19. Applications of Electromagnetism

Electric Motor

An electric motor is a device that converts electrical energy into mechanical energy (rotational motion), and it works on the principle described earlier: a current-carrying conductor placed in a magnetic field experiences a force (governed in direction by Fleming's left-hand rule). In its basic construction, a motor consists of a rectangular coil of insulated wire (the armature) mounted on an axle so that it can rotate freely, placed between the two poles of a permanent magnet (or an electromagnet) so that the coil lies within a magnetic field; the ends of the coil are connected to a split-ring commutator, which in turn touches two fixed carbon brushes connected to a source of current. When current is passed through the coil, the two sides of the coil (being on opposite sides of the axle and therefore effectively carrying current in opposite directions relative to the field) experience forces in opposite directions, and this pair of oppositely directed forces creates a turning effect (torque) that rotates the coil. The crucial role of the split-ring commutator is to automatically reverse the direction of current in the coil every half-rotation, which correspondingly reverses the direction of the force on each side of the coil at exactly the right moment, ensuring that the torque continues to act in the same rotational sense and the coil keeps spinning continuously in one direction rather than merely oscillating back and forth. Electric motors are used in an enormous range of devices, including electric fans, washing machines, mixers and grinders, electric vehicles, and countless industrial machines.

Electric Generator (Dynamo)

An electric generator, also called a dynamo, is essentially the reverse of a motor: it converts mechanical energy into electrical energy, working on the principle of electromagnetic induction (Faraday's law). In its basic construction, a generator likewise consists of a coil of wire mounted on an axle that can be rotated within a magnetic field, but here the coil is turned by an external mechanical source of energy — for example, falling water in a hydroelectric plant, steam pressure in a thermal or nuclear plant, or wind in a wind turbine. As the coil rotates within the magnetic field, the magnetic flux linked with the coil continuously changes, and by Faraday's law this induces an EMF (and hence a current, if the circuit is closed) in the coil. The direction of the induced current at each instant is given by Fleming's Right-Hand Rule (note: right-hand rule for generators/induced current, as opposed to the left-hand rule used for motors/force on a current) — with the thumb, forefinger, and middle finger again mutually perpendicular, if the forefinger points in the direction of the field and the thumb points in the direction of motion of the conductor, the middle finger gives the direction of the induced current.

AC generator vs DC generator: The essential difference between an AC generator and a DC generator lies not in the coil or the magnet, but in the type of contact used to connect the rotating coil to the external circuit. An AC generator uses two separate slip rings (continuous, unbroken metal rings, one connected to each end of the coil), which means that as the coil rotates, the current it delivers to the external circuit periodically reverses direction along with the reversing direction of the induced EMF, producing alternating current (AC) — this is the type of generator used in almost all commercial power stations. A DC generator, by contrast, uses a split-ring commutator (the same type of device used in a motor) instead of slip rings; the commutator automatically reverses the connections to the external circuit at exactly the same moment that the EMF within the coil itself reverses, so that current always flows out of the generator in the same direction through the external circuit — the current is direct current (DC), though its magnitude does still pulse up and down (it is not perfectly steady like a battery's output) unless further smoothed by additional circuitry.

Transformer

A transformer is a device used to change (step up or step down) the voltage of an alternating current supply, and it works entirely on the principle of mutual electromagnetic induction between two coils. A transformer consists of two separate coils of insulated wire, called the primary coil and the secondary coil, both wound on a common soft iron core (the core is laminated — made of thin, electrically insulated sheets — to reduce energy losses from unwanted induced currents within the core itself, called eddy currents). When an alternating current is passed through the primary coil, it produces a continuously changing magnetic flux in the iron core; because this flux is also linked with the secondary coil (via the shared core), the continuously changing flux induces an alternating EMF in the secondary coil as well, by Faraday's law. The relationship between the voltages and the number of turns in the two coils is given by:

