Which way is the force?

The idea

The direction of the force on a current-carrying conductor in a magnetic field depends on the directions of the current and of the field; reversing either one reverses the force.

A rod pushed up

In a magnetic field that points into the page, a rod with a current to the right is pushed up the page. Now the current is made to flow to the left.

What happens to the push?

• It is reversed, so the rod is pushed down the page
• It stays up the page
• It becomes zero

Reverse and see

Press the buttons to reverse the current or the field. Watch the force.

Two directions matter

The force on the rod depends on which way the current flows and on which way the field points. Reverse the current, and the force reverses. Reverse the field, and the force reverses. Reverse both and the force is back where it started.

The direction of the force on a current-carrying conductor in a magnetic field depends on the direction of the current and on the direction of the field; reversing either one reverses the force.

Notes

The direction of the force depends on the direction of the current and of the field; reversing either one reverses the force.

Check yourself

The current in a rod in a magnetic field is reversed, and the field is not changed. The force on the rod…

The poles of the magnet are interchanged, so the field is reversed, and the current is not changed. The force on the rod…

Both the current and the field are reversed. The force on the rod is…

The direction of the force on a current-carrying conductor in a magnetic field depends on…

• stays in the same direction. The direction of the force is reversed when the direction of the current is reversed.
• is reversed — correct. Yes!
• becomes zero. Reversing the current reverses the direction of the force. It does not remove the force.
• stays in the same direction. If the direction of the field is changed by interchanging the two poles, the direction of the force on the rod is reversed.
• is not affected by the magnet at all. The direction of the force depends on the direction of the field, so interchanging the poles reverses the force.
• is reversed — correct. Yes!
• in the same direction as at the start — correct. Yes!
• reversed. Reversing the current reverses the force, and reversing the field reverses it again, so it ends up as it started.
• zero. Both reversals only change the direction of the force. The force acts in the same direction as at the start.
• the direction of the current only. Reversing the field also reverses the force, so the direction of the field matters too.
• the direction of the current and the direction of the field — correct. Yes!
• the shape of the magnet only. The direction of the force depends on the directions of the current and of the field.