# How does a magnetic field “push” charges?

I was under the assumption that a magnetic field acts similar to that of the normal force in mechanics; both affect the path of the object, but do no work. So now suppose that we have a rectangular circuit with the left side in an uniform magnetic field that is pointing towards the computer screen and the right side being moved at a velocity $v$ Since the whole circuit is moving with a velocity $v$ there is a magnetic force perpendicular to $v$, but still in the direction of the wire. Therefore the magnetic field pushes the charges along. If we assume the charges to be that of protons and electrons, the magnetic field will separate electrons from protons and cause current to flow. My confusion is on the very fact that the magnetic field is causing the current to flow. I simply thought the magnetic field would change the direction of current flow. How can this be?

• Magnetic fields do no work in free space, but an electron (e.g.) redirected such that it hits the surface of the wire will transfer momentum. Or maybe you're asking how a magnetic field induces current, in which case a varying field is required. – Carl Witthoft Jan 18 '15 at 17:55
• @carlwitthoft So one needs a varying magnetic field and an external force to make the magnetic field change? – Oscar Flores Jan 18 '15 at 19:00
• @CarlWitthoft A varying field induces electric field which can contribute to an emf. But time varying fields are not required to generate an emf when a circuit is moving. Though you are correct in the sense that an emf is different than work. – Timaeus Feb 10 '15 at 7:31