Electromagnetic Induction

It is the production of an emf by a changing magnetic field. What is electromagnetic induction? It is the production of an emf by a changing magnetic field. Faradays Law of electromagnetic induction The induced emf in a conductor is directly proportional to the rate of change of magnetic flux through the conductor. This is the principle behind the electric generator or dynamo. What is magnetic flux? Magnetic flux φ = B X A The unit of magnetic flux is the weber.

To demonstrate electromagnetic induction (or Faradays Law)

Method Pass a magnet into a coil of wire attached to a galvanometer. There will be a deflection on the galvanometer that will change direction as the magnet is removed from the coil. Also the faster the magnet is moved the greater the deflection. Changing the pole of the magnet changes the direction of the induced current also. If the magnet is not moving there will be no current. This shows that a changing magnetic field induces an emf in a coil and this emf causes a current to flow in a closed coil.

Lenz’s Law The direction of the induced current opposes the motion or change causing it. To demonstrate Lenz’s law Drop a magnet down a copper pipe and then down a plastic pipe the same size. Note that it takes longer for the magnet to pass down the copper pipe due to the induced emf that produces a current in the copper pipe. The magnetic field due to this current opposes the motion of the magnet causing it to slow down. This demonstrates Lenz’s law.

The formula E = emf dφ = change in flux dt = change in time

Mutual Induction Mutual induction occur when the changing magnetic field surrounding one coil causes an emf to be induced in a nearby coil.

To Demonstrate Mutual Induction Place 2 coils near each other. Connect one to a battery and switch and the other to a galvanometer. When the switched is closed the galvanometer briefly deflects one way. When the switched is opened the galvanometer briefly deflects other way. One coil is inducing an emf in the other.

The transformer is a practical application of this principle

Step down transformer

The Transformer Equation NP = Number of turns in the primary Ns = Number of turns in the secondary Vin = Voltage in Vout = Voltage out

Energy Losses in the transformer Ideally Power in = Power out (VI)IN=(VI)OUT

Energy Losses in the transformer To reduce energy losses. Laminate the core-this reduces currents induced in the core The low voltage coil carries the greatest current so it should be made of thick wire which has low resistance. The core is made of iron which is easy to magnetise and demagnetise.

Self Induction The production of an emf in a coil due to its own changing magnetic field. To demonstrate self induction The neon lamp flashes when the switch is opened as the rapidly collapsing magnetic field induces a large emf in the coil. (self induction)

This trace may be shown using an oscilloscope. Alternating Current This trace may be shown using an oscilloscope.

RMS Values of AC When calculating power with AC always use the rms values not the peak values.

AC and Capacitors A charged capacitor blocks DC but conducts AC. Charging a capacitor Discharging a capacitor

AC and Capacitors A charged capacitor blocks DC but conducts AC.

AC and inductors Inductors and AC Inductors act as resistors to AC. If the power source is DC the current through a coil is greater than if the power source is AC. This is due to the induced emf that opposes the applied emf. Uses of inductors Dimming stage lighting Tuning radios