Revision: Electricity and Magnetism >> Current Electricity Physics (English Medium) ICSE Class 10 CISCE

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

Current is defined as the rate of flow of charge.

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

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

 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 1 C of charge flows through a conductor in 1 s, it called 1 ampere (A) current.
I = `Q/t`

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

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’.

 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.

The obstruction offered to the flow of current by the conductor (or wire) is called its resistance.

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

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

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

The temperature coefficient is defined as the ratio of the increase in resistivity per degree rise in temperature to its resistivity at T₀.

One coulomb is the amount of electric charge transferred by a current of one ampere in one second.

One ohm is the resistance of a component when the potential difference of one volt applied across the component drives a current of one ampere through it.

Current density is a vector quantity, often known as an area vector or cross-sectional area vector, whose value is equal to the electric current flowing per unit area.

J = `"I"/"A"`

S.I unit is A/m².

The reciprocal of resistance is called conductllnce. It is denoted by the letter G. 

The conductors which obey Ohm's law are called ohmic resistors (or linear resistances).

The conductors which do not obey Ohm's law are called non ohmic resistors (or non-linear resistances).

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

The reciprocal of specific resistance is known as conductivity.

A superconductor is a substance of zero resistance (or infinite conductance) at a very low temperature.

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).

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.

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.).

When current is drawn from a cell i.e., when the cell is in closed circuit, the potential difference between the electrodes of the cell is known as its terminal voltage.

The terminal voltage of a cell is defined as the work done per unit charge in carrying a positive test charge around the external circuit connected across the terminals of the cell.

The resistance offered by the electrolyte inside the cell, to the flow of current, is called the internal resistance of the cell.

When two dry bodies are rubbed together, they get charged due to the movement of free electrons from one body to the other body, so they possess electrical energy.

Electric power (P) is the rate at which electrical energy is transferred or consumed in an electrical circuit.

In an electrical circuit, electric power is defined as the rate at which electrical energy is supplied by the source.

The unit in which the consumer pays the cost of electrical energy consumed is kWh.

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.

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

or

W = QV

Voltage drop in the Cell v = \[\frac {w}{q}\]

Electric Power P = \[\frac {W}{t}\] = VI = \[\frac {V^2}{R}\] = I²R

R = \[\frac {V^2}{P}\]

or

R = \[(\text{voltage rating on the appliance})^2 \over \text{power rating on the appliance}\]

Energy (in kWh) = power (in kW) × time (in h)

= \[\frac{\text{power (in watt)}\times\text{time (in hour)}}{1000}\]

= \[\frac{V(\mathrm{volt})\times I(\mathrm{ampere})\times t(\mathrm{hour})}{1000}\]

Cost of electricity = electrical energy in kWh × cost per kWh

According to Ohm’s law, the current flowing in a conductor is directly proportional to the potential difference across its ends, provided the physical conditions and temperature of the conductor remain constant.
No, it is not always true. E.g., Diode valve, junction diode, etc., do not obey Ohm’s law.

Statement: Ohm’s Law

"The electric current flowing through a conductor is directly proportional to the potential difference across its ends, provided the temperature and other physical conditions of the conductor remain constant."

Mathematically,

I ∝ V or V = I R

where:

• V = Potential difference (in volts)
• I = Current (in amperes)
• R = Resistance of the conductor (in ohms, Ω)

Explanation:

When two conductors at different electric potentials are joined by a metallic wire, electrons flow from the conductor at a lower potential (excess electrons) to the one at a higher potential (deficit of electrons). This movement of electrons results in an electric current.

• The current continues to flow until both conductors reach the same potential.
• For continuous current flow, a constant potential difference must be maintained across the ends of the conductor (e.g., using a battery or power supply).

Derivation / Mathematical Proof:

From Ohm’s Law:

I ∝ V ⇒ \[\frac {V}{I}\] = constant

This constant is defined as the resistance (R) of the conductor. Therefore,

V = I R   ---(1)

This is the mathematical form of Ohm’s Law.

Special Case:

If the current I = 1 A, then:

V = R

This implies that the resistance of a conductor is numerically equal to the potential difference across it when 1 ampere of current flows through it.

Conclusion:

Ohm's Law provides a fundamental relationship between voltage, current, and resistance in an electric circuit. It is widely used in the design and analysis of electrical and electronic systems.

• 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 (q = ±ne,  e = 1.6 × 10⁻¹⁹ C); there are two types of charges (positive and negative), as charges repel, unlike charges attract, and the SI unit of charge is coulomb (C).

• 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.

• 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.

• 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.

• In a parallel combination, the potential difference across each resistor is the same as that across the terminals of the battery.
• The total current in a parallel circuit is equal to the sum of the currents in the individual branches, and the equivalent resistance is less than the smallest resistance connected.

• Electrical energy supplied by a source equals the work done in moving a charge through a potential difference and is given by W = QV = VIt.
• Using Ohm’s law, electrical energy can also be expressed as W = I²Rt or W = (V²/R) t, and its S.I. unit is joule (J).

• Electrical power represents the rate at which electrical energy is supplied by the source in an electric circuit.
• The S.I. unit of electrical power is a watt (W), and larger units such as kilowatt, megawatt, and gigawatt are used for measuring higher power.

• Watt-hour (Wh) and kilowatt-hour (kWh) are the commercial units of electrical energy, used instead of joule for practical purposes.
• Electrical energy consumed by household and industrial appliances is measured in kilowatt-hours (kWh) and used to calculate electricity costs.
• One kilowatt-hour represents a large amount of energy, equivalent to the energy used by a high-power appliance in one hour.