Chapter 4 - Moving Charges and Magnetism

Electric charges at rest create electric fields. Moving charges also produce magnetic fields.

Moving Charges and Magnetism Class 12 explains the relationship between electric current and magnetism. Students study magnetic forces, the motion of charged particles, fields produced by currents and the working of a moving-coil galvanometer.

Magnetic field and Oersted’s experiment

Oersted’s experiment showed that a current-carrying conductor affects a nearby compass needle.

This established that electric current produces a magnetic field.

Magnetic field is a vector quantity. Its direction can be represented using magnetic field lines and right-hand rules.

Lorentz force

A charge moving through electric and magnetic fields experiences the Lorentz force.

The magnetic part of the force depends on:

• Charge

• Speed

• Magnetic-field strength

• Angle between velocity and magnetic field

The magnetic force is perpendicular to both the velocity and magnetic field.

A stationary charge experiences no magnetic force.

Work done by magnetic force

Because magnetic force is perpendicular to velocity, it does no work on a charged particle.

It can change the direction of velocity but not its speed or kinetic energy.

This is a critical distinction between electric and magnetic forces. An electric field can change both speed and direction.

Motion of charged particles

If a charged particle enters a uniform magnetic field perpendicular to the field, it moves in a circular path.

If it also has a velocity component parallel to the field, the path becomes helical.

The direction of curvature depends on the sign of the charge.

Students connect magnetic force with centripetal force to calculate radius and time-related quantities.

Biot–Savart law

The Biot–Savart law gives the magnetic field produced by a small current element.

Students apply it to calculate the field due to a current-carrying circular loop.

The law involves a vector product, so direction is as important as magnitude.

Ampere’s circuital law

Ampere’s law relates the circulation of magnetic field around a closed path to the current enclosed.

It is especially useful for symmetric current distributions.

Students use it for:

• An infinitely long straight conductor

• A long straight solenoid, treated qualitatively as prescribed

Force on a current-carrying conductor

A conductor carrying current in an external magnetic field experiences force.

The force depends on current, conductor length, magnetic field and relative orientation.

Students must distinguish between:

• The magnetic field produced by the conductor

• The external magnetic field acting on the conductor

Parallel current-carrying conductors

Two parallel currents exert magnetic forces on each other.

Currents in the same direction attract. Currents in opposite directions repel.

This interaction historically contributed to the definition of the ampere.

Torque on a current loop

A current loop in a uniform magnetic field may experience torque.

The loop behaves like a magnetic dipole with a magnetic dipole moment.

The torque tends to align the dipole moment with the magnetic field.

Moving-coil galvanometer

A moving-coil galvanometer detects and measures small currents using the torque on a current-carrying coil.

Students study:

• Principle and construction

• Current sensitivity

• Role of radial magnetic field

• Conversion into an ammeter

• Conversion into a voltmeter

An ammeter requires very low resistance and is connected in series. A voltmeter requires very high resistance and is connected in parallel.

Common student difficulties

Students often:

• Apply magnetic force to a stationary charge

• Forget the sign of the moving charge

• Use the right-hand rule directly for electrons without reversing direction

• Assume magnetic force changes speed

• Confuse the source field with the external field

• Use the wrong current direction in force questions

• Connect ammeters and voltmeters incorrectly

• Mix the resistances required for galvanometer conversion

Teachoo resources

Teachoo provides:

• Direction diagrams

• Charged-particle motion questions

• Biot–Savart and Ampere-law derivations

• Current-loop problems

• Galvanometer-conversion numericals

• NCERT solutions

• MCQs and competency-based questions

Frequently Asked Questions

Can a magnetic field change the speed of a charged particle?

A magnetic field alone cannot change its speed because the magnetic force does no work. It can change the direction.

What happens when velocity is parallel to magnetic field?

The magnetic force is zero because the vector product of parallel vectors is zero.

Why does a charged particle move in a circle?

When velocity is perpendicular to a uniform magnetic field, magnetic force acts as the centripetal force.

Why is an ammeter connected in series?

It must measure the current passing through the circuit branch and has very low resistance.

Why is a voltmeter connected in parallel?

It measures potential difference across a component and has very high resistance.

Is this chapter connected with Class 11 mechanics?

Yes. Circular motion, vector products, force and torque are directly used.