Chapter 11: Electricity and Magnetism

This chapter explains how electricity and magnetism are related. It covers direct current (d.c.), alternating current (a.c.), the magnetic effect of current, solenoids, motor effect, electromagnetic induction, generators, and transformers.

The current which flows in only one fixed direction is called direct current (d.c.). In a d.c. source, the positive and negative terminals are fixed.

Direct current obtained from a dry cell — current flows in one fixed direction through the bulb.

Direct current obtained from a solar panel lighting a bulb.

The current whose magnitude and direction change continuously at a fixed interval of time is called alternating current (a.c.).

A bicycle dynamo is a source of alternating current — a magnet rotates near a coil to light a lamp.

In the time–current graph of d.c., the current stays constant (a straight horizontal line) even as time increases.

In the time–current graph of a.c., the current rises from zero to a maximum, falls back to zero, then rises to the same maximum in the opposite direction, and returns to zero. This completes one cycle.

Time-current graph of direct current: a constant horizontal line.

Time-current graph of alternating current: a sine wave completing one cycle.

The number of cycles of a.c. completed in one second is called frequency. If one cycle is completed in one second, the frequency is 1 hertz (1 Hz). Since d.c. does not change direction, its frequency is zero.

A rectifier is a device used to convert alternating current into direct current. Devices such as mobile phones and computers use d.c. internally, so a rectifier is needed inside their chargers.

Block diagram: alternating current (AC) passed through a rectifier gives direct current (DC).

In 1820, Hans Christian Oersted, a Danish physicist, discovered that electricity and magnetism are related. He observed that a magnetic compass needle deflects near a current-carrying wire. The deflection of the compass needle is due to the magnetic field produced by the electric current.

Very Important for SEE: The direction of deflection reverses when the direction of current in the wire reverses.

A compass needle placed near a current-carrying wire deflects; reversing the current reverses the deflection.

When a straight wire carries current, a magnetic field is formed around it in a circular pattern. This can be shown using plotting compasses or iron dust sprinkled on a cardboard pierced by the wire.

Magnetic compasses placed around a current-carrying straight wire show a circular magnetic field pattern.

Iron dust settles in a circular pattern on a cardboard around a straight current-carrying wire.

If a current-carrying straight wire is gripped with the right hand such that the thumb points in the direction of current flow, the curled fingers indicate the direction of the magnetic field around the wire.

Maxwell's right-hand thumb rule: thumb shows current direction, curled fingers show magnetic field direction.

A solenoid is a cylindrical coil made by wrapping insulated wire around a cylindrical object. When current flows through a solenoid, a magnetic field is produced in and around it, similar to the field of a bar magnet — strong at the two ends and weak in the middle.

A solenoid is made by winding insulated copper wire into a coil and connecting it to a battery.

Magnetic field around a current-carrying solenoid, similar to the field of a bar magnet, with N and S poles formed.

Very Important for SEE: One end of a current-carrying solenoid becomes the North Pole and the other the South Pole. If the current direction is reversed, the poles also reverse.

If a solenoid is held in the right hand such that the fingers point in the direction of current flow, the thumb points toward the North Pole of the magnetic field developed in the solenoid.

Maxwell's right-hand grip rule for a solenoid: fingers show current direction, thumb shows North Pole.

Since the magnetic field created by a solenoid is temporary (it exists only while current flows), it is used to make an electromagnet.

The total number of magnetic lines of force passing through a surface area within a magnetic field is called magnetic flux. It is a measure of the magnetic field passing through a given surface area.

Magnetic field lines around different sources: a straight wire, a wire loop, a solenoid, a bar magnet, and the Earth.

Magnetic lines of force of a bar magnet — denser (stronger flux) near the poles, less dense in the middle.

The production of motion in a current-carrying wire placed inside a magnetic field is called the motor effect. Fans, water pumps, and mixer grinders work because of the motor effect.

A current-carrying wire hung between the poles of a U-shaped magnet moves due to the motor effect.

The force of attraction and repulsion between the magnetic field of the permanent magnet and the magnetic field produced around the current-carrying wire produces motion (this force is called the force on a current-carrying conductor in a magnetic field).

