Caroline Chisholm College Physics

1. 2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage Caroline Chisholm College Physics

2. Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet Caroline Chisholm College Physics 1820 - Oersted - Electric current produces a magnetic field 1821 - Faraday - current carrying conductor in a magnetic field experiences a force The motor effect 1831 - Faraday - electric current or emf can be generated through the use of a magnetic field Electromagnetic induction

3. Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet G G Caroline Chisholm College Physics Electromagnetic induction 1831 - Faraday - electric current or emf can be generated through the use of a magnetic field Faraday wanted to see if the magnetic field from one coil would set up a current in another. Faraday wound coils of wire around wood and attached a battery (PRIMARY COIL) Then he wound another circuit of coils which was attached to a galvanometer (SECONDARY COIL) He noticed that a temporary current registered when the battery was switched on or off but no current registered while the battery was left on M.Edwards 6/5/02 THE GALVANOMETER MOVED IN OPPOSITE DIRECTIONS FOR OFF AND ON When he placed a needle in a secondary coil, he found that the needle became magnetised WHEN THE MAGNETIC FIELD OF THE PRIMARY COIL IS CHANGING, A CURRENT IS INDUCED IN THE SECONDARY CONCLUSION: His experiment with coils on opposite sides of an iron ring produce even more noticeable currents when switching off and on M.Edwards 6/5/02

4. Perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet G Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet Caroline Chisholm College Physics Faraday also showed that when a magnet was moved near a coil, an electric current was set up in the coil Opposite poles produced opposite currents N Opposite movement directions produced opposite currents Faster movements produced larger currents Jacaranda Experiments 7.1 & 7.2

5. Define magnetic field strength B as magnetic flux density M.Edwards 23/5/02 M.Edwards 23/5/02 Caroline Chisholm College Physics Describe the concept of magnetic flux in terms of magnetic flux density and surface area Magnetic flux, F, is the amount of magnetic field that passes through an area Magnetic field strength, B, is the amount of flux per unit area, or ‘magnetic flux density’ So if the same number of field lines pass through a smaller area, the magnetic field is stronger; like this: The closer (more dense) the field lines are, the stronger the magnetic field is ‘Magnetic field strength’ or ‘magnetic flux density’ is measured in Tesla (T) Magnetic flux is measured in Webers (Wb)

6. Describe generated potential difference as the rate of change of magnetic flux through a circuit Caroline Chisholm College Physics Review Text P.137-138 Faraday's Law of Induction "The induced emf in a circuit is equal in magnitude to the rate at which the magnetic flux through the circuit is changing with time"

7. Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when:- the distance between the coil and magnet is varied- the strength of the magnet is varied- the relative motion between the coil and the magnet is varied N G Caroline Chisholm College Physics Experiment using the bar magnet and the solenoid It eventually worked when the solenoid was fixed!

8. Caroline Chisholm College Physics • A student wanted to determine what factors would influence the current generated when a permanent magnet was moved inside a coil of wire as shown in the diagram below. • (a) Outline an experimental procedure that the student could use to determine what factors influence the induced current. • (b) Design a table that clearly shows and includes a quantitative sample of expected results for the experiment in part (a).

9. Caroline Chisholm College Physics

10. Gather, analyse and present information to explain how induction is used in cooktops in electric ranges Caroline Chisholm College Physics Induction heating uses changing magnetic fields to induce eddy currents which cause an increase in temperature of the conductor (e.g. saucepan), due to collisions between moving electrons and atoms, and agitation of the atoms due to rapidly changing B field. The induction coils and the saucepan are separated by a ceramic top plate The eddy currents cause the saucepan to heat up directly (which then cooks the food), without the loss of energy to the air that occurs with gas heating. Induction cookers are 80% efficient, compared to 43% for gas. Click here and find the relevant links in "motors and generators links"

11. The diagram below shows an induction cooktop. (a) Explain how the coil induces a current in a saucepan placed on the cooktop.(b) Explain how this would reduce fire hazards in the kitchen. Caroline Chisholm College Physics

12. Caroline Chisholm College Physics

13. Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors G I I I I Caroline Chisholm College Physics Lenz's Law "An induced emf always gives rise to a current that creates a magnetic field that opposes the original change in flux through the circuit" This means that if a North pole of a magnet is brought near a solenoid, current will flow in the solenoid to produce a North pole that opposes the approaching North pole. N N Or if a conducting ring is moving into or out of a field, a current will be set up in the ring that produces a magnetic field that opposes the change in flux. current will stop when the ring has completely left the field e.g. this ring has a decreasing number of field lines into the page as it moves right. So the current will be clockwise to produce more field into the page.

14. Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors Caroline Chisholm College Physics Lenz’s Law must be true in terms of conservation of energy, otherwise if a change in flux produced a flux in the SAME direction as the original change, this would produce an even greater change in that direction, producing an even GREATER change etc. etc.... The current would keep increasing as this happened, thereby CREATING ENERGY WITHOUT DOING ANY WORK...... This clearly contradicts CONSERVATION OF ENERGY: Energy cannot be created nor destroyed, but it can be transformed from one form to another To create electrical energy, work must be done - e.g. moving the magnet or the ring from the examples on the previous page Explain that, in electric motors, back emf opposes the supply emf A rotating coil in a motor also sets up a current that opposes the change in flux. This will be in the opposite direction to the supply emf, so is called back emf. The back emf increases as the motor armature speed increases until the back emf equals the supply emf. Then the armature rotates at a constant rate.

15. Explain the production of eddy currents in terms of Lenz’s Law Caroline Chisholm College Physics The magnetic fields set up by eddy currents oppose the change in flux that is occurring in that region of the conductor. For example if an aluminium disc is moving to the left out of a B field which is directed into the page..... What change is happening to the flux through the disc? A. The amount of magnetic flux into the page (through the disc) is DECREASING So how will the disc OPPOSE this change? A. A current will be induced that produces a magnetic field into the page! Which way will it go? Which way will it go? USE YOUR RIGHT HAND GRIP RULE!!! POINT THE FIELD INTO THE PAGE!! Which way will it go? Which way will it go? CLOCKWISE!

16. Gather secondary information to identify how eddy currents have been utilised in switching devices and electromagnetic braking Caroline Chisholm College Physics Review Text P.141 “GATHER” Click here and find the relevant links in "motors and generators links"

17. Caroline Chisholm College Physics The following diagram represents a disk spinning in a magnetic field. Explain what will happen to the disk and how this may be overcome.

18. Caroline Chisholm College Physics