PHYSICS • ELECTRICITY AND MAGNETISM

Magnetic Effect of Electric Current

An electric current does more than heat a wire: it also produces a magnetic field around the conductor. This connection between electricity and magnetism is used in electric motors, relays, loudspeakers, bells and many other devices.

This guide explains the main ideas with labelled diagrams, practical examples and the rules commonly studied in school Physics.

1. What Is the Magnetic Effect of Electric Current?

The magnetic effect of electric current is the production of a magnetic field around a conductor when electric current flows through it. In 1820, Hans Christian Ørsted observed that a compass needle near a current-carrying wire deflected, showing that electric current can produce a magnetic effect.

A magnetic field is the region around a magnet or current-carrying conductor where magnetic forces can be detected. We represent it using magnetic field lines.

Key idea: When the current is switched off, the magnetic field produced by that current disappears. Reversing the current reverses the direction of the magnetic field.

2. Magnetic Field Around a Straight Current-Carrying Wire

The magnetic field around a long, straight wire forms a set of concentric circles centred on the wire. The field is stronger near the wire and becomes weaker farther away. Its direction depends on the direction of current.

Magnetic field around a straight wire carrying current out of the page A top view of a wire shown as a dot at the centre, meaning current comes out of the page. Concentric circular field lines have arrows pointing anticlockwise according to the right-hand thumb rule. Top view of the wire Current out • ⊙ Magnetic field lines Concentric circlesaround the wire Closer to the wire:stronger field The drawing is a top view,not a side view of the wire.
Top view: ⊙ means current comes out of the page. For current out of the page, the magnetic field circles anticlockwise; for current into the page (⊗), it circles clockwise.

Right-hand thumb rule

Hold the wire in your right hand with your thumb pointing in the direction of conventional current. Your curled fingers show the direction of the magnetic field around the wire.

Right-hand thumb rule A schematic hand with thumb pointing upward along a wire to show current direction and curled fingers around the wire to show magnetic field direction. Thumb: current (I) Curled fingers:magnetic field (B) Use the right handfor conventional current. Schematic illustration ofthe rule, not an anatomical drawing.
Use the thumb for conventional current and the curled fingers for the magnetic field direction.

3. Magnetic Field Due to a Circular Loop

When current flows through a circular loop, every small part of the wire produces a magnetic field. Near the centre, the fields combine to make a stronger field that is approximately along the axis of the loop. Increasing the current or adding more turns makes the field stronger, provided the other conditions are similar.

Magnetic field pattern of a circular current-carrying loop A circular loop shown in perspective as an ellipse, with current arrows and magnetic field lines passing through its centre. Circular loop carrying current Magnetic field Field lines combinenear the centre. Red loop:current in the wire Field direction depends onthe current direction.
Schematic field pattern for a circular loop. The exact shape depends on the viewpoint and current direction.

4. Magnetic Field of a Solenoid

A solenoid is a long coil made by winding insulated wire into many close turns. When current passes through it, the solenoid produces a magnetic field similar to that of a bar magnet. One end behaves like a north pole and the other like a south pole.

A solenoid represented by repeated copper-coloured loops around a tube. Magnetic field lines run through the inside and return outside, with north and south poles marked. Magnetic field of a solenoid SN Coil of insulated wire Field inside is strong and nearly uniform Pole labels shown for this chosen current direction; reversing current swaps the north and south ends.
A solenoid produces a field similar to a bar magnet. The north and south labels depend on the direction of current.

Electromagnet: If a soft iron core is placed inside a current-carrying coil, the magnetic field becomes much stronger. This arrangement is called an electromagnet. Its strength can be changed by changing the current or the number of turns.

5. What Happens When a Current-Carrying Conductor Is Placed in a Magnetic Field?

A current-carrying conductor placed in an external magnetic field experiences a force. The force direction depends on both the current direction and the magnetic field direction. If either is reversed, the force reverses. If the conductor is parallel to the field, the magnetic force on it is zero in the ideal school-level model.

Force on a current-carrying conductor in a magnetic field A conductor carries current out of the page between a north pole on the left and a south pole on the right. The magnetic field points from left to right, so the force points upward, perpendicular to both. NS Force (F) Current out of page (⊙) Magnetic field (B): from N to S Schematic orientation; the force is perpendicular to both I and B.
With current out of the page and magnetic field from left to right, the force is upward by Fleming's left-hand rule.

Fleming's left-hand rule

Stretch the thumb, forefinger and middle finger of your left hand so that they are mutually perpendicular:

  • Forefinger: direction of magnetic field (from north to south).
  • Middle finger: direction of conventional current.
  • Thumb: direction of force or motion.

This rule is used to determine the direction of force in an electric motor. Do not confuse it with Fleming's right-hand rule, which is used for induced current in a generator.

6. Electric Motor: Electrical Energy to Mechanical Energy

An electric motor uses the force on a current-carrying coil in a magnetic field to produce rotation. A simple DC motor contains a coil, a magnet, a split-ring commutator, brushes and a power source.

Simplified labelled diagram of a DC electric motor A rectangular coil between north and south magnet poles is connected to a split-ring commutator and brushes supplied by a battery. Labels identify the main parts. NS Coil (armature) Axle Split-ringcommutator Brush Brush Battery / DC supply
Simplified schematic of a basic DC motor. Practical motor designs vary, but the diagram identifies the main parts and their roles.

How it works: The two sides of the current-carrying coil experience forces in opposite directions, producing a turning effect. The split-ring commutator reverses the current in the coil every half-turn so that the coil continues rotating in the same overall direction.

