Notes: Electricity and Magnetic Effect of Current | Science & Pedagogy Paper 2 for CTET & TET Exams - CTET & State TET PDF Download

Electricity involves the movement of electric charges and stands as one of the most widely used sources of power, encompassing both static and current electricity. Heating, chemical reactions, and magnetic effects are all connected to various relationships between current, voltage, and resistance. Electromagnetism, a fundamental interaction, finds extensive applications in our daily lives.

Electric Charges and Their Properties

In the sixteenth century, the English scientist William Gilbert discovered that certain substances can attract light objects when rubbed with specific materials. These substances are said to be charged, and charges can be either positive or negative. Like charges repel each other, while unlike charges attract.

The SI unit of charge is the Coulomb (C). The force between charged particles is governed by Coulomb's law, which states:

F = k (q₁ q₂) / r²

where F is the force, q₁ and q₂ are the charges, r is the distance between them, and k is a proportionality constant. This force is known as the electrostatic force.

A comb attracting small pieces of paper is an example of electrostatic force in action.

1 Coulomb = 6.24 × 10¹⁸ electrons

Conductors, Insulators, and Semiconductors

Substances are classified based on their ability to conduct electricity:

• Conductors: Materials that allow electricity to flow freely, e.g., copper, silver, graphite, and salt solutions.
• Insulators: Materials that do not allow electricity to flow easily, e.g., rubber, glass, and plastics.
• Semiconductors: Materials with electrical conductivity between that of conductors and insulators, e.g., silicon and germanium.

1 Coulomb of charge flowing through a circuit in 1 second constitutes 1 Ampere (A) of current. Other units of current include milliampere (mA) and microampere (µA).

Insulators lack free electrons, which is why they do not conduct electricity, whereas conductors have many free electrons.\

Electric Current

Electric current is the flow of electric charge (electrons) through a conductor over time. The current I is given by:

I = q / t

where q is the charge and t is the time. The SI unit of current is the Ampere (A), with other units including milliampere (mA) and microampere (µA).

The direction of conventional current is opposite to the flow of electrons, from the positive to the negative terminal.

Types of Current

• Direct Current (DC): Current flows in one direction with zero frequency.
• Alternating Current (AC): Current changes direction at regular intervals.

An ammeter is used to measure electric current in a circuit. Household electricity is AC with a frequency of 50 Hz. If E₀ is the peak emf of AC, the root mean square emf is E₀ / √2.

Electric Potential and Potential Difference

For charges to flow in a conductor, there must be a potential difference, typically provided by a battery. When connected to a circuit, this potential difference causes charges to move, creating an electric current.

The electric potential difference (V) between two points in a circuit is defined as the work done (W) to move a unit charge (Q) from one point to the other:

V = W / Q

The SI unit of electric potential difference is the volt (V). Electric potential is the work done to move a unit positive charge from infinity to a point, and it is a scalar quantity measured in volts (V).

Ohm's Law and Resistance

Ohm's Law describes the relationship between the current flowing through a metallic wire and the potential difference across its terminals. According to Ohm's Law:

V ∝ I

or

V = IR

where V is the potential difference, I is the current, and R is the resistance of the wire.

The motion of electrons through a conductor is hindered by its resistance. A good conductor of a given size has low resistance, while an insulator of the same size has very high resistance. The SI unit of resistance is the ohm (Ω).

Factors Affecting the Resistance of a Conductor

The resistance of a conductor depends on the following factors:

• Length of the conductor (l)
• Area of cross-section (A)
• Nature of the material

The resistance of a metallic conductor is given by:

R = ρ (l / A)

where ρ (rho) is the electrical resistivity or specific resistance of the material, a characteristic property. The SI unit of resistivity is ohm-meter (Ω-m).

Example Problem

Problem: How much current will an electric heater draw from a 220 V line if the resistance of the heater is 50 Ω?

Solution: Given, V = 220 V and R = 50 Ω. Using Ohm's Law,

I = V / R

I = 220 / 50 = 4.4 A

Electric Circuits

An electric circuit is a continuous path for the flow of electricity through various electrical components, connected to a source such as an electric cell, bulb, switch, and wires.

A schematic diagram of the circuit represents different components using conventional symbols.

Resistors in Series

In a series circuit, the current is the same through each resistor, and the total voltage across the combination is the sum of the voltage drops across each resistor. The equivalent resistance (R_equivalent) is given by:

R_equivalent = R₁ + R₂ + R₃

where R₁, R₂, and R₃ are the individual resistances.

Resistors in Parallel

In a parallel circuit, the total current is the sum of the currents through each branch, and the voltage across each resistor is the same. The equivalent resistance (R_equivalent) is given by:

1 / R_equivalent = 1 / R₁ + 1 / R₂ + 1 / R₃

where R₁, R₂, and R₃ are the individual resistances.

Heating Effect of Electric Current

When electric current passes through a conductor, it generates heat due to the resistance it offers to the current flow. For example, an electric fan becomes warm if used continuously for a long time. This phenomenon is known as the heating effect of electric current.

Since the conductor resists the flow of current, work must be done to maintain the current. The amount of work done by a current flowing through a wire of resistance R  during time  ( t ) is given by:

W = qV

Since 

Using Ohm's law  V = IR , this becomes 

This work or energy is dissipated in the resistor as heat. Therefore, the amount of heat \( H \) produced in time \( t \) is:

This is known as Joule’s law of heating. The law implies that heat produced in a resistor is directly proportional to the square of the current for a given resistance, directly proportional to the resistance for a given current, and directly proportional to the time for which the current flows through the resistor.

Electric Power

Electric power is the rate at which electrical energy is dissipated into other forms of energy. It is given by:

P  = I²R

Since V = IR , power can also be expressed as \P = VI .