A DC motor's rotor coil placed between the poles of a permanent magnet.

Force acting on a DC motor coil due to the interaction of two magnetic fields, producing rotation.

A motor coil is wound around a core and placed between two opposite magnetic poles. When alternating current passes through the coil, the direction of its magnetic field keeps changing, and the coil rotates continuously due to the interaction with the permanent magnet's field.

In 1831, Michael Faraday discovered that when magnetic lines of force are cut perpendicularly by a conducting wire, a voltage (electromotive force, e.m.f.) is induced in the wire, and current flows if the circuit is completed.

The process of inducing an e.m.f. (voltage) in a conductor due to a change in the magnetic flux linked with it is called electromagnetic induction.

A solenoid connected to a galvanometer — moving a bar magnet in and out induces a current.

As the N or S pole of a bar magnet enters or exits a solenoid, the galvanometer needle deflects, showing induced current.

When there is relative motion between a conductor and a magnet, an e.m.f. is induced in the conductor. The magnitude of this e.m.f. is directly proportional to the rate of change of magnetic flux linked with the conductor.

A dynamo induces current on a small scale (used in bicycles, motorcycles). An a.c. generator produces current on a large scale for domestic and industrial use.

In both, a magnet or coil is rotated so that the magnetic flux linked with the coil changes, inducing an e.m.f. The magnitude of induced voltage depends on the number of turns in the coil, the strength of the magnetic field, and the speed of rotation.

A turbine used to spin a generator at high speed in a hydroelectric plant.

SEE Focus: According to Nepal Electricity Authority data (2022 AD), Nepal's hydroelectric potential is 2200 MW, with additional capacity from thermal plants and upcoming projects like Upper Arun, Uttar Ganga, and Dudh Kosi. Nuclear technology is not used in Nepal.

In an a.c. generator, a rectangular coil is placed in a magnetic field and rotated. As it rotates, it cuts the magnetic field, and the magnetic flux linked with the coil changes. This induces an e.m.f. whose magnitude is directly proportional to the rate of change of flux linkage.

The coil of an a.c. generator rotates between magnetic poles; slip rings and carbon brushes connect it to the external circuit.

The voltage induced in the coil of an a.c. generator varies sinusoidally with time.

A transformer is a device used to increase or decrease the voltage of an alternating current. It cannot be used with direct current.

A mobile charger uses a small transformer to reduce 220V a.c. to a low voltage a.c. such as 5.3V.

A transformer consists of two separate coils of insulated copper wire wound on a common iron core. They are not electrically connected.

Mutual induction: a changing current in coil 1 induces a current in nearby coil 2, detected by a galvanometer.

The transformer core is built from laminated (insulated) E and I shaped iron sheets to reduce heating.

The core is built from thin E, I, or U-shaped iron sheets, each coated with insulation, clamped together. This process is called core lamination and it reduces excessive heating of the core caused by induced currents (eddy currents).

Step-down transformer: A transformer with fewer turns in the secondary coil than in the primary coil. It reduces the voltage of alternating current.

Step-down transformer block diagram: primary 220V with more turns, secondary 110V with fewer turns.

Step-up transformer: A transformer with more turns in the secondary coil than in the primary coil. It increases the voltage of alternating current.

Step-up transformer block diagram: primary 110V with fewer turns, secondary 220V with more turns.

Primary turns (Np) / Secondary turns (Ns) = Primary Voltage (Vp) / Secondary Voltage (Vs)

Given: Primary voltage (Vp) = 220 V, Secondary voltage (Vs) = 12 V, Primary turns (Np) = 500.

Required: Secondary turns (Ns).

Formula: Np / Ns = Vp / Vs

Calculation: 500 / Ns = 220 / 12, so Ns = (500 × 12) / 220 = 27.27 ≈ 28.

Answer: The secondary winding has about 28 turns.

Electricity is produced at a generating station and converted to high voltage (e.g., 132 kV) using a step-up transformer for transmission through transmission lines with minimum loss. Before reaching customers, the voltage is reduced step by step using step-down transformers at substations (e.g., to 220V for homes).

Electricity generation, transmission substation, distribution substation, and distribution lines to customers.