 Electromagnetic Induction – The production of an emf (the 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 magnitude of the induced emf depends on the velocity of the wire through the magnetic field, the length, and the strength of the magnetic field.  No emf is induced if the loop is static or the magnetic field is constant.

 Move the loop into or out of the magnetic field (B through loop changing with t).  Rotating the loop within the magnetic field (θ changing with t).  Changing the strength of the magnetic field through a stationary loop (B changing with t).  Altering loop’s shape (A changing with t).

 Lenz’s Law – The magnetic field of the induced current opposes the change in the applied magnetic field.  Lenz’s law allows you to determine the direction of the induced field, but not the magnitude.  Faraday’s Law of Induction Equation: emf = -N ∆[AB(cosθ)]/∆t average induced emf = - of the number of loops in the circuit x the rate of change of (circuit loop area x magnetic field component normal to the plane of the loop) ÷ time change

 A coil with 25 turns of wire is wrapped around a hollow tube with an area of 1.8 m 2. 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 form 0.00 T to 0.55 T in 0.85 s, find the magnitude of the induced emf in the coil. If the resistance in the coil is 2.5 Ω, find the magnitude of the induced current in the coil.

 Generator – A device consisting of a coil of wire that is mechanically rotated in a magnetic field that uses induction to convert mechanical energy to electrical energy.  Alternating Current – An electric current that changes direction at regular intervals.  U.S. frequency of AC is 60 Hz.  Motors – Devices that convert electrical energy to mechanical energy by using current to supply a magnetic field that causes the loop to rotate.  Back emf – The emf induced in a motor’s coil that tends to reduce the current powering the motor.

 Mutual Inductance – A measure of the ability of one circuit carrying a changing current to induce an emf in a nearby circuit.  Transformer – A device that changes one ac potential difference to a different ac potential difference.  In its simplest form, a transformer consists of two coils of wire wound around a core of soft iron.  The coil connected to the source of potential difference is called the primary coil.  The coil connected to the resistor is called the secondary coil.

∆V 2 = (N 2 /N 1 ) ∆V 1 potential difference in secondary = (number or turns in secondary coil ÷ number of turns in primary coil) x potential difference in primary  Because the strength of the magnetic field in the iron core and the cross-sectional area of the core are the same for both the primary and secondary windings, the potential differences across the two windings differ only because of the different number of turns of wire for each.

 When N 2 is greater than N 1, the secondary potential difference is greater than that of the primary, and the transformer is called a step-up transformer.  When N 2 is less than N 1, the secondary potential difference is less than that of the primary, and the transformer is called a step- down transformer.

 A transformer is used on a 120 V line to provide a potential difference of 2400 V. If the primary has 75 turns, how many turns must the secondary have and is it a step-up or step-down transformer?

 Homework worksheet