The Characteristics of a Magnetic Field

Magnetism produces lines of force known as flux. These can be detected by compass needle which shows the direction you are travelling as the earth’s magnetic north pole attracts the north ends of other magnets. The magnetic lines of force start from the north pole and end at the south pole. They are continuous throughout the body of the magnet. Magnetic lines of force can pass through iron more easily than air which is the reason why iron is used as the core in electromagnets. Two magnetic lines of force cannot intersect each other which is why when you try to push two of the same facing magnets (pushing north and north towards each other) it will repel. Magnetic field lines tent to contract longitudinally and tend to expand laterally. The magnetic field lines are crowded near the pole where the field is strong and far from the magnet where the field is weak.

Flux density is the amount of magnetic, electric or other flux passing through a unit area and magnetic field strength is the force field that is created by moving electric charges and magnetic dipoles. Magnetic field lines fictitious objects, they are conceptualized to indicate the magnetic field on a magnetic material. They give a visual representation of the magnetic field around a magnetic material. They originate on the north and end on the south pole of a magnet. A tangent to a magnetic line gives the direction of force on a north pole. The magnitude of the force is giving in terms of the density of magnetic lines at the point. The relationship between flux density and field strength is that the higher the density of the flux lines the larger the magnetic force. These are called lines of force. Electromagnets also have these lines however they are electric lines of force and they are similar, although the magnetic poles on electromagnetic are replaced by electric charges.

Electromagnetic induction is an incredibly useful phenomenon with a wide variety of applications. Induction is used in power generation and power transmission.

An eddy current is a swirling current set up in a conductor in a response to a changing magnetic field. The current swirls in such a way as to create a magnetic field opposing the change, to do this in a conductor, electrons swirl in a plane perpendicular to the magnetic field. Because eddy currents tend to oppose they cause energy to be lost. Eddy currents transform more useful forms energy such as kinetic energy into heat. This is normally less useful in most situations as the loss of useful energy is not desirable however there are some practical applications for the loss of useful energy. One of these applications is in brakes of some trains. When a train brakes the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. This magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels are spinning the stronger the effect. This in-turn means that as the train slows down the braking force is reduced which produces the smooth stopping motion you feel in trains.

When a conductor is made to pass through a magnetic field, a voltage is generated in it. This voltage will be reduced slightly by the resistance of the conductor so we talk about the theoretical voltage as though the conductor had no resistance and this is called the electro motive force or as it is commonly known as EMF. The EMF is directly proportional to the flux density, the velocity and the length of conductor within the flux. The direction of the current generated is found from Flemings right hand rule. When you point the index finger of your fight hand in the direction of the flux (North to South), point your thumb in the direction of the velocity and bend over the second finger it will point in the direction of the current.