Magnetism and Electromagnetism: Properties, Circuits, and Induction

Magnetic Properties

A magnetic field is the region around a magnet where its magnetic action is exerted. Within this region, ferrous materials are attracted to the magnet. Outside this region, no attraction is observed.

The magnetic field is represented by lines of force, conventionally going from the north to the south pole. Like poles repel each other, while opposite poles attract.

Magnetic flux is the number of lines of force passing through a surface within a magnetic field.

Materials that allow magnetic flux to pass through them easily are called permeable. They exhibit low resistance to this flux, a property known as reluctance. When a permeable material is placed in a uniform magnetic field, the lines of force concentrate within it, a phenomenon called induction.

Electromagnetism

Magnetic effects can be generated by applying an electric current.

Oersted's experiment demonstrated that an electric current creates a magnetic field in the surrounding space. The strength of the magnetic field is directly proportional to the current.

The direction of the magnetic field lines depends on the current's direction and can be determined using the right-hand rule: grasp the wire with your right hand, your thumb pointing in the current's direction; your fingers then curl in the direction of the magnetic field lines.

A coil produces a stronger magnetic field than a straight wire. Increasing the number of turns in a coil (N turns) increases the magnetic field strength by a factor of N. Adding a magnetic core further enhances the magnetic field.

Electromagnets have wide applications, such as in relays. Relays are devices consisting of an electromagnet and a soft iron piece called an armature. The armature is attracted to the electromagnet when current flows through the coil.

Magnetic Circuits

A magnetic circuit is a closed path followed by magnetic field lines, typically channeled through a ferromagnetic material. Air gaps may be included in the circuit.

In magnetic circuits, the magnetic flux is analogous to current flow. It is driven by the magnetomotive force and encounters resistance in the form of reluctance.

Electromagnetic Induction

When a conductor moves within a magnetic field, an electric current is induced. This phenomenon is called electromagnetic induction. A coil will induce a higher current, and the current will be greater with a stronger magnetic field or a faster coil movement. The same effect occurs whether the magnet or the coil moves; the key is the change in magnetic flux. The coil's movement must cut across the magnetic field lines; parallel movement does not generate current.

The direction of the induced electromotive force (EMF) can be determined using Fleming's right-hand rule: extend your thumb, index finger, and middle finger perpendicularly to each other. Align your thumb with the conductor's motion and your index finger with the magnetic field direction; your middle finger will then point in the direction of the induced EMF.

Electromagnetic induction also occurs in solenoids. If a coil connected to a generator is switched on, current flows, creating a magnetic field proportional to the current. Placing a second coil nearby, connected to a galvanometer, subjects it to the magnetic field lines, inducing a current. This induction between two coils is the principle behind transformers.

A transformer consists of two coils wound around a common magnetic core. To induce current in the secondary coil, a change in magnetic flux is necessary, often achieved using alternating current (AC). The secondary coil will also output AC. To obtain direct current (DC), a rectification process using diodes is required. The coils' cores are made of iron, a conductor; therefore, they are also subject to magnetic field variations.