Recall and use the relative directions of force, magnetic field and current

Objective

Recall and use the relative directions of force (F), magnetic field (B) and current (I) in a conductor.

Key Idea

The force on a current‑carrying conductor in a magnetic field is given by the vector product

$\mathbf{F}=I\,\mathbf{L}\times\mathbf{B}$

where L is the length vector of the conductor. The direction of F is perpendicular to both I (direction of current) and B (direction of magnetic field).

🔁 Right‑hand rule – Point your fingers in the direction of the current, then curl them toward the magnetic field; your thumb points in the direction of the force.

Direction Table

⚡ Remember: F is always perpendicular to both I and B.

Analogy: The Magnetic Field as a Crowd

Imagine the conductor as a line of people (current) walking straight ahead. The magnetic field is like a crowd of people moving in a different direction. When the crowd pushes against the line, the line is pushed sideways – that sideways push is the force.

👣 If the crowd moves left to right and the line walks up, the line will be pushed to the right.

Exam Tip

When a question asks for the direction of the force, always sketch the current and magnetic field directions first and then apply the right‑hand rule. A quick diagram saves time and reduces errors.

📝 Practice: Draw a conductor, label I, B and then use the right‑hand rule to find F. Check your diagram before writing the answer.

Quick Practice Problem

1. Current flows horizontally to the right in a wire.
2. Magnetic field points vertically upward.
3. Determine the direction of the magnetic force on the wire.

Answer: The force is directed downward (use the right‑hand rule).