A sheet of copper is placed between the poles of an electromagnet with the magnetic field perpendicular to the sheet's surface. When the sheet is pulled out, a considerable force is required, and the necessary force is larger with increased speed. Explain why.
As soon as the copper conductor moves through the field the changing flux linked to it creates an induced emf within it. This emf (called a back emf) is generated in opposition to the flux change causing it .. in accordance with Lenz's law. The 'opposition' is due to the back emf driving an induced current that creates an electromagnetic field in opposition to the existing field .. the repelling forces create the opposition to the conductor's movement. Induced currents occurring like this, within relatively large areas of a conductor, are known as 'eddy currents' as they tend to flow in circular paths. Because copper is a good conductor, there is hardly any resistance to an induced current flow .. even a small emf can create a large induced current .. so creating a large opposing force. Moving the conductor faster increases the rate of flux change linking with the copper .. so the back emf is higher, giving a greater induced current and opposing force.
