Induction and Alternating Current Induced Current

Magnetic Fields and Induced EMFs Electromagnetic induction – the production of an emf (energy per unit charge supplied by a source of electric current) in a conducting circuit by a change in the strength, position, or orientation of an external magnetic field The circuit must be closed The circuit and magnetic field must move relative toward each other According to the right-hand rule, magnetic force, the magnetic field, and the motion of the circuit are all three perpendicular to one another Magnetic field is directed out of the fingers Magnetic force is directed out of the palm The circuit moves in the direction of the extended thumb The magnitude of the induced emf depends upon the velocity with which the circuit is moved through the magnetic field, the length of the wire, and the strength of the magnetic field

Magnetic Fields and Induced EMFs The magnitude of the induced emf and current depend upon the orientation of the loop with respect to the magnetic field Largest when the plane of the loop is perpendicular to the magnetic field Zero when the plane of the loop is parallel to the field Changing the size of the loop will induce an emf Changing the strength of the magnetic field induces an emf Basically, emfs are induced when something changes

Characteristics of Induced Current The induced current in a circuit produces its own magnetic field Lenz’s Law – The magnetic field of the induced current opposes the change in the applied magnetic field The direction of the new magnetic field opposes that of the original magnetic field If the circuit and magnet are getting closer together, the new magnetic field will be in the opposite direction If the circuit and magnet are getting further apart, the new magnetic field will be in the same direction

Characteristics of Induced Current Faraday’s Law of Magnetic Induction Average induced emf = -(the number of loops in the circuit)*[the rate of change of (circuit loop area)*(magnetic field component normal to the plane of the loop)] emf = -N[AB(cos)]/t N is always a whole number A is area in square meters (m2) B is magnetic field strength in Teslas (T) or N/(A*m) or (V*s)/m2  is the angle between the magnetic field and the loop in degrees t is the time interval in seconds (s) emf is equivalent to voltage or potential difference and is measured in volts (V) Either A, B, or  will change. Subtract the two values Once you calculate the emf value using Faraday’s law, you can use Ohm’s law to find either current or resistance or vice versa

Characteristics of Induced Current Example: A coil with 25 turns of wire is wrapped around a hollow tube with an area of 1.8m2. Each turn has the same area as the tube. A uniform magnetic field is applied at a right angle to the plane of the coil. If the field increases uniformly from 0.00T to 0.55T in .85s, find the magnitude of the induced emf in the coil. If the resistance in the coil is 2.5, find the magnitude of the induced current in the coil.

Characteristics of Induced Current

Applications of Induction Use electromagnetic induction to produce temporary or continuously changing current Door bells – pressing the button interrupts an electric circuit This causes the circuit’s magnetic field to decrease In response this induces a current in a nearby circuit The induced current generates an opposing magnetic field that causes a plunger to strike a chime

Applications of Induction Tape recorders – magnetic tape moves past a recording head and a playback head A microphone transforms sound into a fluctuating electric current The current is amplified and passes through a wire coiled around an iron ring This is the recording head A gap is cut in the ring The magnetic field does not pass through the gap as easily as it does the rest of the coil The magnetic field magnetizes the metal oxides on the tape in a pattern that corresponds to the frequency and intensity of the sound entering the microphone In playback mode, the reverse happens The pattern on the tape generate variable magnetic fields which correspond to certain frequencies and intensities of sound waves