lesson
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In 1820, Danish physicist Hans Christian Ørsted was setting up a battery demonstration when he noticed a nearby compass needle flick sideways every time he closed the switch.
That accidental discovery revealed a fundamental law of nature: an electric current (moving charge) creates an invisible magnetic field around itself.
What does this invisible magnetic field actually look like in the space surrounding a straight wire?
The Circular Field Pattern
If you poke a vertical copper wire through a sheet of white card and sprinkle iron filings on top, tapping the card makes the filings align into smooth, nested rings.
📊A clean, modern 3D-angled educational diagram. A vertical copper wire passes through the center of a flat horizontal gray card (#f8f9fa with #e2e8f0 border). A bold red arrow labeled 'Current (I)' points vertically upward along the wire. On the card surface, show 3 concentric circular magnetic field lines (in blue #0284c7) centered on the wire. The inner circle is tight, the middle circle is medium, and the outer circle is widely spaced to visually represent field strength dropping with distance. Place small magnetic compass needles along the circles, with their red North tips pointing counterclockwise along the circular paths. Label: 'Concentric circular field lines' and 'Field lines closer = stronger field'. Text color #1e2945, clean sans-serif typography, responsive to 350px width.
The magnetic field forms continuous concentric circles centered on the wire, with no distinct north or south poles like a bar magnet.
Notice how the rings are packed tightly near the wire and spread further apart as you move away — the spacing shows that the field is strongest near the wire.
How do you know whether these circular field lines travel clockwise or counterclockwise around the wire?
The Right-Hand Grip Rule
We determine the direction of the magnetic field using a simple physical trick called the Right-Hand Grip Rule.
Imagine gripping the wire with your right hand: point your thumb in the direction of conventional current, and your curled fingers point in the direction of the magnetic field lines.
📊An interactive-style dual panel comparison showing the Right-Hand Grip Rule. Left panel: A vertical wire with current flowing UPWARD. A stylized right hand grips the wire with the thumb pointing straight up (labeled 'Thumb = Current I'). Blue circular arrows wrap around the wire in the direction of the curled fingers (labeled 'Curled Fingers = Magnetic Field B, Counterclockwise'). Right panel: The same vertical wire with current flowing DOWNWARD. The right hand points thumb down, with fingers curling clockwise (labeled 'Clockwise'). Below both, show standard cross-section dot-and-cross notation: Circle with a dot (⊙) = Current toward you (anti-clockwise field), Circle with a cross (⊗) = Current away from you (clockwise field). High contrast #1e2945 text, smooth vector lines, responsive layout.