Induction and Electricity Generation

Outline Michael Faraday’s discovery of the generation of an electric current by a moving magnet

Following O¨ersted’s discovery that moving charge produced a magnetic field, Faraday wondered if the reverse was true- if a moving magnetic field could produce electromotive force (EMF). Faraday initially assembled a crude transformer, running a current through a primary coil and checking for current in a secondary coil that was immersed in the magnetic field produced by the primary coil. Faraday found that when the primary coil was connected and disconnected to a battery, the galvanometer needle moved slightly, but the effect was only momentary. Faraday then took an iron ring and wound a wire around it. When he moved a magnet in and out of the ring, a galvanometer showed a constantly changing current. From this, Faraday concluded that a moving magnet could be used to generate electric current.

figure 18

Remember- Faraday built a DC transformer, but it did not work as transformers require AC current for operation. He then moved a magnet in and out of a wire loop and detected a current.

Perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet (including ”Plan, choose equipment or resources for, and perform a first hand investigation to demonstrate the production of an alternating current”)

Note that this experiment addresses two dot points- creating an electric current by moving a magnet, and producing an alternating current.

In this experiment, we moved a magnet into a hollow coil. The coil was a length of thin wire wrapped around a cardboard cylinder, with around 200 turns. The coil was connected to a galvanometer so that current passing through the coil would be detected. We attached a strong ceramic magnet to the end of a small wooden stick, and then moved the magnet in and out of the coil repeatedly. This causes the coil to experience changing flux, and therefore a current was induced, which was detected by the galvanometer. In this way we modelled the generation of an electric current. Further, because we moved the coil in and out, in both directions, the current produced was an alternating current. This was shown by the needle on the galvanometer oscillating between positive and negative readings, showing the current changing direction.

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, and the relative motion between the coil and the magnet is varied

This experiment used the same setup as that described previously, with a ceramic magnet on a stick being moved in and out of a coil attached to a galvanometer. To test the effect of distance, we moved the magnet with a constant amplitude and period, while changing the centre of motion. When the centre of motion was at the very edge of the coil, so that the magnet passed in and out of the coil, current flow was maximised. When the centre of motion was the middle of the coil, some current was produced but it was diminished, because the coil experienced less change in flux (since the magnet was inside the coil the whole time). When the centre of motion was outside the coil, induced current also dropped. Using a stronger magnet induced a larger current, and when we altered the relative motion between the coil and the magnet by changing the speed of the magnet, we found that the induced current was larger when the magnet was moved faster.

figure 19

Remember- Current generation is maximised with the coil and magnet close together, with a strong magnet, and with a fast relative movement between the coil and magnet.

Define magnetic field strength B as magnetic flux density

Magnetic Flux Density is another name for magnetic field strength, which is measured in Teslas or in webers per square metre. It refers to how much magnetic flux is passing through a unit area.

Describe the concept of magnetic flux in terms of magnetic flux density and surface area

 

Magnetic flux density can be thought of as a measure of how many magnetic field lines pass through a square metre. Magnetic flux is a measure of how many magnetic field lines pass through a different area. It is found by multiplying the magnetic flux density by the area in question. So essentially, magnetic flux is a measure of the amount of magnetic field passing through an area.

figure 20

Describe generated potential difference as the rate of change of magnetic flux through a circuit

According to Faraday, the voltage, the potential difference, or the emf (which all mean the same thing) is dependant on how fast magnetic flux is changed, that is, how quickly the amount of magnetic field passing through an area changes. According to Faraday’s law, ǫ = −∆Φb , where ∆Φb

is the change in magnetic flux over time t. So the generated potential difference is essentially the rate of change of flux. The negative sign accounts for the direction of the induced emf, as explained in the next dotpoint.

Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors

This is an important dotpoint that needs to be understood correctly. Make sure you thoroughly study this section and that you are able to clearly justify the existence of back emf. Also, make sure you practice writing explanations of Lenz’s Law, because it is a concept that many students find difficult to understand.

Lenz’s Law states that an induced emf always produces a magnetic field that opposes the change that produced it. This essentially means that when an induced emf is generated by moving a magnet, the induced emf will produce a magnetic field that exerts a force on the magnet in the opposite direction to the original movement of the magnet.

It comes about because of the law of conservation of energy. When a magnet is moved into a coil, the resulting current can flow in one of two directions. Each direction produces a magnetic field through the coil. This magnetic field can either attract the magnet further into the coil, or repel the magnet.

If the magnet was attracted, it would be sucked into the coil. This further movement would result in further change in flux, which would induce more emf. Effectively, the magnet would be sucked through the coil, generating energy.

