Magnetic Field Around a Straight Wire
Pass current through a vertical wire and move a compass around it to explore the right-hand thumb rule.
- NCERT Activities 12.4 and 12.5
- Right-hand thumb rule
Drag the purple probe compass, or tap anywhere to move it there. Tap the wire to reverse the current.
Field lines
Anticlockwise
Probe distance r
4.5 cm
Wire's field at probe
13.4 μT
B ∝ I / r
Needle deflection
Turn on Earth's field
Blue circles are drawn where the wire's field is above 8 μT, like iron filings that line up only where the field is strong enough.
About the Magnetic Field Around a Straight Wire
This simulation shows the magnetic field around a straight current-carrying conductor from CBSE Class 10 Magnetic Effects of Electric Current. The field lines appear as concentric circles around the wire, and the compass needles turn to follow them when you switch on the current.
Reverse the current to see every needle flip, apply the right-hand thumb rule, and drag a probe compass to see how the field weakens with distance. Turn on the Earth's magnetic field to see why the deflection grows as you increase the current, as in NCERT Activity 12.5.
Try this
- 1Reverse the current. What happens to every compass needle?
- 2Drag the probe compass away from the wire. How does the field strength change?
- 3Turn on the Earth's field and set the current to 1 A. Why do far-away needles point north?
What you will learn
- Show that field lines around a straight wire are concentric circles.
- Find the direction of the field with the right-hand thumb rule.
- Relate the field strength to the current and the distance from the wire.
Key ideas
- Right-hand thumb rule
- Hold the wire with the thumb along the current. Your curled fingers show the direction of the field lines.
- Field strength
- The field increases with current and decreases as you move away from the wire.
Board exam tip
When asked for the field pattern, draw concentric circles that get farther apart as you move away from the wire.
Frequently asked questions
What is the right-hand thumb rule?
Hold a current-carrying straight wire in your right hand with the thumb pointing along the current. Your curled fingers show the direction of the magnetic field lines around the wire.
What is the shape of the field lines around a straight conductor?
They are concentric circles centred on the wire, in a plane perpendicular to it. They are closer together near the wire, where the field is stronger.
How does the field change with current and distance?
The magnetic field becomes stronger when the current increases and weaker as you move away from the wire.