Right-hand thumb rule

The idea

If a current-carrying straight conductor is held in the right hand with the thumb pointing along the current, the fingers wrap around it in the direction of the magnetic field lines (Maxwell's corkscrew rule says the same).

Holding a wire

A straight wire carries a current. You imagine holding it in your right hand with your thumb pointing the way the current goes.

What do your curled fingers show?

• The direction of the magnetic field lines around the wire
• The direction of the current
• The size of the current

Thumb and fingers

Press Next step. See the thumb, the fingers and the field lines.

A rule for direction

The thumb points along the current. The fingers curl round the wire in the direction of the field lines. If the current is reversed, the thumb is reversed, and so are the field lines. This is the right-hand thumb rule. It is also called Maxwell's corkscrew rule: drive a corkscrew along the current, and the way it turns is the way of the field.

If a current-carrying straight conductor is held in the right hand with the thumb pointing along the current, the fingers wrap around it in the direction of the magnetic field lines. (Maxwell's corkscrew rule says the same.)

Notes

Right-hand thumb rule: hold the wire in the right hand with the thumb along the current; the fingers wrap round it the way the field lines go.

Check yourself

You hold a straight current-carrying wire in your right hand with the thumb along the current. The curled fingers show…

The current in a straight wire is reversed. The thumb now points the other way, so the field lines…

A horizontal power line carries current from east to west. Seen from the east end, the field turns…

In Maxwell's corkscrew rule, a corkscrew is driven in the direction of the current. Its rotation shows…

• the direction of the current. The thumb shows the direction of the current. The curled fingers show the direction of the field lines.
• the direction of the field lines — correct. Yes!
• the direction of the force on the wire. The force on a wire in a magnetic field is found with a different rule, Fleming's left-hand rule. Here the fingers show the field lines.
• go round the wire the same way as before. The fingers curl the way the field lines go. With the thumb reversed, they curl the other way.
• become straight lines along the wire. The field lines about a straight wire are concentric circles whose direction is given by the right-hand thumb rule.
• go round the wire the other way — correct. Yes!
• clockwise — correct. Yes!
• anticlockwise. Seen from the east end it is clockwise. It turns anticlockwise when seen from the west end.
• towards the east end, in a straight line. The field lines form circles round the wire. Seen from the east end they turn clockwise.
• the direction of the current. The corkscrew moves along the current. Its rotation shows the direction of the magnetic field.
• the direction of the magnetic field — correct. Yes!
• the size of the current. The corkscrew rule is about direction. Its rotation shows the direction of the magnetic field.