The unit of electric power is the watt (W). The commercial unit of energy is kilowatt-hour (kWh).

Applications of Heating Effect of Electric Current

• Electric heating is used in devices like electric toasters, ovens, and heaters.
• Electric bulbs use the heating effect to produce light. The filament, usually made of tungsten (melting point 3380°C), retains heat and produces light.
• Bulbs are filled with inert gases like nitrogen and argon to prolong the filament's life.

Overloading

When too many electrical appliances are connected to a single socket, they draw a large current, causing overheating and potentially a fire. This is known as overloading.

Electric Fuse

An electric fuse is placed in series in domestic circuits to protect them by stopping the flow of excessively high current. It consists of a wire made of an alloy or metal with a low melting point, which melts in case of short-circuiting or overloading.

Electric heaters or room heaters contain a coil of wire called an element, often made of nichrome due to its high resistance.

Traditional electric bulbs also waste energy by producing heat along with light. This wastage can be reduced by using fluorescent tube lights or compact fluorescent lamps (CFLs), though CFLs contain the toxic substance mercury.

The ISI mark (Bureau of Indian Standards) ensures the appliance is safe and minimizes energy wastage.

MCB (Miniature Circuit Breakers)

Modern circuits use Miniature Circuit Breakers (MCBs) instead of fuses. MCBs are switches that automatically turn off when the current exceeds a safe limit.

In household wiring, all electrical appliances are connected in parallel.

The live wire is covered with red insulation, while the neutral wire is covered with black insulation. To avoid electrical shocks, the metal body of electrical appliances is earthed, connecting to a metal plate deep in the earth near the house.

Short Circuit

A short circuit occurs when the live wire and neutral wire touch directly, causing a large current to flow through the household wiring.

Magnets

Magnetic Material

Materials that can attract magnetic substances (such as cobalt, iron, and nickel) are called magnets. A magnet has two poles: the North pole (N) and the South pole (S).

• Non-magnetic materials: Materials that do not get attracted to magnets (e.g., plastic, rubber, glass).
• Magnetic materials: Materials that get attracted to magnets (e.g., iron, cobalt, nickel).

Types of Magnets

• Natural Magnets: Stones or minerals with magnetic properties, known for low magnetism, brittleness, and irregular shapes.
• Man-made Magnets: Artificially made magnets, often stronger than natural ones, created by magnetizing certain metals or alloys. They come in various shapes such as bar magnets, circular magnets, U-shaped magnets, ring magnets, and horse-shoe magnets.

Pieces of lodestone, called magnetite, can attract iron and are naturally occurring magnets.

Magnetic and Non-Magnetic Materials

• Materials strongly attracted to magnets are magnetic materials (e.g., iron, nickel, cobalt).
• Materials not attracted to magnets are non-magnetic materials (e.g., plastic, paper, rubber).
• The distance between the assumed poles of a magnet is called the magnetic length, while the distance between the ends of the magnet is the geometric length.

Magnetic length = 5/6 geometric length

Magnetic Field & Field Lines

The region around a magnet where the forces of attraction and repulsion can be detected is called a magnetic field. The most convenient way to describe a magnetic field is by drawing magnetic field lines.

The direction of the field is the direction that a North pole (N) would take if placed in the field. Magnetic field lines of a bar magnet originate from the North pole and terminate at the South pole.

• Like poles of a magnet repel each other, while unlike poles attract each other.
• Magnetic field lines never intersect each other.
• The strength of the magnetic field is indicated by the closeness of the field lines. The field is strongest at the poles of the magnet, where the field lines are most crowded. Hence, the magnetic power of a bar magnet is maximum at the poles.

Magnetic Flux

Magnetic flux is the total number of magnetic lines of force passing through a surface. The SI unit of magnetic field is Tesla (T) and the CGS unit is Gauss (G), with 1 Tesla = 10⁴ Gauss. The SI unit of magnetic flux is Weber (Wb), so the magnetic field can also be expressed in Weber per square meter (Wb/m²).

Magnetic Effect of Current

When electric current passes through a wire, it behaves like a magnet and produces a magnetic field around it. This phenomenon is known as the magnetic effect of electric current.

• The magnetic field around a current-carrying straight wire or conductor is in the form of concentric circles around the wire. The direction of the field lines is given by the right-hand thumb rule.
• The magnetic effect of current was first observed by Hans Christian Oersted.

Electromagnet

A strong magnetic field produced in a coil can magnetize a piece of magnetic material placed within the coil carrying a current. If an insulated wire is wound around a cylindrical object, it is called a solenoid. The cylindrical object is called the core.

A solenoid with a soft iron core is called an electromagnet. Electromagnets are used in various applications of the magnetic effect of current, such as in dynamos, transformers, telegraphs, and telephones.

When a current-carrying conductor is placed in a magnetic field, it experiences a force. The direction of this force is given by Fleming’s left-hand rule.

Electromagnetic Induction

Electromagnetic induction is the process by which a changing magnetic field in a conductor induces a current in it. This was first discovered by Michael Faraday. He demonstrated that the motion of a magnet relative to a coil can induce an electromotive force (emf) and produce an induced current in the circuit. The direction of the induced current is given by Fleming’s right-hand rule.

Applications of Magnetic Effects of Current

Electric Motors

An electric motor is a device that converts electrical energy into mechanical energy. It operates on the principle that a current-carrying coil placed in a magnetic field experiences a torque, causing the coil to rotate. Electric motors are essential components in electric fans, washing machines, refrigerators, and other appliances.

Electric Generators or Dynamos

Electric generators are based on the phenomenon of electromagnetic induction. They convert mechanical energy into electrical energy. When a coil of wire is rotated in a magnetic field, current is induced in the coil, producing electricity.