# Physics OCR B A2 unit 2 listening notes part 1

?
• Created by: Rhys
• Created on: 28-05-12 17:55

Magnetic Fields. Flux.  A coil of wire creates magnetic flux. The amount of magnetic flux created is dependant on three things: the number of coils in the wire, the amount of current flowing through the wire, and the permeance of the object through which the flux is flowing. So. ,Flux equals permeance times no of coils times current (current turns0. where  flux (in webers, denoted Wb),   This is the total flux induced. "current-turns", is also known as the flux linkage. Permeance is related to permeability (a material property) by the following equation: permeance equals mue times area divided by the length, where mue is permeability,  A permanent magnet is just like a coil, except that a current does not need to be generated to maintain the flux. Over smaller areas, we need to know the flux density B. This is the amount of flux per. unit area: Flux density equals flux over area The flux around a coil of wire varies  only gives the total flux, not the flux across a certain area. To show this, we use lines of flux. These obey the following rules: 1. Lines of flux go from the north pole of a permanent magnet to the south pole. 2. Lines of flux go clockwise about wires carrying current away from you. 3. Lines of flux never touch, intersect, or cross. The direction of the flux is shown with an arrow. Flux is a bit like electricity in that it must have a complete circuit. The lines of flux always take the route of least permeance. An iron core has around 800 times as much permeability as some air. So, flux goes through the iron core, and not the air. Induction.  A magnetic field creates a current in a wire moving through it. This process is known as induction. Flux Linkage.  A magnetic field going through a coil of wire has a property known as flux linkage. This is the product of the flux phi and the number of coils in the wire . Faraday's Law.  Electric current is only induced in a coil of wire if the magnetic field is moving relative to the coil. Faraday's Law gives the electromotive force (emf) ε produced in a coil by a magnetic field: e.m.f equals minus no of coils change in flux over time. In other words, the emf (electric potential) induced in the coil is proportional to the rate of change of flux linkage. In practice, this means that if the coil is stationary relative to the magnetic field, no emf is induced. In order to induce emf, either the coil or the magnetic field must move. Alternatively, we may change the number of coils, for example, by crushing the coil, or pressing a switch which added more coils into the circuit, or moving more of the coils into the magnetic field. Faraday's Law also works the other way. If we were to integrate both sides and rearrange the formula in terms of