Chapter 3 - Current Electricity

Electrostatics studies charges at rest. Current Electricity Class 12 studies what happens when charges move continuously through a conductor.

Students learn how electric current arises, why conductors resist current, how cells supply energy and how complex circuits can be analysed.

Electric current

Electric current is the rate of flow of charge through a cross-section.

The conventional current direction is taken as the direction in which positive charge would move. In metallic conductors, electrons drift in the opposite direction.

A large number of electrons move randomly inside a metal even without an electric field, but their random motion produces no net current.

Drift velocity

When an electric field is applied, free electrons acquire a small average drift velocity.

Students derive the relationship between current and:

• Number density of charge carriers

• Charge on each carrier

• Cross-sectional area

• Drift velocity

The drift velocity is usually small even though an electrical effect is established through a circuit rapidly.

Mobility and conductivity

Mobility describes how easily a charge carrier responds to an electric field.

Conductivity measures a material’s ability to conduct current. Resistivity measures its opposition to current flow.

Conductivity is the reciprocal of resistivity.

Ohm’s law

Ohm’s law states that potential difference is proportional to current for an ohmic conductor under constant physical conditions.

Not every device obeys Ohm’s law. Students compare linear and non-linear voltage-current characteristics.

A straight V-I graph through the origin indicates ohmic behaviour under the relevant conditions.

Resistance and resistivity

Resistance depends on:

• Length of the conductor

• Cross-sectional area

• Material

• Temperature

Resistivity is a material property at a specified temperature and does not depend on the conductor’s dimensions.

Students must distinguish between resistance, resistivity and conductivity.

Temperature dependence of resistance

For metallic conductors, resistance generally increases with temperature over the studied range.

This happens because increased lattice vibrations obstruct the motion of charge carriers.

Students apply the temperature coefficient of resistance in numerical problems.

Electrical energy and power

When charge moves through a potential difference, electrical energy is transferred.

Electrical power measures the rate of this energy transfer.

Students use different equivalent power expressions depending on which circuit quantities are known.

Cells, EMF and internal resistance

A cell converts chemical or another form of energy into electrical energy.

Electromotive force, or EMF, is energy supplied by the cell per unit charge. It is not literally a mechanical force.

A real cell has internal resistance. Therefore, terminal potential difference may differ from EMF when current flows.

Students study:

• EMF

• Terminal voltage

• Internal resistance

• Cells in series

• Cells in parallel

• Charging and discharging conditions

Kirchhoff’s rules

Kirchhoff’s rules are used for circuits that cannot be simplified using only series and parallel combinations.

The junction rule follows conservation of charge.

The loop rule follows conservation of energy.

Students must select assumed current directions and a loop-traversal direction consistently. A negative current means the actual direction is opposite to the assumed one.

Wheatstone bridge

A Wheatstone bridge compares resistances using a balanced network.

At balance, no current flows through the galvanometer branch. The balance condition allows an unknown resistance to be calculated.

This topic also connects with Class 12 Physics practical work.

Common student difficulties

Students often:

• Confuse electron flow with conventional current

• Assume individual electrons move through the circuit almost instantly

• Mix resistance and resistivity

• Apply Ohm’s law to non-ohmic devices

• Confuse EMF with terminal voltage

• Ignore internal resistance

• Use incorrect signs in Kirchhoff equations

• Apply series or parallel rules to circuits that are not truly series or parallel

Learning with Teachoo

Teachoo provides:

• Current and drift-velocity explanations

• V-I graph analysis

• Resistance and resistivity numericals

• Cell-combination questions

• Kirchhoff circuit solutions

• Wheatstone-bridge problems

• NCERT solutions

• MCQs, assertion-reason and case-based questions

Circuit solutions are redrawn clearly before equations are written.

Frequently Asked Questions

What is electric current?

Electric current is the rate at which electric charge flows through a cross-section.

In which direction does current flow in a metal?

Conventional current is opposite to electron drift.

What is the difference between resistance and resistivity?

Resistance depends on the material and dimensions of a particular conductor. Resistivity is an intrinsic material property at a specified temperature.

Is EMF a force?

No. EMF is energy supplied per unit charge by a source.

Why can terminal voltage be less than EMF?

When a cell supplies current, some potential is lost across its internal resistance.

What does a negative current in a circuit solution mean?

It means the actual current direction is opposite to the initially assumed direction.