State that the direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point

4.1 Simple Phenomena of Magnetism

Learning Objective

State that the direction of a magnetic field at a point is the direction of the force on the north (N) pole of a magnet placed at that point.

Key Concepts

• A magnetic field is a region around a magnet where other magnetic materials experience a force.
• Magnetic field lines are used to represent the field; they emerge from the north pole and enter the south pole.
• The direction of the magnetic field at any point is defined by the direction a north‑pole would be pushed if it were placed at that point.

Defining the Direction of a Magnetic Field

When a small test magnet is placed in a magnetic field, the north pole of the test magnet experiences a force. The direction of that force is taken as the direction of the magnetic field at that location.

Experimental Determination

1. Place a bar magnet on a flat surface so that its poles are clearly marked.
2. Lay a small compass (or a tiny bar magnet free to rotate) near the magnet, but not touching it.
3. Observe the direction in which the north pole of the compass needle points.
4. The line drawn from the compass to the magnet shows the direction of the magnetic field at the compass’s position.

Using LaTeX for Mathematical Representation

The magnetic field strength is often denoted by \$B\$ and measured in tesla (T). The force \$F\$ on a magnetic pole of strength \$m\$ (in weber, Wb) placed in a magnetic field \$B\$ is given by

\$F = mB\$

Since \$m\$ for a north pole is positive, the force \$F\$ points in the same direction as the field \$B\$.

Summary Table

Key Points to Remember

• The magnetic field direction is the direction of the force on a north pole.
• Field lines exit the north pole and enter the south pole of a magnet.
• At any point, the tangent to a field line gives the direction of the magnetic field.
• Using a compass is a practical way to visualise the field direction.

Common Misconceptions

• Misconception: The magnetic field points from south to north.
  

  Correction: Field lines are drawn from north to south; the direction of the field is the direction a north pole would move, i.e., from north to south.

• Misconception: The field is strongest at the centre of a bar magnet.
  

  Correction: The field is strongest near the poles, where the field lines are most concentrated.

Practice Question

A small compass is placed near the north pole of a bar magnet. The north end of the compass needle points directly towards the magnet’s north pole. What does this indicate about the direction of the magnetic field at the compass’s location?

Answer: The magnetic field at that point is directed towards the magnet’s north pole, i.e., the same direction as the force that would act on a north pole placed there.