However, this violates conservation of energy. If the magnet was sucked in, there would be energy output in the form of emf and charge movement, but there would be no energy input, no work done. This means that energy is being created, and according to conservation of energy, energy cannot be created nor destroyed.

So if the magnetic field induced by the magnet cannot attract it further into the coil, it must repel the magnet. In this case, in order to continue energy production the magnet must be forced into the coil- this kinetic energy input is then converted into emf, obeying conservation of energy. This gives rise to the production of back emf in motors.

As shown before, moving charge in a magnetic field experiences force. This force causes the motor to turn, and the charge movement is as a result of the supply emf. However, by Faraday’s law, changing flux induces emf. The rotating coils of a motor experience changing flux, and this causes a current to be induced in them.

According to Lenz’s law, the direction of emf must oppose the original change that produced it. In this case, it means that if the motor is spinning clockwise, the induced emf applies a force in an anticlockwise direction, slowing the motor.

Now the movement of the motor is the result of supply emf, and the supply emf causes the motor to spin in a particular direction, either clockwise or anticlockwise. As the induced emf causes torque in the opposite direction, the direction of emf must be opposite to the supply emf, hence the induced emf in a motor is referred to as back emf.

Remember- In accordance with conservation of energy, back emf always opposes supply emf in a motor.

Explain that, in electric motors, back emf opposes the supply emf

As shown before, back emf is induced in a rotating coil as found in a motor. The back emf causes torque that opposes the movement of the armature. However, the initial movement of the armature is caused by the supply emf running through the circuit in a particular direction. As back emf produces force in the opposite direction to the supply emf, which caused the armature to rotate in the original direction, the back emf must act in the opposite direction to the supply emf, therefore opposing it. The effect of back emf opposing supply emf is to reduce the net emf, thereby reducing the current. This means that a motor spinning rapidly uses far less current than a stationary motor, since the spinning motor has induced back emf that reduces the current.

Remember- Motors generate an emf while they operate, which opposes the supply emf

Explain the production of eddy currents in terms of Lenz’s Law

Eddy currents are formed when there is a change in flux and therefore induced emf, but no conducting path in which electrons can flow. In this case, rather then flowing around in a circuit the electrons flow around in a circle. This circular movement of charge is an eddy current. When there is a change in flux, emf is induced, and according to Lenz’s Law this emf must oppose the original movement that created it. This means that the charge experiencing the field produces a force that opposes the original movement of the conductor. As the charge moves, it gradually forms into a circular flow of electrons. This circular flow of electrons produces a magnetic field that will repel the original magnetic field that induced the eddy currents.

Remember- Eddy currents form according to Lenz’s Law.

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

Induction cooktops use electromagnetic induction to produce heat. Beneath the cooktop is an induction coil consisting of a solenoid placed horizontally (i.e. with circular coils lying horizontally). When alternating current (AC) is passed through the coil, a magnetic field is formed that is aimed straight up through the cooking surface. However, AC is fed into the induction coil, and this causes the magnetic field formed to constantly change. This means that when a saucepan or pot is placed above the induction coil, it experiences changing flux. This causes eddy currents to be formed in the pan, which results in the pan heating up due to electrical resistance. An induction cooktop is much more efficient at converting electrical energy to heat because the pan is heated directly, rather than indirectly.

Remember- Induction cooktops use induction from a coil beneath the cooktop to heat pans.

Gather secondary information to identify how eddy currents have been utilised in electromagnetic braking

Electromagnetic braking is mainly used in trains, where it can silently and efficiently slow down the train without the noise and wear and tear of conventional friction braking. With electromagnetic braking, an electromagnet is placed underneath the train. When turned on while the train is moving, it causes the metal rails below it to experience a change in flux. According to Faraday’s law, this causes eddy currents to be induced in the rails. According to Lenz’s law, these eddy currents must have a magnetic field that opposes that change which produced them. This means that the eddy currents that form repel the electromagnet as it moves, exerting a force opposite to the direction it is moving in. This is therefore a braking force, acting against the movement of the train. By inducing eddy currents in tracks, trains can slow down. The other advantage is that because emf is dependant on rate of change of flux, when the train is travelling quickly there is a greater production of emf than when it is travelling slowly. This means that electromagnetic braking is most efficient at high speeds which are when it is needed most, because it is at high speeds that the most noise and heat are produced by frictional braking, which also results in wear and tear on the braking system. Alternatively, a magnetic field can be applied directly to the wheels of the train by electromagnets placed near the metal wheels. This also provides a braking effect.

Remember- Magnets mounted on a train are used as electromagnetic brakes.