Induction by changing magnetic fields

Does a magnet inside a solenoid cause electricity to flow in the solenoid’s wire? It depends! When the magnet is moving into the solenoid, an ammeter connected to the circuit shows electric current called an induced current. When the magnet stops moving, the current stops, too. When the magnet is moved backward out again, current starts up again—but flows in the opposite direction! The magnet induces a current in the wire, but only when the magnet is moving relative to the wire. The phenomenon of a moving magnetic field inducing an electric current is called electromagnetic induction. Read the text aloud
Electric current induced in a solenoid by a moving bar magnet
What was happening when the magnet moved in and out of the solenoid? The movement of the magnetic field caused a change in the magnetic flux through every loop of wire in the solenoid. Changing magnetic flux induces electric current in a conductor. Since electric current will flow only when there is an electric field inside the conducting wire, we could also say that changing magnetic flux induces an electric field in the wire. Read the text aloud
In magnetic induction, work is being done to move the electric charges around the coil—i.e., to create the electric current. The work done per unit charge is called the electromotive force (emf), although it is not really a force. The units of emf are joules per coulomb or volts. In magnetic induction, a changing magnetic flux creates an induced emf. Read the text aloud
In 1831, Michael Faraday conducted a series of experiments similar to the illustration above to discover electromagnetic induction. Faraday’s law of induction (equation 19.2) states that the induced emf in a circuit is proportional to both the rate of change of the magnetic flux and the number of coils in the wire. Read the text aloud Show Who actually discovered induction?
(19.2) emf=N Δ Φ B Δt
emf  = induced emf (V or J/C)
N  = number of turns of coil
ΔΦB  = change in magnetic flux (T m2)
Δt  = change in time (s)
Faraday’s law
What is the direction of the induced current?When a current is induced, in what direction will it flow? Lenz’s law states that the direction of the induced current is such that the current’s magnetic field acts to oppose the original change in magnetic field. Lenz’s law describes how the induced current acts to resist changes in magnetic flux—similar to how the inertia of an object causes it to resist changes in its velocity. Lenz’s law therefore provides the minus sign in equation (19.2)! Read the text aloud Show What if Lenz’s law were opposite?
What is the direction of the induced current?A wire loop is located in the plane of the page. If a magnetic field across the page is increasing in the direction into the page, what is the direction of the current induced in the wire loop? (Recall that the notation for a circle with an “X” inside it means that the magnetic field vector is pointing into the page.) Show

Previous Page Next Page561