Quick answer: Around a current-carrying circular loop, the magnetic field lines are concentric circles close to the wire, and they become larger further away. At the centre of the loop, they appear as straight lines. The field at the centre is stronger when the current is larger, the radius is smaller or the number of turns is greater. A coil of turns produces a field times as large as a single turn, because the fields of all the turns add up in the same direction.
When a straight current-carrying wire is bent into a circular loop, its magnetic field changes shape. Each small part of the loop produces circular field lines around it, and inside the loop these all point in the same direction, so they combine.
In CBSE Class 10 Science, Chapter 12 (Magnetic Effects of Electric Current), the magnetic field of a circular loop, the factors affecting its strength and why a coil of turns gives a field times stronger are frequently asked board exam topics.
What You Will Learn
- The pattern of magnetic field lines around a circular loop
- Why the field lines at the centre appear straight
- How to tell which face of a loop acts as a north or south pole
- The factors that affect the strength of the field at the centre
- Why a coil of turns gives a field times as large
1. Field Pattern Around a Circular Loop
Imagine a circular loop of wire passing through a horizontal cardboard sheet sprinkled with iron filings. When current flows:
Step 1: Close to the wire, the field lines form concentric circles around the wire, just like for a straight wire.
Step 2: Further from the wire, these circles become larger.
Step 3: At the centre of the loop, the arcs are so large that they appear as straight lines.
Step 4: Every part of the loop produces field lines that pass through the inside of the loop in the same direction, so the fields add up inside the loop.
Key point: At the centre of a current-carrying circular loop, the magnetic field lines are straight and pass through the loop at right angles to its plane.
2. Which Face Acts as North or South?
A current-carrying loop behaves like a thin, flat magnet. A commonly used way to find its poles is the clock rule:
| Looking at one face of the loop, the current appears to flow | That face acts as a |
|---|---|
| Clockwise | South pole |
| Anticlockwise | North pole |
You can also use the right-hand thumb rule on any part of the loop: curl your fingers around the wire with the thumb along the current; your fingers show the direction of the field lines.
3. Factors Affecting the Field at the Centre
| Factor | Effect on the magnetic field at the centre |
|---|---|
| Current () | Larger current gives a stronger field: |
| Radius of the loop () | Smaller radius gives a stronger field: |
| Number of turns () | More turns give a stronger field: |
Combining these:
4. Why Is the Field of n Turns n Times as Large?
Common board question: A coil of turns produces a magnetic field times as large as a single turn. Why?
Answer:
- A coil of turns is made of loops wound closely together.
- The current flows in the same direction in every turn.
- So the magnetic field produced by each turn at the centre is in the same direction.
- These fields add up, so the total field is times the field of a single turn.
5. Summary Table
| Feature | Key point |
|---|---|
| Field lines near the wire | Concentric circles |
| Field lines at the centre | Appear as straight lines |
| Clockwise current (seen from a face) | That face acts as a south pole |
| Anticlockwise current (seen from a face) | That face acts as a north pole |
| Strength at the centre | Increases with current and number of turns; decreases with radius |
6. Exam Tips and Common Mistakes
Exam Tip: When drawing the field of a circular loop, show concentric circles near the wire and straight lines at the centre.
Exam Tip: For the turns question, the key words are same direction and add up.
Common Mistake: Saying a larger loop gives a stronger field. A smaller radius gives a stronger field at the centre.
Common Mistake: Mixing up the clock rule. Clockwise means south; anticlockwise means north. A memory aid: the ends of the letter N can be traced anticlockwise, and those of S clockwise.
Frequently Asked Questions
What is the pattern of the magnetic field of a circular loop?
Near the wire, the field lines are concentric circles. They get larger further from the wire, and at the centre of the loop they appear as straight lines.
What factors affect the magnetic field at the centre of a circular coil?
The field increases with the current and the number of turns, and decreases as the radius of the coil increases.
Why does a coil of n turns produce n times the field of one turn?
The current in every turn flows in the same direction, so the fields of all the turns add up at the centre.
How do you find the north and south poles of a current loop?
Look at one face of the loop. If the current appears clockwise, that face is a south pole; if it appears anticlockwise, that face is a north pole.