AP Physics ST Faraday’s Law teachnet.ie

Faraday’s Law So far we’ve seen how an electric field can produce a magnetic field… can a magnetic field produce an electric field??? Hmmmm. – Electric field is produced by a static charge. – Magnetic field is produced by a charge in motion. whs.wsd.wednet.edu

Overhead Induction Coils Demonstration #1 Isolated circuit… no power supply. Galvanometer measures extremely small currents; micro-amperes. Deflection of the needle signifies the presence of an electric current. electricalandelectronics.org

Overhead Induction Coils Demonstration #1 Observations: – Motion vs. no motion of magnet. – Fast vs. slow motion penetration. – IN vs. OUT movement. – Movement along axis vs. twisting motion. – Number of coils. – Strength of magnet. – North vs. South pole. electricalandelectronics.org

Overhead Induction Coils Demonstration #1 Conclusions: – A current is produced in the circuit as long as there is relative motion of the magnet within the coil. – An induced current is said to be set up as a result of a changing magnetic field. – We say the induced current is produced by an induced emf. electricalandelectronics.org

Electromagnetic Induction Coils Demonstration #2 Electromagnetic set-up – Power supply, two coils, iron core, switch, galvanometer. – Power supply is connected to and energizes the primary coil. – Iron core simply “streamlines” the magnetic field to the secondary coil; not connected to the power supply. chinalabsupplies.com

Electromagnetic Induction Coils Demonstration #2 Observations: – Switch closed vs. switch open. – Switch closed to open vs. open to closed. – Vary voltage. – Vary number of loops. chinalabsupplies.com

Electromagnetic Induction Coils Demonstration #2 Conclusions: – Current induced only when the switch moves from closed to open or open to closed. – Magnitude of current varies with voltage of primary coil. – Magnitude of current varies with number of loops in either the primary or secondary coil. chinalabsupplies.com

Faraday’s Conclusions “An electric current can be induced in a circuit by a changing magnetic field.” As a current is produced by an emf source it is quite customary to state … “An induced emf is produced by a changing magnetic field.” grahamsdownunderthoughts.blogspot.com

Faraday’s Conclusions Faraday’s Law of Induction The emf induced in a circuit is directly proportional to the time rate of change of the magnetic flux through the circuit. grahamsdownunderthoughts.blogspot.com

Faraday’s Law Faraday’s Law: Recall flux… N = number of loops…

Faraday’s Law Emf can be induced in the circuit in a number of ways: – Magnitude of B can change with time. – Cross sectional area A of loop can change with time. – Angle θ between B and A can change with time. – LAME!... But all these can happen simultaneously. The negative sign has MAJOR physical significance (Lenz’s Law) but discussed in next lesson.

Applications of Faraday’s Law Ground Fault Interrupter (GFI) – Wire TO appliance and wire FROM appliance pass through a iron core. – Sensing coil wrapped around iron core. – AC current creates a changing flux. – If flux varies “out of range” the sensing coil triggers a circuit breaker within outlet. azpartsmaster.com

Applications of Faraday’s Law Pick-up coil – Permanent magnet inside coil (you can see the core) magnetizes a small portion of the string. – Plucked string vibrates over coil changing the magnetic flux through coil. – Changing flux induces an emf in coil which is fed to the amplifier which sends to speakers. rip-n-guitar.com alpha-music.com

Applications of Faraday’s Law Induction Cooking – Cooking with Induction Cooking with Induction – Coil under cook top runs ac current which induces a current in the metal pan (conductor) heating only the pan/contents. cookswares.com choice.com.au

Induction Cooking inductionovens.com

Lesson Summary

Example #1: 31-4 Serway 5 th ed A rectangular loop of area A is placed in a region where the magnetic field is perpendicular to the plane of the loop. The magnitude of the field is allowed to vary in time according to the expression where value B max and τ are constants. The field has the constant value B max for t < 0. a.Use Faraday’s law to show that the E induced in the loop is given by: b.Obtain a numerical value for E at t = 4.00 s when A = m 2, B max = T, and τ = 2.00 s. c.For the values of A, B max, and τ given in part b what is the maximum value of E ?

Example #2 The primary coil of a long solenoid has 200 turns/cm and carries a current I = 1.5 A. The diameter of the primary coil is 3.2 cm. At its center is placed a secondary coil having 130 turns of diameter d = 2.1 cm. The current in the primary coil is reduced to zero at a steady rate in 25 ms. What is the induced emf in the secondary coil?