2 The relative motion between a conductor and

2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage

Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet 1820 - Oersted - Electric current produces a magnetic field 1821 - Faraday - current carrying conductor in a magnetic field experiences a force 1831 - Faraday - electric current or emf can be generated through the use of a magnetic field

Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet 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) G 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 His experiment with coils on opposite sides of an iron ring produce even more noticeable currents when switching off and on G When he placed a needle in a secondary coil, he found that the needle became magnetised M. Edwards 6/5/02 WHEN THE MAGNETIC FIELD OF THE PRIMARY COIL IS CHANGING, A CURRENT IS INDUCED IN THE SECONDARY

Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet N G Perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet Jacaranda Experiments 7. 1 & 7. 2

Define magnetic field strength B as magnetic flux density Magnetic flux, F, is the amount of magnetic field that passes through an area Describe the concept of magnetic flux in terms of magnetic flux density and surface area M. Edwards 23/5/02 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: M. Edwar ds 23/5/02 Magnetic flux is measured in Webers (Wb) ‘Magnetic field strength’ or ‘magnetic flux density’ is measured in Tesla (T)

Describe generated potential difference as the rate of change of magnetic flux through a circuit Review Text P. 137 -138

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

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).

Gather, analyse and present information to explain how induction is used in cooktops in electric ranges 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.

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.

Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors 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. 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. G I current will I stop I when I the ring has completely left the field

Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors 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. . . . 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 A rotating coil in a motor also sets up a current that opposes the motors, back emf opposes 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.

Explain the production of eddy currents in terms of Lenz’s Law 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? USE YOUR RIGHT HAND GRIP RULE!!! POINT THE FIELD INTO THE PAGE!! Which way will it go? CLOCKWISE!

Gather secondary information to identify how eddy currents have been utilised in switching devices and electromagnetic braking Review Text P. 141 “GATHER”

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