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Induced Fields
Magnetic fields may vary in time. Experiments conducted in 1831 showed that an emf can be induced in a circuit by a changing magnetic field. ▪ Experiments were done by Michael Faraday and Joseph Henry. The results of these experiments led to Faraday’s Law of Induction. An induced current is produced by a changing magnetic field. There is an induced emf associated with the induced current. A current can be produced without a battery present in the circuit. Faraday’s law of induction describes the induced emf. Introduction
Michael Faraday
British physicist and chemist Great experimental scientist Contributions to early electricity include: ▪ Invention of motor, generator, and transformer ▪ Electromagnetic induction ▪ Laws of electrolysis Introduction
EMF Produced by a Changing Magnetic Field, 2
When the magnet is held stationary, there is no deflection of the ammeter. Therefore, there is no induced current. ▪ Even though the magnet is in the loop
EMF Produced by a Changing Magnetic Field, 3
The magnet is moved away from the loop. The ammeter deflects in the opposite direction.
EMF Produced by a Changing Magnetic Field, Summary
The ammeter deflects when the magnet is moving toward or away from the loop. The ammeter also deflects when the loop is moved toward or away from the magnet. Therefore, the loop detects that the magnet is moving relative to it. ▪ We relate this detection to a change in the magnetic field. ▪ This is the induced current that is produced by an induced emf.
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 ammeter changes from zero in one direction and then returns to zero. When the switch is opened, the ammeter changes in the opposite direction and then returns to zero. The ammeter 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 loop by a changing magnetic field. ▪ This would be the current in the secondary circuit of this experimental set-up. The induced current exists only while the magnetic field through the loop is changing. This is generally expressed as: an induced emf is produced in the loop by the changing magnetic field. ▪ The actual existence of the magnetic flux is not sufficient to produce the induced emf, the flux must be changing.
Faraday’s Law – Example
Assume a loop enclosing an area A lies in a uniform magnetic field. The magnetic flux through the loop is ΦB = BA cos θ. The induced emf is ε = - d/dt (BA cos θ).
Ways of Inducing an emf
The magnitude of the magnetic field can change with time. The area enclosed by the loop can change with time. The angle between the magnetic field and the normal to the loop can change with time. Any combination of the above can occur.
Applications of Faraday’s Law – Pickup Coil
The pickup coil of an electric guitar uses Faraday’s law. The coil is placed near the vibrating string and causes a portion of the string to become magnetized. When the string vibrates at some frequency, the magnetized segment produces a changing flux through the coil. The induced emf is fed to an amplifier.
Motional emf
A motional emf is the emf induced in a conductor moving through a constant magnetic field. The electrons in the conductor experience a force, that is directed along ℓ. F = q v B
Sliding Conducting Bar
A conducting bar moving through a uniform field and the equivalent circuit diagram. Assume the bar has zero resistance. The stationary part of the circuit has a resistance R.
Moving Conductor, Variations
Use the active figure to adjust the applied force, the electric field and the resistance. Observe the effects on the motion of the bar.
