The Compass Flinch
Oersted's accident — currents make magnetic fields
What this lesson covers
Why it matters
1820. Oersted is mid-lecture when he closes a switch and a compass on the bench FLINCHES. Electricity and magnetism had been strangers for all of history — until that twitch.
The idea in plain words
The rule that reads every wire. Tap the terms.
right-hand thumb rule
Current flows NORTH in a horizontal wire; a compass sits ABOVE it. Which way does the needle's N pole swing?
The needle's N swings west
- the rule: thumb — points along the current
- the rule: curled fingers — give the circular field direction around the wire
- the upgrade: loop & solenoid — coil the wire and the circles stack into a bar-magnet-like field with N and S faces
- Thumb north, fingers curl: above the wire the field points WEST
Predict first
Switch on a current in a wire lying over a compass. The needle…
A current is surrounded by circular field lines (right-hand thumb rule). The needle swings to follow them — perpendicular tendencies, not parallel.
- deflects — the current creates a magnetic field around the wire — correct
- does nothing — wires aren't magnets
- points along the wire
What you do
Recreate the famous accident: switch on, watch the flinch, then reverse the current.
Check yourself
The field lines around a long straight current are…
Iron filings draw perfect rings — tighter and stronger near the wire.
Reversing the current…
Flip the thumb, the fingers' curl flips: your compass swings the other way, as you saw.
A solenoid with current behaves like…
The stacked circular fields add into a through-going field — the heart of every electromagnet.
- concentric circles centred on the wire — correct
- straight lines parallel to the wire
- radial spokes
- reverses the field everywhere around the wire — correct
- doubles the field
- cancels the field
- a bar magnet, with N and S ends you can switch off — correct
- a charged rod
- nothing — coiling cancels the field