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Faraday’s Law
Michael Faraday
 Great experimental physicist
 1791 – 1867
 Contributions to early electricity include:
• Invention of motor, generator, and transformer
• Electromagnetic induction
• Laws of electrolysis
Induced emf
 A current can be produced by a changing magnetic field [B=f (t)], i.e., B varies over time
• First shown in an experiment by Michael Faraday
Faraday’s Experiment
 The purpose of the secondary circuit is to detect current that might be produced by the magnetic field
 When the switch is closed, the ammeter deflects in one direction and then returns to zero
 When the switch is opened, the ammeter deflects in the opposite direction and then returns to zero
 When there is a steady current in the primary circuit, the ammeter reads zero
Faraday’s Experiment – Set Up
 A primary coil is connected to a switch and a battery
 The wire is wrapped around an iron ring
 A secondary coil is also wrapped around the iron ring
 There is no battery present in the secondary coil
 The secondary coil is not directly connected to the primary coil
Faraday’s Experiment – Findings
 At the instant the switch is closed, the galvanometer (ammeter) needle deflects in one direction and then returns to zero
 When the switch is opened, the galvanometer needle deflects in the opposite direction and then returns to zero
 The galvanometer reads zero when there is a steady current or when there is no current in the primary circuit
Faraday’s Experiment – Conclusions
 An electric current can be induced in a circuit by a changing magnetic field
• This would be the current in the secondary circuit of this experimental set-up
 The induced current exists only for a short time while the magnetic field is changing
 This is generally expressed as:
“an induced emf is produced in the secondary circuit by the changing magnetic field”
Faraday’s Law – Statements
 Faraday’s law of induction states that “the emf induced in a circuit is directly proportional to the time rate of change of the magnetic flux through the circuit”
 Mathematically,
 Remember FB is the magnetic flux through the circuit and is found by
 If the circuit consists of N loops, all of the same area, and if FB is the flux through one loop, an emf is induced in every loop and Faraday’s law becomes
What is Magnetic Flux ?
 The emf is actually induced by a change in the quantity called the magnetic flux Magnetic flux is defined in a manner similar to that of electrical flux
 Magnetic flux is proportional to both the strength of the magnetic field passing through the plane of a wire loop wire and the area of the loop
Magnetic Flux
 The flux can be visualized with respect to magnetic field lines
• The value of the magnetic flux is proportional to the total number of lines passing through the loop
 When the area is perpendicular to the lines, the maximum number of lines pass through the area and the flux is a maximum
 When the area is parallel to the lines, no lines pass through the area and the flux is 0