One Chapter 7

THE ELECTROMAGNETIC AGE

• The discovery of electromagnetism led to the development of new technology.
• Many devices had electric motors replace other types of motors or even man or animal power.
• The use of electric motors requires a continuous source of electric potential.
• During the 19th century, the discoveries were made to produce electricity by using generators.

FARADAY'S DISCOVERY

• Oersted (1819) showed that moving electrons create magnetic fields.
• Investigations began to see if a magnetic field could produce a current.
• 1831 Faraday discovered the basic principle of electromagnetic induction.
• Faraday's investigations involved 3 situations.

CASE 1 - Moving a wire through the jaws of a horseshoe magnet.

· Result:

o Electrons only flow while the conductor moves through the magnetic field.

CASE 2 - Plunging a bar magnet into and out of the core of a coil.

· Result:

o Electrons only flow when the bar magnet moves in relation to the coil (magnetic field).

CASE 3 - Touching the iron core of a coil with a bar magnet and then removing the magnet.

· Result:

o Electrons only move through the coil when the iron core was being

magnetized or demagnetized.

• These three cases are combined into one law.

Faraday's law of electromagnetic induction:

Whenever the magnetic field in the region of a conductor is moving or changing in magnitude, electrons are induced to flow through the conductor.

• Induction can be demonstrated by using a device like the one shown above. When the switch is closed, the magnetic field builds up in the iron core. Current is induced in the secondary coil. The galvanometer (G) detects current and its direction. Once the field has stabilized at its maximum value, current is no longer induced, because the field is no longer changing.
• In the same apparatus, when the switch is opened, the magnetic field in the ring decreases to zero. While it is decreasing, current is induced in the secondary coil (in the opposite direction). When the field is zero, the current reaches zero.

MUTUAL INDUCTION - A changing current in one coil, produces a changing current in a nearby

coil.

• the iron cores or rings only strengthen the effect (they are not needed).

MAGNITUDE OF THE INDUCED ELECTRIC POTENTIAL

• A moving or changing magnetic field, causes an electric current to flow through a conductor.
• The factors affecting the magnitude of the induced current are:
  • The number of turns on the induction coil.
  • The rate of change, or rate of motion, of the inducing magnetic field.
  • The strength of the inducing magnetic field.
• The resistance of the conductor in most cases remains constant.
• Ohm's law states:

V= IR or

I = V/R

• Therefore if we have an induced current, resistance stays constant, then the 3 factors previously mentioned must affect not only current, but also potential difference.

The Direction of the Induced Current

• There is a relationship between the action of an inducing field and the direction of the induced current. Heinrich Lenz stated this relationship.
• Lenz's Law - An induced current flows in such a direction that the induced field it creates opposes the action of the inducing field.
  • The explanation of why this occurs lies in the law of conservation of energy.

• Consider

• The magnet is plunged into the coil.
• The inducing field is the field of the magnet
• The induced field must oppose the action of the inducing field (i.e. oppose the motion of the magnet into the coil).
• Therefore, the right end of the coil must be a N-pole to repel (oppose the motion) the magnet.
• By using our right hand rule, our thumb must point in the direction of the magnetic field inside the coil (point towards the N on the magnet) and our fingers curl around in the direction of the current.
• Therefore, the current is as shown below.

• Work is done on the magnet to slow it down (it loses energy).
• Work is done on the electrons to speed them up (they gain energy lost by the magnet).
• If the right side was not a N-pole, then the magnet would be pulled into the coil. That is, work is done on the magnet (gains energy), work is done on the electrons (to cause them to move), and no where is energy lost. Therefore energy would have been created, and as we know, energy cannot be created or destroyed. This means that the right end of the coil must be a N-pole not a S-pole.

January 17, 2014