CH 31: FARADAYS LAW

If current can generate a magnetic field is it possible to generate a current from a magnetic field? Yes, but not from a constant magnetic field! If you connect a ammeter to a coil of wire you will be able to detect any current in that wire. What happens if you hold a magnet in front of the coil? Nothing – The ammeter will read zero current. What happens if you move the magnet into the coil? A current will be generated in the coil. When the magnet is moved into the coil the number of magnetic field lines that pass through the coil is changed. This change in the magnetic field strength causes the current. A changing magnetic field, or more specifically a changing magnetic flux, will cause an induced emf, which in turn will cause a current to be induced in the coil. – Emf induced by changing flux [V] B – Magnetic flux [Wb] The coil resists the changing flux The induced emf depend on changing the flux, which is either changing the magnetic field strength, changing the area, changing the angle between B and A or some combination of the three.

In figure (a), a solenoid produces a magnetic field whose strength increases into the plane of the page. An induced emf is established in a conducting loop surrounding the solenoid, and this emf lights bulbs A and B. In figure (b), points P and Q are shorted. After the short is inserted, 1. bulb A goes out; bulb B gets brighter. 2. bulb B goes out; bulb A gets brighter. 3. bulb A goes out; bulb B gets dimmer. 4. bulb B goes out; bulb A gets dimmer. 5. both bulbs go out. 6. none of the above

Explanation