Magnetic Effect of Electric Current

When electric current flows through a conductor, a magnetic field is produced around the conductor. This phenomenon is called the magnetic effect of electric current.

Danish scientist Hans Christian Ørsted first demonstrated the relationship between electricity and magnetism experimentally in 1820.

👉 Oersted's Experiment

A compass needle is placed horizontally in the north-south direction. A straight conducting wire is placed close to and parallel to the compass needle. The wire is connected to a cell or battery, a switch, and the rest of the circuit.

When the switch is closed and electric current flows through the wire, the north pole of the compass needle is seen to deflect from its normal north-south position.

When the current is switched off, the needle returns approximately to the north-south direction. If the direction of current is reversed, the direction of deflection of the compass needle also reverses.

This shows that electric current produces a magnetic field around the wire.

Direction of deflection in Oersted's experiment

The following table applies to a specific arrangement in which the wire is parallel to the compass needle along the north-south direction and the A-B ends are defined as shown in the arrangement.

The east-west direction of deflection can change if the position of the wire or the orientation of the diagram is changed. Therefore, it is more useful to understand the rule for determining field direction than to memorize the table alone.

Conclusion

Oersted's experiment shows that:

• A magnetic field is produced around a current-carrying conductor.
• Reversing the direction of current reverses the direction of the magnetic field.
• The deflection of the compass needle provides evidence for the presence of this magnetic field.
• When the current is switched off, the magnetic field produced by the current disappears.

Thus, electricity and magnetism are closely related.

👉 Ampere's Swimming Rule

Historically, Ampere's swimming rule has been used to explain the direction of deflection of a compass needle near a current-carrying wire.

Imagine a person swimming along the current-carrying wire with the person's head pointing in the direction of current and the face turned towards the compass needle. The north pole of the compass needle will deflect towards the person's left-hand side.

This rule is especially useful for understanding compass deflection when the relative positions of the wire and compass needle are specified.

👉 Right-Hand Thumb Rule

The most commonly used rule for determining the direction of the magnetic field around a straight current-carrying conductor is the Right-Hand Thumb Rule.

Hold the straight wire in your right hand so that the extended thumb points in the direction of electric current. The curled fingers then show the direction of the magnetic field around the wire.

In short:

• Thumb → direction of electric current
• Curled fingers → direction of magnetic field

👉 Magnetic Field Around a Straight Current-Carrying Wire

A long vertical copper wire is passed through a horizontal cardboard sheet. Iron filings are sprinkled on the cardboard. When electric current is passed through the wire and the cardboard is gently tapped, the iron filings arrange themselves in patterns resembling concentric circles around the wire.

This shows that the magnetic field lines around a long straight current-carrying conductor are circular, with the wire at their center.

At any point on a magnetic field line, the tangent drawn to the field line gives the direction of the magnetic field at that point.

What factors affect the strength of the magnetic field?

For a straight current-carrying conductor:

• Increasing the current makes the magnetic field stronger.
• Increasing the distance from the wire makes the magnetic field weaker.

Thus, at the same distance, a larger current produces a stronger magnetic field.

What happens when the current direction is reversed?

If the direction of electric current in the straight wire is reversed, the direction of the magnetic field around the wire also reverses.

Therefore, changing the current direction also changes the magnetic field direction.

This idea becomes especially important when studying the force on a current-carrying conductor and the working principle of an electric motor.

👉 Magnetic Field of a Circular Current-Carrying Conductor

When electric current flows through a circular wire, a magnetic field is produced around the loop.

Each small section of the circular wire may be considered as a small straight current-carrying conductor. At the center of the loop, the magnetic fields due to the different parts of the wire act in the same direction and combine, making the field relatively strong near the center.

When viewed from one face of the circular loop:

• If the current appears clockwise, that face behaves like a south pole.
• If the current appears anticlockwise, that face behaves like a north pole.

The opposite face has the opposite magnetic polarity.

How can the magnetic field of a circular loop be made stronger?

The magnetic field at the center of a circular current-carrying conductor becomes stronger when:

• The electric current is increased.
• The number of turns of the coil is increased.
• The radius of the circular turns is decreased while other conditions remain unchanged.

In a multi-turn circular coil, the magnetic fields produced by the individual turns add together because they act in the same direction.

👉 Identifying the Poles of a Cell Using a Compass Needle

A compass needle may be placed in the north-south direction, and the two ends of a wire may be connected to the two terminals of a cell. When current flows through the wire, the direction of deflection of the compass needle's north pole can be observed.

Using Ampere's swimming rule or an appropriate magnetic-field direction rule, the direction of current in the wire can be found.

Conventional current flows through the external circuit from the positive terminal to the negative terminal. Therefore, once the direction of current is known, the positive and negative terminals of the cell can be identified.

🧠 Quick Revision

⭐ A magnetic field is produced around a current-carrying wire.

⭐ In Oersted's experiment, the deflection of a compass needle shows the presence of this magnetic field.

⭐ The magnetic field lines around a straight wire are concentric circles.

⭐ The Right-Hand Thumb Rule gives the direction of the magnetic field around a straight current-carrying wire.

⭐ Increasing current increases magnetic field strength, while increasing distance from the wire decreases it.

⭐ Reversing the direction of current reverses the direction of the magnetic field.

⭐ For a circular current-carrying loop, the face from which current appears clockwise behaves like a south pole, while the face from which it appears anticlockwise behaves like a north pole.

✍ Try It Yourself

🔹 1. Why does the compass needle stop deflecting when the current through the nearby wire is switched off?

🔹 2. Why does the direction of compass deflection reverse when the current direction is reversed?

🔹 3. What is the shape of the magnetic field lines around a straight current-carrying wire?

🔹 4. What can be determined using the Right-Hand Thumb Rule?

🔹 5. How does the magnetic field strength change if the current in a straight wire is increased?

🔹 6. How does the magnetic field strength change as the distance from the wire increases?

🔹 7. If current in a circular loop appears anticlockwise from one face, which magnetic pole does that face represent?

🔹 8. What happens to the magnetic field at the center of a circular coil if the number of turns is increased?