Magnetic fields due to currents
A magnet you can switch off
- Coil some wire, switch on a current, and you have an electromagnet.
- Switch it off and the magnetism vanishes.
- Currents make magnetic fields — and we can predict their shape.
Field around a straight wire
- The field forms circles around the wire.
- Find the direction with the right-hand grip rule: thumb along the current, fingers curl the way the field points.
The magnetic field around a long straight wire forms:
Concentric circles, with the direction given by the right-hand grip rule.
The right-hand grip rule gives the direction of the field around a current-carrying wire.
Thumb along the current, curled fingers show the way the circular field points.
Coils and solenoids
- A flat coil acts like a small bar magnet.
- Inside a long solenoid the field is nearly uniform; an iron core boosts it (used in electromagnets and transformers).
Inside a long solenoid the magnetic field is nearly ____.
Like a stretched bar magnet — the field inside is uniform and parallel to the axis.
Adding an iron core greatly increases a solenoid's magnetic field.
The iron's atomic magnets line up and add to the field — used in electromagnets and transformers.
Two parallel currents
- Each wire sits in the other's field. Using the left-hand rule:
- same direction → attract; opposite directions → repel. (This defines the ampere.)
Two parallel wires carrying currents in the same direction:
Same-direction currents attract; opposite-direction currents repel. This is the basis of the ampere.
You've got it
- a straight wire makes circular field lines — right-hand grip rule
- a solenoid's field is uniform inside; an iron core strengthens it
- parallel currents: same way attract, opposite ways repel