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

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

OR

The fundamental property of subatomic particles that gives rise to the phenomenon of experiencing force in the presence of electric and magnetic fields is called electric charge.

• Positive charge: Deficiency of electrons
• Negative charge: Excess of electrons
• SI unit: Coulomb (C)
• Dimension: [M⁰L⁰T¹A¹]

A unit positive charge used to test the strength of electric fields is called a test charge.

An electric charge which can be considered to exist at a single point is called a point charge.

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.

A field whose magnitude and direction is the same at all points is called a uniform electric field.

A field whose magnitude and direction are not the same at all points is called a non-uniform electric field.

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.

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.

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.

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 e.m.f. of a cell is defined as the energy spent (or the work done) per unit charge in taking a positive test charge around the complete circuit of the cell (i.e., in the circuit outside the cell as well as in the electrolyte inside the cell).

When no current is drawn from a cell i.e., when the cell is in open circuit, the potential difference produced by the chemical reaction between the terminals of the cell is called its electro-motive force (or e.m.f.).

The e.m.f of an electrical energy source is one volt if one joule of work is done by the source to drive one coulomb of charge completely around the circuit.

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.

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

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]

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.

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

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.

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.

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

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

• 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.

• 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.