PHYS 012A Lecture Notes - Lecture 22: Ammeter, Residual-Current Device, String Vibration
Chapter 31: Faraday’s Law
EMF Produced by a Changing Magnetic Field
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.
o We relate this detection to a change in the magnetic field.
o This is the induced current that is produced by an induced emf.
Farada’s Eperiet
- Findings: at the instant the switch is closed, 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
returns to zero. The ammeter reads zero when there is a steady current or when there is no
current in the primary circuit.
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- Conclusions:
o 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
o This is 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.
- Statements
o Emf is proportional to the time rate of change of the magnetic flux through the circuit
Induced fields
- Magnetic fields vary in time
- 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.
- Farada’s la of idutio desries the idued ef.
Farada’s La: Eaple
- Flux through the loop is
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.
Appliatios of Farada’s La - GFCI
- GFCI – (ground fault circuit interrupter) protects users of electrical appliances against electric
shock
- When the currents are opposite, flux is zero
- When the return current in wire 2 changes, the flux is no longer zero.
- Resulting induced emf can trigger a circuit breaker
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