Quick answer: The magnetic field lines around a straight current-carrying conductor are concentric circles centred on the wire. Their direction is given by the right-hand thumb rule, and it reverses if the current is reversed. The field is stronger for a larger current and weaker farther from the wire.
Oersted's discovery
In 1820, Hans Christian Oersted noticed that a compass needle placed near a wire was deflected when current flowed through the wire. This showed that an electric current produces a magnetic field.
Activity: iron filings around a straight wire
| current
|
.----+----. cardboard
( ( ( | ) ) ) filings form
'----+----' circles
|
Step 1: Pass a thick copper wire vertically through a hole in the middle of a horizontal sheet of cardboard.
Step 2: Connect the wire in a circuit with a battery, a key and a rheostat.
Step 3: Sprinkle iron filings evenly on the cardboard.
Step 4: Switch on the current and tap the cardboard gently.
Observation: The iron filings arrange themselves in concentric circles around the wire. These circles show the magnetic field lines.
Placing a small compass on a circle shows the direction of the field at that point.
Direction: right-hand thumb rule
Rule: Imagine holding the current-carrying wire in your right hand with the thumb pointing in the direction of the current. Your fingers wrap around the wire in the direction of the magnetic field lines.
| Current direction (vertical wire, seen from above) | Field direction |
|---|---|
| Upwards (towards you) | Anticlockwise |
| Downwards (away from you) | Clockwise |
If the current is reversed, the compass needle turns the other way, showing that the field direction has reversed.
What affects the strength of the field?
| Change | Effect on the field at a point | Evidence |
|---|---|---|
| Increase the current | Field becomes stronger | Compass deflects more |
| Move farther from the wire | Field becomes weaker | Compass deflects less |
Beyond NCERT: In Class 12 you learn that the field is directly proportional to the current and inversely proportional to the distance from the wire (). NCERT Class 10 describes these effects in words.
The field lines are closer together near the wire, where the field is stronger, and farther apart away from the wire.
Summary table
| Feature | Straight current-carrying wire |
|---|---|
| Shape of field lines | Concentric circles centred on the wire |
| Direction | Right-hand thumb rule |
| Reverse the current | Field direction reverses |
| Larger current | Stronger field |
| Greater distance | Weaker field |
Exam tips
- In diagrams, draw the circles centred on the wire and put arrows on them to show direction.
- State the right-hand thumb rule in full; it is often asked as a definition.
- Give both factors: current (more current, stronger field) and distance (farther away, weaker field).
- Mention that the field lines are closer together near the wire.
Common mistakes
- Drawing straight or radial lines instead of circles around the wire.
- Using the left hand for the right-hand thumb rule.
- Saying the field gets stronger farther from the wire.
- Confusing the right-hand thumb rule (field direction) with Fleming's left-hand rule (force direction).
Frequently Asked Questions
What is the pattern of magnetic field lines around a straight current-carrying wire?
The field lines are concentric circles with the wire at their centre.
How do you find the direction of the magnetic field around a wire?
Use the right-hand thumb rule: point the right thumb along the current, and the curled fingers show the direction of the field.
What happens to the field if the current is reversed?
The direction of the magnetic field reverses, so a compass needle near the wire turns the other way.
How does distance from the wire affect the field?
The magnetic field becomes weaker as the distance from the wire increases.
Who discovered that a current produces a magnetic field?
Hans Christian Oersted discovered it in 1820 when a current-carrying wire deflected a compass needle.