Revision: Term - 2 >> Electricity Science SSLC (English Medium) Class 7 Tamil Nadu Board of Secondary Education

Current is defined as the rate of flow of charge.

 Semiconductors: Substances whose resistance decreases with the increase in temperature are named as semiconductors. E.g. manganin, constantan etc.

An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit.

The magnitude of an electric current is the number of electric charges flowing through a conductor in one second.

Electromotive force: When no current is drawn from a cell, when the cell is in open circuit, the potential difference between the terminals of the cell is called its electromotive force (or e.m.f.).

The movement of the positive charge is called conventional current.

The unit of electric current is ampere (A). When one coulomb charge flows through an electric circuit in one second, then the electric current flowing through the circuit is said to be an ampere. 

The resistivity of a material is the resistance of a wire of that material of unit length and unit area of cross-section.

Substances whose resistance decreases tremendously with decreasing temperature and reaches nearly zero near absolute zero are called superconductors; e.g., lead, tin, etc.

A continuous and closed path of an electric current is called an electric circuit.

The potential at a point is defined as the amount of work done per unit charge in bringing a positive test charge from infinity to that point.

The potential difference (p.d.) between two points is equal to the work done per unit charge in moving a positive test charge from one point to the other.

OR

The work done per unit positive charge in moving a charge from one point to another in an electric field, is called potential difference between those two points.

 Potential difference: The potential difference between two points may be defined as the work done in moving a unit positive charge from one point to the other.

Electric potential is a measure of work done on the unit's positive charge to bring it to that point against all electrical forces. It is represented as ‘V’.

Specific resistance of a material is the resistance of a wire of that material of unit length and unit area of cross-section.

S.I. Unit of resistivity is ohm-metre, i.e., Ω·m.

\[\rho=R\left(\frac{A}{l}\right)\]

A fixed resistor has a resistance of a fixed value. Common types of fixed resistors include carbon film resistors and wire-wound resistors.

A variable resistor has a resistance that can be varied. It is used to vary the amount of current flowing in a circuit.

The resistance of a conductor is defined as the ratio of the potential difference V across the conductor to the current I flowing through it.

• S.I. unit of resistance is ohm (Ω)
• Dimensional formula: [M L² T⁻³ A⁻²]

Resistance is the obstacle that the wire presents to the current flow.

A switch is an on-off device for current in a circuit (or in an appliance). It is connected in the live wire.

The range of energies possessed by conduction electrons is called conduction band.

The solids which have a large number of free electrons are called conductors. (e.g. Iron, Aluminium)

The solids which have very small number of free electrons are called insulators. (e.g. Glass, Wood)

The range of energies possessed by valence electrons is called valence band.

The energy difference between the valence band and the conduction band is called forbidden energy gap.

The material with electrical conductivity between that of a conductor and an insulator, whose number of charge carriers can be controlled as per requirement, is called a semiconductor. (e.g. Silicon, Germanium)

The different energy levels with continuous energy variation are called energy bands.

Synaptic signals: Extremely weak electric current is produced in the human body by the movement of charged particles. These are called synaptic signals. These signals are produced by the electrochemical process. They travel between the brain and the organs through the nervous system.

Magnetic effect of current: A wire or a conductor carrying current develops a magnetic field perpendicular to the direction of the flow of current. This is called the magnetic effect of current.

When a resistor is connected in an electrical circuit, heat is produced in it due to the current. This is known as the heating effect of current.

The solution through which the electricity passes is called an electrolyte.

Electric fuse is a safety device which is used in household wiring and in many appliances.

A current-carrying conductor is always associated with a magnetic field around it is called the magnetic effect of current. It was first discovered by Hans Christian Oersted in 1820.

V = \[\frac {W}{Q}\]

or

W = QV

The mathematical expression of Joule’s Law of heating is: H = I² Rt

Where,

H = Produced Heat 
I = Current flowing through the device
t = Time taken
r = Resistance of the appliance

• Electricity is a convenient and controllable form of energy widely used in homes, industries, schools, and hospitals.
• Electric current is produced when electric charges flow through a conductor, and it flows only through a closed, continuous electric circuit.
• A switch completes or breaks the circuit; when the circuit is broken, current stops flowing, and devices like bulbs do not glow.
• Electric current is the rate of flow of charge, given by the relation I = Q / t, where Q is charge and t is time.
• In metallic wires, electrons are the charge carriers, but by convention, current flows from the positive to the negative terminal, in the opposite direction to electron flow.

• Specific resistance is a characteristic property of a substance and differs among metals, semiconductors, and insulators.
• Specific resistance depends on temperature: it increases with temperature for metals and decreases with temperature for semiconductors, while it remains nearly constant for some alloys.
• Specific resistance does not depend on the shape and size of the conductor and remains unchanged when a wire is stretched or doubled.

• Free electrons in a metal move randomly; without a potential difference, there is no net flow of current.
• When a potential difference is applied, electrons drift towards the positive terminal, but collide with fixed positive ions, losing energy.
• These collisions cause resistance, and the number of collisions determines the amount of resistance in the conductor.

• Conductors → E_g = 0 - bands overlap, electrons flow freely.
• Semiconductors → E_g < 3 eV — small gap, conducts at room temperature.
• Insulators → E_g > 5 eV — large gap, no conduction.
• Ge = 0.72 eV, Si = 1.1 eV — both semiconductors.
• Metal conductivity decreases with temp. Semiconductor conductivity increases with temp. 

• Electric current creates a magnetic field, shown by compass needle deflection.
• Oersted discovered the link between electricity and magnetism in 1820.
• Reversing current changes the direction of the magnetic field.
• Iron filings form circular patterns, showing magnetic field lines around the wire.
• Magnetic field strength increases with current and decreases with distance.