Direction, orientation and rules for orientation

Three vectors, three directions

Laplace’s force F = I * dl ^ B is perpendicular to the plane formed by the conductor (dl) and the magnetic field B. Its direction and sense are determined using the rules for the cross product.

Right-hand rule

Orientate the thumb, index finger and middle finger according to the three-finger rule:

• the index finger points in the direction of the current I (dl)
• the middle finger points in the direction of the field B
• the thumb then points in the direction of the force F

An equivalent method is the corkscrew rule: rotate dl towards B along the shortest path; the direction in which the corkscrew turns gives the direction of F.

Practical example

A horizontal wire pointing eastwards carries a current. It is immersed in a vertical magnetic field B, directed upwards. The cross-product dl ^ B then points north or south depending on the direction of the current: the force is horizontal, perpendicular to the wire.

Common pitfall

Do not confuse the direction of B with the direction of F: the force is always perpendicular to B, never parallel to B. Furthermore, if the direction of the current is reversed, F changes direction; if B is reversed, F also changes direction; if both are reversed at the same time, F retains the same direction.