# Q & A: What are magnetic field lines?

Q:
What are magnetic field lines? I know they are imaginary lines and point in the direction a compass would when place on one. But what does this direction imply? What is moving along this line? Why does it (also please tell me what) 'GO' from N. pole to S. pole?
- Savyasachi (age 15)
New Delhi, India
A:
The magnetic field B at any place is a vector, meaning it has a specific direction. The magnetic force on a (non-magnetic) particle of charge q moving at velocity v is proportional to the vector cross product qvXB. That implies that the charged particle moving along the direction that the field points will not experience a magnetic force. So you can think of a magnetic field line as being the path the particle could follow without feeling any magnetic force. Usually, however, nothing is really moving along the lines.

These lines have some interesting properties.

With our current physics (no magnetic monopoles) the lines never end. Instead they all form loops.

When we draw a picture of a magnetic field using field lines, we use dense lines where the field is strong and less dense where the field is weak.

When iron filings are sprinkled in a field, they tend to line up end-to-end along the directions of the field. So they make a sort of picture of the abstract field lines.

The choice of saying which direction the lines go (N to S or S to N) doesn't have any physical meaning. It's just a convention.

Mike W.

(published on 08/12/2012)