# Classroom Demo

### Mutual induction and path dependent electric fields

This is a vivid demonstration of the concept of mutual inductance. A function generator and audio amplifier drive current through a homemade solenoid. The frequency range of interest is up to 20 KHz. Another piece of wire is wound around the solenoid, which picks up an induced emf. This emf is filtered and amplified. Both the primary and induced voltages are compared on an oscilloscope.
The real fun is when you see the induced emf grow as you keep on winding multiple turns of wire around the solenoid. You start unwinding or winding a few turns in the opposite sense, the induced emf starts to drop. When the same loop’s position is changed so that it does not enclose the primary solenoid, the induced emf drops again to zero. If the bunch of wires are placed vertically inside the primary, no emf is developed. Furthermore, change the frequency and notice the dependence of the induced emf. Last, the phase difference between the source and induced voltages is a clear demonstration of the time derivative of a sinusoid.

By Azeem Iqbal | Electricity and Magnetism

### Magnetic Braking

In this demonstration we exhibit the phenomena of magnetic braking. In the apparatus we have two metallic plates (one with slots), an electromagnet and a power supply. The metallic plate is a conductive surface which is oscillated in between an electromagnet. During this motion it will have circular electric currents called eddy currents induced in it by the magnetic field, due to Faraday’s law of induction. By Lenz’s law, the circulating currents will create their own magnetic field which opposes the field of the magnet. Thus the moving conductor will experience a drag force from the magnet that opposes its motion, proportional to its velocity and the plate gradually stops oscillating.

However, the metallic plate with slots experiences very less effects of the magnetic field and takes longer to come to a stop. This is because due to inconsistent surface it will not have circular electric currents and thus will not have the right magnetic field to oppose its movement against the field of electromagnet.

By Azeem Iqbal | Electricity and Magnetism

### Electromagnetic Induction

This demonstration exhibits the relationship between induced EMF and magnetic flux. In the apparatus, we have a rotating disk which has magnets placed on it, a Hall probe, and two solenoids with different number of turns. As the magnets are swept from beneath the sensors, the solenoids register the rate of change of flux while the Hall probe directly measures the flux. These variables are displayed on simultaneous graphs, all in real time.

By Azeem Iqbal | Electricity and Magnetism

### RC Circuit

In this demonstration we measure the voltages across the resistor and the capacitor to investigate the time needed to discharge a capacitor and determine the RC time constant of an RC series circuit. In the apparatus, we have an RC circuit, a data acquisition hardware and data displayed through a Labview program. We can see that when an alternating input voltage (on and off) is applied to the circuit, the capacitor gets charged and displays a gradual increase and decrease in the voltage.

By Azeem Iqbal | Electricity and Magnetism

### Barkhausen Effect

Discovered by German physicist Heinrich Barkhausen in 1919, the Barkhausen effect is a name given to the noise which a ferromagnetic material makes when the magnetic force applied to it is changed. To demonstrate this a coil of wire wound on the ferromagnetic material is affected by sudden, discontinuous jumps in magnetization using a hand held magnet. The sudden alterations in the magnetization of the material produces current pulse in the coil. This is amplified to produce a series of clicks in a loudspeaker; this is also called as Barkhausen noise. Similar effects can be observed by applying only mechanical stresses (e.g. bending) to the material placed in the detecting coil.

Barkhausen effect has many important applications today. For example, the amount of Barkhausen noise for a given material is linked with the amount of impurities, crystal dislocations, etc. and can be a good indication of mechanical properties of such a material. Therefore, it is used as a method of non-destructive testing for the degradation of mechanical properties in magnetic materials. It can also indicate physical damage in a thin film structure due to various nano-fabrication processes such as reactive ion etching.

By Azeem Iqbal | Electricity and Magnetism

### Meissner Effect

The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered this phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples.

In this demonstration, we have placed a yttrium-barium-copper-oxygen (123) superconductor inside liquid nitrogen (-196°C). As it is cooled below the superconducting transition temperature, the material becomes a superconductor and a perfect diamagnet, expelling the applied magnetic field. Because of this, when a magnet is held above the material it starts to levitate and hangs suspended in air. Gradually when the liquid nitrogen boils off and the superconductor returns to temperatures above its critical point, the magnet eventually loses its levitation and falls.

By Azeem Iqbal |

### Motional EMF

This demonstration exhibits the concept of motional emf. When a conductor is moved through a uniform magnetic field,
an emf is generated across its end. This emf can be detected. In this demonstration, a conductor that is connected to conducting rails is moved inside the field of an electromagnet, in an oscillatory manner. The emf generated is filtered, amplified and seen on an oscilloscope.

By Azeem Iqbal | Electricity and Magnetism

### Forces on conductors inside magnetic fields and between current carrying conductors

This demonstration exhibits forces on current carrying wires when placed inside a magnetic field. The magnetic field can come in fact, from another current carrying wire. This is the principle of the first demonstration. A direct current is passed through the two wires in opposing directions, causing the wires to stick together.

In the second variant of this demo, permanent magnets are placed below a coil of wire. When the current is switched on, the coil experiences a force that is perpendicular to the magnetic field. The force moves the coil either in the backward or forward direction depending upon the polarity of the magnets. If the polarity switches periodically, the coil could be rotated like a pendulum.

By Azeem Iqbal | Electricity and Magnetism
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