Revision: Electric Charge and Electric Current Science SSLC (English Medium) Class 9 Tamil Nadu Board of Secondary Education

Electric charge is an intrinsic property of certain fundamental particles (like electrons and protons) that gives rise to electric and magnetic forces, causing them to experience a force when placed in an electromagnetic field.

OR

Electric charge is the physical property of matter that causes it to experience a force when placed in an electric field.

Key facts:

• SI Unit: Coulomb (C)
• Dimensional Formula: [A T] (Ampere × Time)
• Two types exist: positive and negative

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

The electric field intensity at any point is the strength of the electric field at that point.

• It is defined as the force experienced by a unit positive charge placed at that point.

\[\vec{E}=\frac{\vec{F}}{q_0}=\frac{kq}{r^2}\hat{r}=\frac{kq}{r^3}\vec{r}\]

• The SI unit of E is NC⁻¹ (newtons per coulomb).

The region in which the charge experiences an electric force is the electric field around the charge.

The charge Q that produces the electric field is called the source charge.

The charge q that tests the effect of the source charge is called the test charge.

The space surrounding an electric charge q in which another charge q₀ experiences a (electrostatic) force of attraction or repulsion, is called the electric field of the charge q.

OR

Electric field due to a charge Q at a point in space may be defined as the force that a unit positive charge would experience if placed at that point.

OR

The region surrounding an electric charge or a group of charges in which another charge experiences a force is called an electric field.

 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’.

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 the potential difference between those two points.

OR

The potential difference between any two points in the circuit is the amount of energy needed to move one unit of electric charge from one point to the other.

Electromotive Force (emf) is the work done by a cell (or any energy source) in driving a unit positive charge around the complete circuit, including through the cell itself.

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

Electric resistance is the property of a conductor by virtue of which it opposes the flow of electric current through it.

R = \[\frac {V}{I}\]

Conductance is the reciprocal of resistance — a measure of how easily current flows through a conductor.

G = \[\frac {1}{R}\]

Resistivity is a material-dependent property that measures how strongly a material opposes current flow, independent of its shape or size.

ρ = \[\frac {RA}{l}\]

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.

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.

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

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

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.

The phenomenon in which a current-carrying conductor produces a magnetic field around itself is called the magnetic effect of electric current.

Alternating current is a current that changes in magnitude and direction after equal intervals of time.

Direct current is a non-oscillatory current that flows in one direction in a circuit, from the cell to the cell.

Rectifier: The device used to convert ac to dc is called a rectifier.

Frequency is the number of the complete cycle of variations, gone through by the ac in one second.

\[\vec{E}=\frac{1}{4\pi\varepsilon_0}\frac{Q}{r^2}\hat{r}\]

The dimensional formula of the electric field E is:

E = \[\frac {F}{q_0}\] = \[\frac{[LMT^{-2}]}{[IT]}=[MLT^{-3}I^{-1}]\]

\[V_A-V_B=\frac{W}{Q}\]

ε = \[\frac {dW}{dq}\]

SI Unit: volt (V), where 1 V = 1 J C⁻¹
Dimensional Formula: [ML²T⁻³A⁻¹]

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