Vs / Vp = Ns / Np (secondary voltage / primary voltage = secondary turns / primary turns)

• Step-up transformer: The secondary coil has more turns than the primary coil (Ns > Np), so it increases (steps up) the voltage from primary to secondary, while correspondingly decreasing the current (since, ideally, input power equals output power: Vp×Ip = Vs×Is). Step-up transformers are used at power generating stations to raise the voltage to very high levels for efficient long-distance transmission (higher voltage means lower current for the same power, which reduces resistive (I²R) power losses in the long transmission lines).
• Step-down transformer: The secondary coil has fewer turns than the primary coil (Ns < Np), so it decreases (steps down) the voltage while increasing the current. Step-down transformers are used at substations near consumers to reduce the very high transmission voltage down to the safer 220–240 V level suitable for household and industrial use, and small step-down transformers are found inside chargers/adapters for laptops, mobile phones, and similar low-voltage devices.

A crucial and very frequently asked exam fact is that a transformer works only on alternating current (AC) and cannot work on direct current (DC) at all. This is because the underlying principle, electromagnetic induction, fundamentally requires a changing magnetic flux to induce an EMF; a steady direct current produces a constant, unchanging magnetic flux in the core, and since there is no change in flux, no EMF at all is induced in the secondary coil, so a transformer connected to a pure DC supply produces no output in its secondary coil (and, in fact, since a DC supply also sees only the primary coil's very low pure resistance rather than its normal AC impedance, it can draw an excessively large current and burn out the primary coil).

Electromagnet and Its Uses

An electromagnet is a temporary magnet created by winding an insulated wire into a coil (typically a solenoid) around a core of soft iron and passing an electric current through the coil; the soft iron core becomes strongly magnetised as long as current flows through the coil (because soft iron is easy to magnetise but does not retain its magnetism once the current is switched off, unlike steel, which retains magnetism and is therefore used for permanent magnets instead). The strength of an electromagnet can be increased by increasing the current through the coil, increasing the number of turns in the coil, or using a core material with better magnetic properties, and — importantly — an electromagnet's magnetism can be switched on or off at will simply by switching the current on or off, and even its polarity can be reversed by reversing the direction of current flow, giving it a flexibility that permanent magnets do not have.

Electromagnets have a vast range of practical applications, several of which are commonly cited in exams: large industrial electromagnetic cranes used in scrapyards and steel plants to lift and move heavy iron and steel objects and scrap; electric bells (described below); loudspeakers and microphones, which use the interaction between a permanent magnet and a current-carrying coil to convert electrical signals into sound vibrations and vice versa; electric motors and generators, which use electromagnets (or permanent magnets) as described above; magnetic resonance imaging (MRI) machines in medical diagnostics, which rely on extremely powerful electromagnets; and magnetic relays and circuit breakers, which use an electromagnet to mechanically operate a switch.

Electric Bell

The electric bell is a classic, frequently diagrammed application of the electromagnet, illustrating a simple self-interrupting circuit. It typically consists of an electromagnet positioned close to a flexible metal strip called the armature, which has a small hammer attached to it and a metal contact (a springy metal strip) that touches a fixed contact screw to complete the circuit when at rest. When the bell's push-button switch is pressed, current flows through the circuit and energises the electromagnet, which attracts the soft iron armature toward itself; this movement causes the attached hammer to strike the bell's gong, producing sound, but it also simultaneously pulls the armature's contact strip away from the fixed contact screw, thereby breaking the circuit. With the circuit broken, the electromagnet immediately loses its magnetism (since it depends entirely on the flow of current), so it no longer attracts the armature, which then springs back to its original position under its own elasticity — but this return movement re-establishes contact with the screw, completing the circuit once again, re-energising the electromagnet, and repeating the whole cycle. This rapid make-and-break cycle repeats many times per second for as long as the push-button is held down, causing the hammer to strike the gong repeatedly and producing the familiar continuous ringing sound.