S B A Wire moving up N S B A S N A B Wire moving out of the paper Wire moving up S N B A S N A B Wire into of the paper

8 C) Output graph of an AC Generator (Induced e.m.f. versus time) Interpretation of the output graph 1. Maximum output (E0 and -E0) always correspond to the time when the coil is parallel to the magnetic field. This is becau