Moving charges feel the force

A current-carrying wire lies along the field lines. Does it feel a force?

In a vacuum tube, a narrow beam of electrons flies between two magnet poles. No wire and no circuit: just charged particles in flight. The beam bends.

Why does the field push on a current-carrying wire at all? Back in the Circuits topic, we saw that an electric current is a flow of charge. The field pushes on the wire because an electric current is a flow of charged particles in motion. The push really lands on the moving charges themselves.

So the wire is not needed. Any moving charged particle in a magnetic field experiences a force — unless it moves parallel to the field lines.

Watch the bend. But the picture needs care: the force on a moving charge is perpendicular to both the field and the charge's motion. In a side-on drawing, with the field lines drawn in the plane of the page, the bend would point out of the page or into it, and a flat drawing cannot show that truthfully. So the honest view looks straight along the field lines.

Looking along a field line, the line's arrow either comes straight toward you or heads straight away. An arrow heading away from you shows you its tail feathers: a cross. An arrow coming toward you shows you its tip: a dot. So a grid of crosses marks a field heading into the page, and a grid of dots marks a field coming out of the page.

The dot is an arrow's tip, toward you. The cross is its tail feathers, away from you.

Here is the electron beam in that view. Each cross marks the field heading into the page, away from you. The beam enters at the bottom of the picture, travelling up the page, crossing the field. Its track bends: each electron feels a force.

The field heads into the page. The beam crosses it, and the track bends.

An electron flies across the field lines between magnet poles. Does it feel a force?

Now the same beam is aimed along the field lines. It flies straight on: no force, because it moves parallel to the field lines.

Aimed parallel to the field lines: no force, and no bend.

A proton moves parallel to the field lines. Does it feel a force?

A charged dust speck drifts across the field lines. It feels a force. Any moving charge does.

Any moving charge: a force.

An uncharged atom crosses the field lines and feels no force. It has no charge, so the field has nothing to push on.

No charge: no force.

A magnetic field pushes on a particle that has charge and is moving, as long as its motion is not parallel to the field lines.

A beam of protons crosses the field lines between magnet poles. What happens to the beam?

Each proton is charged and moving.

The protons move across the field lines, not parallel to them.

A moving charged particle in a magnetic field experiences a force.

Each proton feels a force, so the beam bends.

The same proton beam is redirected to run along the field lines. What happens now?

The protons now move parallel to the field lines.

A charge moving parallel to the field lines feels no force.

No force acts. The beam flies straight on.

Suki says a magnet can deflect a beam of electrons even though no wire carries them. Is Suki correct?

Your turn

Which particle experiences a magnetic force?

In a vacuum tube, an electron beam crosses the field lines between magnet poles. What happens to the beam?

One proton crosses the field lines; an identical proton moves along them at the same speed. Which statement is correct?