Examples: Electric fans, mixers, water pumps and many battery-powered toys use electric motors.

7. Electromagnetic Induction and Electric Generator

Electromagnetic induction is the production of an induced voltage when the magnetic field linked with a conductor or coil changes. If the circuit is closed, an induced current can flow. The change may be produced by moving a magnet near a coil or moving the coil in a magnetic field.

Demonstration of electromagnetic induction A bar magnet moves toward a coil connected to a galvanometer. The galvanometer needle deflects while the magnetic field through the coil changes. NS Moving magnet Coil Galvanometer Changing magneticfield through the coil Movement produces aninduced voltage.
When the magnet moves relative to the coil, the galvanometer can deflect, indicating an induced current. If the magnet is held still, the reading returns to zero after the transient.

Fleming's right-hand rule

Stretch the thumb, forefinger and middle finger of your right hand so that they are mutually perpendicular:

  • Thumb: direction of motion of the conductor.
  • Forefinger: direction of magnetic field (north to south).
  • Middle finger: direction of induced conventional current.

An electric generator uses electromagnetic induction to convert mechanical energy into electrical energy. Power stations use generators driven by turbines; the turbine may be turned by steam, flowing water, wind or another energy source.

8. Motor and Generator: Do Not Confuse Them

FeatureElectric motorElectric generator
Main principleForce on a current-carrying conductor in a magnetic fieldElectromagnetic induction
Energy conversionElectrical → mechanicalMechanical → electrical
Rule commonly used for directionFleming's left-hand ruleFleming's right-hand rule
ExampleElectric fanGenerator in a power station

9. Worked Examples

Example 1: Direction of the magnetic field

Question: A straight wire carries conventional current out of the page. What is the direction of its magnetic field when viewed from the front?

Answer: Apply the right-hand thumb rule. Point your right thumb towards yourself, out of the page. Your fingers curl anticlockwise, so the magnetic field is anticlockwise.

Example 2: Increasing the strength of a solenoid

Question: Give two ways to increase the magnetic field of a solenoid.

Answer: Increase the current through the coil or increase the number of turns per unit length. Inserting a suitable soft iron core also makes the electromagnet much stronger.

Example 3: Choosing the correct Fleming rule

Question: Which rule is used to find the direction of force on a current-carrying wire in a magnetic field?

Answer: Fleming's left-hand rule. Fleming's right-hand rule is used for the direction of induced current in a generator.

Example 4: What if the current is reversed?

Question: What happens to the magnetic field around a straight wire if the current is reversed?

Answer: The direction of the magnetic field reverses. If the magnitude of current stays the same, the field strength at the same distance from the wire stays the same in the ideal case.

10. Uses in Everyday Life

  • Electric bells and relays: an electromagnet attracts a metal armature.
  • Electric motors: fans, mixers, pumps and many machines convert electrical energy into motion.
  • Loudspeakers: electric signals interact with a magnetic field to move a coil and diaphragm, producing sound.
  • Generators: mechanical motion is used to produce electrical energy by electromagnetic induction.
  • Scrap-yard cranes: powerful electromagnets lift and release magnetic metals by switching the current on and off.
  • Magnetic door locks and switching devices: electromagnets can control moving parts.

11. Common Mistakes to Avoid

  • Do not mix up the right-hand thumb rule with Fleming's left-hand or right-hand rules.
  • Magnetic field lines outside a bar magnet go from north to south; around a straight wire, the lines are concentric circles.
  • A solenoid behaves like a bar magnet, but its poles swap when the current is reversed.
  • A motor uses electrical energy to produce motion; a generator uses motion to produce electrical energy.
  • An induced current is produced when the magnetic flux linked with a circuit changes, not simply because a magnet is nearby and stationary.
  • In circuit diagrams, current direction means conventional current, from the positive terminal to the negative terminal through the external circuit.

Quick Revision

  • Electric current produces a magnetic field around a conductor.
  • Right-hand thumb rule: thumb = current; curled fingers = magnetic field.
  • A solenoid produces a magnetic field similar to that of a bar magnet.
  • Fleming's left-hand rule gives the direction of force in the motor effect.
  • Electromagnetic induction produces voltage when magnetic flux changes.
  • Fleming's right-hand rule gives the direction of induced current in a generator.

Memory tip: Left = Motor (force/motion); Right = Generator (induced current).

Practice Questions

  1. What is meant by the magnetic effect of electric current?
  2. Describe the shape of magnetic field lines around a long, straight current-carrying conductor.
  3. State the right-hand thumb rule.
  4. What is a solenoid? How can its magnetic field be strengthened?
  5. State Fleming's left-hand rule and name one device that uses it.
  6. What is electromagnetic induction?
  7. Write two differences between an electric motor and an electric generator.
  8. Why does a compass needle deflect when it is placed near a current-carrying wire?

Frequently Asked Questions

Does every current-carrying wire produce a magnetic field?

Yes. An electric current produces a magnetic field around the conductor. The field may be weak or strong depending on current, distance and the conductor's arrangement.

Why is a solenoid useful?

A solenoid can produce a strong, controllable magnetic field, especially when it has many turns and a suitable soft iron core.

What is the difference between the two Fleming rules?

Fleming's left-hand rule is used for the force or motion in a motor. Fleming's right-hand rule is used for the direction of induced current in a generator.

Can a magnet induce current without moving?

A stationary magnet next to a stationary coil does not continuously induce current in the coil. Induction requires a change in magnetic flux through the circuit.