Revision: Electricity Science SSLC (English Medium) Class 8 Tamil Nadu Board of Secondary Education

Definitions [13]

Define the term atomic number.

The number of protons in the nucleus is known as the atomic number of the element and is denoted by Z.

Definition: Mass Number

The mass number of an atom is equal to the total number of nucleons (i.e., the sum of the number of protons and the number of neutrons) in its nucleus.

Definition: Atomic Number

The atomic number of an atom is equal to the number of protons in its nucleus (which is same as the number of electrons in a neutral atom).

Define the term mass number.

The total number of neutrons and protons in the nucleus is called the mass number of the element and is denoted by A.

Definition: Electric Charge

The basic property of matter due to which it experiences electric force and shows attraction or repulsion, is called electric charge.

Define the following:

Electric force

The force existing between the charges is called an ‘electric force’.

Define the term current.

An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire.

Define the term potential difference.

The amount of work done in moving a unit positive charge from one point to another point in an electric field is called the potential difference.

Define the following:

Synaptic signals

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.

Define the following:

Magnetic effect of current

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.

Definition: Heating effect of electric 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.

Define fuse.

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

Define the following:

Electrolyte

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

Formulae [1]

Write the mathematical expression for Joule’s law of heating.

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

Theorems and Laws [1]

State Ohm’s law.

At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.

In formula form: V = IR

Where:

V = Voltage (potential difference)
I = Current
R = Resistance

Key Points

Key Points: Structure of the Atom and Nucleus

The structure of an atom and its nucleus was developed from the discovery of electrons by J.J. Thomson and alpha particle scattering experiments by Rutherford.
An atom consists of electrons, protons, and neutrons, with protons and neutrons in the nucleus and electrons revolving in stationary orbits.
The maximum number of electrons in a shell is given by 2n², and the shells are named K, L, M, N, O, P, and Q.

Key Points: Concept of Charge

Thales (≈2500 years ago) observed that amber rubbed with wool attracts light objects like paper and straw.
William Gilbert (1600) showed that many materials, such as glass, ebonite, and sulphur, also show this effect.
This attractive property is produced by rubbing (friction); a material showing it is said to be electrified, and the process is called frictional electricity.
An electrified material possesses electric charge and is therefore called a charged body.
Electric charge is quantised ; there are two types of charges (positive and negative), as charges repel, unlike charges attract, and the SI unit of charge is coulomb (C).

Key Points: Electric Circuit

Electric energy from a cell is used to do work on charges, which is converted into heat in a resistor or other forms, like motion in a motor.
The work done (energy) in moving a charge $Q$ across a potential difference is:
Heat produced (H) in a resistor over time $t$ is given by Joule’s Law of Heating:
According to Ohm’s Law:
Electrical power (P) is the rate of energy transfer and is calculated as:

Its unit is watt (W), where .

Key Points: Heating Effect of Electric Current

Heating Effect: Current through a resistor produces heat, calculated by H = I²Rt or H = VIt (Joule’s Law).
Heat Applications: Used in devices like irons, heaters, bulbs, and fuses (to stop excess current).
Power Unit: 1 kWh = 3.6 × 10⁶ J, called 1 unit of electrical energy in electricity bills.
Short Circuit & Fuse: A short circuit causes a large current; the fuse wire melts to break the circuit and prevent fire.
MCBs: Miniature Circuit Breakers automatically cut off power during overload or fault conditions.