Year 12 / Electromagnetism
Currents make curling fields.
A long straight current-carrying wire produces circular magnetic field lines. Point your right thumb along conventional current; curled fingers show the magnetic field direction. Reversing current reverses the field.
Inside a long ideal solenoid the field is approximately uniform and proportional to turns per metre and current. This approximation works away from the ends. More tightly packed turns increase the field without changing current.
Field around a straight wire
Explore the relationship
Long straight wire in vacuum; B = μ₀I/(2πr). Field lines circle the wire.
Magnetic field / μT
Tap or focus a plotted point to read its values.
View plotted data as a table
| Series | Distance from wire / cm | Magnetic field / μT |
|---|---|---|
| Magnetic field / μT | 1 | 100 |
| Magnetic field / μT | 1.15 | 86.9565 |
| Magnetic field / μT | 1.3 | 76.9231 |
| Magnetic field / μT | 1.45 | 68.9655 |
| Magnetic field / μT | 1.6 | 62.5 |
| Magnetic field / μT | 1.75 | 57.1429 |
| Magnetic field / μT | 1.9 | 52.6316 |
| Magnetic field / μT | 2.05 | 48.7805 |
| Magnetic field / μT | 2.2 | 45.4545 |
| Magnetic field / μT | 2.35 | 42.5532 |
| Magnetic field / μT | 2.5 | 40 |
| Magnetic field / μT | 2.65 | 37.7358 |
| Magnetic field / μT | 2.8 | 35.7143 |
| Magnetic field / μT | 2.95 | 33.8983 |
| Magnetic field / μT | 3.1 | 32.2581 |
| Magnetic field / μT | 3.25 | 30.7692 |
| Magnetic field / μT | 3.4 | 29.4118 |
| Magnetic field / μT | 3.55 | 28.169 |
| Magnetic field / μT | 3.7 | 27.027 |
| Magnetic field / μT | 3.85 | 25.974 |
| Magnetic field / μT | 4 | 25 |
| Magnetic field / μT | 4.15 | 24.0964 |
| Magnetic field / μT | 4.3 | 23.2558 |
| Magnetic field / μT | 4.45 | 22.4719 |
| Magnetic field / μT | 4.6 | 21.7391 |
| Magnetic field / μT | 4.75 | 21.0526 |
| Magnetic field / μT | 4.9 | 20.4082 |
| Magnetic field / μT | 5.05 | 19.802 |
| Magnetic field / μT | 5.2 | 19.2308 |
| Magnetic field / μT | 5.35 | 18.6916 |
| Magnetic field / μT | 5.5 | 18.1818 |
| Magnetic field / μT | 5.65 | 17.6991 |
| Magnetic field / μT | 5.8 | 17.2414 |
| Magnetic field / μT | 5.95 | 16.8067 |
| Magnetic field / μT | 6.1 | 16.3934 |
| Magnetic field / μT | 6.25 | 16 |
| Magnetic field / μT | 6.4 | 15.625 |
| Magnetic field / μT | 6.55 | 15.2672 |
| Magnetic field / μT | 6.7 | 14.9254 |
| Magnetic field / μT | 6.85 | 14.5985 |
| Magnetic field / μT | 7 | 14.2857 |
| Magnetic field / μT | 7.15 | 13.986 |
| Magnetic field / μT | 7.3 | 13.6986 |
| Magnetic field / μT | 7.45 | 13.4228 |
| Magnetic field / μT | 7.6 | 13.1579 |
| Magnetic field / μT | 7.75 | 12.9032 |
| Magnetic field / μT | 7.9 | 12.6582 |
| Magnetic field / μT | 8.05 | 12.4224 |
| Magnetic field / μT | 8.2 | 12.1951 |
| Magnetic field / μT | 8.35 | 11.976 |
| Magnetic field / μT | 8.5 | 11.7647 |
| Magnetic field / μT | 8.65 | 11.5607 |
| Magnetic field / μT | 8.8 | 11.3636 |
| Magnetic field / μT | 8.95 | 11.1732 |
| Magnetic field / μT | 9.1 | 10.989 |
| Magnetic field / μT | 9.25 | 10.8108 |
| Magnetic field / μT | 9.4 | 10.6383 |
| Magnetic field / μT | 9.55 | 10.4712 |
| Magnetic field / μT | 9.7 | 10.3093 |
| Magnetic field / μT | 9.85 | 10.1523 |
| Magnetic field / μT | 10 | 10 |
Explore: Double current, then double distance. Compare the field readings.
Double the current in a long wire and double distance from it.
- B is proportional to I/r.
- The two factors cancel, leaving B unchanged.
Assumed knowledge
Conventional current and scientific notation.
Learning checkpoints & source
YOUR LEARNING CHECKPOINT- Represent fields around wires and solenoids.
- Use field-strength equations and direction rules.
READY TO TRY IT?