Electromagnetic Induction Magnetism can induce electrical currents in wires You just have to keep motion between the magnets and wires

Michael Faraday 1791 – 1867 Great experimental scientist Invented electric motor, generator and transformers Discovered electromagnetic induction Discovered laws of electrolysis Section 20.1

Faraday’s Experiment – Set Up A current can be produced by a changing magnetic field. –First shown in an experiment by Michael Faraday A primary coil is connected to a battery. A secondary coil is connected to an ammeter. Section 20.1

There is no battery in the secondary circuit. When the switch is closed, the ammeter reads a current and then returns to zero. When the switch is opened, the ammeter reads a current in the opposite direction and then returns to zero. When there is a steady current in the primary circuit, the ammeter reads zero. Section 20.1 Faraday’s Experiment

Faraday’s Conclusions An electrical current is produced by a changing magnetic field. The secondary circuit acts as if a source of electromotive force (emf) were connected to it for a short time. It is customary to say that an induced emf is produced in the secondary circuit by the changing magnetic field. EMF is another word for VOLTAGE Section 20.1

When there is no relative motion between the coils of wire and the magnet there is no current produced

Current is created in the coil when the magnet is moved towards the coil. The current’s direction always opposes the change in the magnetic field Note: here conventional current (+) with RIGHT hand rule is used. The same result for electron flow would come from the left hand rule.

Current also exists when you pull it away from the coil, just in the opposite direction. The current in the coil is called an induced current. The coil itself acts as a source of emf known as induced emf.

Another way to look at it. Changing the area of a coil, in effect, reduces/increases the B field that the coil is subject to. Changing the B field strength experienced by the coil. This will also create a current.

Motional EMF The EMF Induced in a Moving Conductor

A rod is being pushed to the right with constant speed v. Suddenly the bulb lights. Why? Where is the current coming from ? Where is this opposing force coming from?

We have been using the term emf, ε, or electro motive force. ε=BLv Potential Difference

Magnetic Flux Motional EMF and Magnetic Flux

By definitiontherefore Of course the angle with the field is important

Faradays Law actually reads Where N is the # of turns in the coil. But what is the negative all about?

Consider the field created by the counterclockjwise loop in our previous problem. What is the direction of its field?

Lenzs’ Law

Self-inductance Self-inductance occurs when the changing flux through a circuit arises from the circuit itself. –As the current increases, the magnetic flux through a loop due to this current also increases. –The increasing flux induces an emf that opposes the change in magnetic flux. –As the magnitude of the current increases, the rate of increase lessens and the induced emf decreases. –This decreasing emf results in a gradual increase of the current. Section 20.5

Self-inductance, Cont. The self-induced emf must be proportional to the time rate of change of the current. –L is a proportionality constant called the inductance of the device. –The negative sign indicates that a changing current induces an emf in opposition to that change. Section 20.5

Self-inductance, Final The inductance of a coil depends on geometric factors. The SI unit of self-inductance is the Henry – 1 H = 1 (V · s) / A You can determine an expression for L Section 20.5