Electromagnetism

Magnets & Magnetic Fields

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Magnetic Force and Fields

~600 BC, the Greeks discovered that a certain type of iron ore, later known as lodestone, or magnetite, was able to attract other small pieces of iron.

a piece of lodestone would come to rest in a north – south position

widely used for navigational purposes

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Magnetic Force and Fields

Today, artificial magnets are made from various alloys of iron, nickel and cobalt

Magnets have areas of concentrated magnetic force which we call poles

One is called a north seeking pole or the N-pole and the other is the south seeking pole or the S-pole

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The Laws of Magnetic Poles

There are 2 laws:1. Opposite magnetic poles attract

2. Similar magnetic poles repel

Magnets have a field of force surrounding them, which we call the magnetic force field

The magnetic field lines indicate the direction in which the N-pole of the test compass would point

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Magnetism

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Characteristics of Magnetic Field Lines

The spacing of lines indicate the relative strength of the force. The closer the lines, the greater the force

Outside a magnet, the lines are concentrated at the poles. They are closest within the magnet itself.

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Characteristics of Magnetic Field Lines

By convention, the lines proceed from the S to N inside a magnet and from N to S outside a magnet, forming closed loops

The lines do not cross one another

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Magnetic Materials

Ferromagnetic materials (Ni/Co/Fe, alloys) can be induced by placing them in a magnetic field Can be induced temporarily or permanently

Small pieces of iron rubbed in one direction with lodestone become magnetized

Dropping or heating magnet can demagnetize it

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Breaking a Magnet

When a magnet is broken it forms two new magnets each with a N and a S pole

The orientation of the poles in the new magnet will be the same as the orientation in the old magnet

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Principle of Electromagnetism

The principle of electromagnetism was demonstrated when we place an iron nail in the presence of a current running through a wire

The iron nail becomes magnetized similar to when we stroke it with a permanent magnet

The current produces a magnetic field that causes this induction

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Principle of Electromagnetism

The Danish Physicist Hans Christian Oersted was the first person to come up with this principle

He discovered that a magnet got affected by a current carrying wire

Oersted’s principle: whenever an electric current moves through a conductor, a magnetic field is created in the region around the conductor

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Magnetic field in a straight conductor

Right-hand rule (RHR): If a straight conductor

is held in the right hand with the thumb pointing in the direction of the conventional current, the curled fingers will point in the direction of the magnetic field

*Note conventional current is opposite to electron flow

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Drawing current and field lines

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Ampère’s Experiment

Two conductors with current in opposite directions generate a stronger magnetic field

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Solenoid

A coiled conductor is called a solenoid. The field lines generated resemble those of a

magnet.

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Right Hand Rule for a Solenoid

Ampere's Rule for a solenoid (RHR for solenoid) states that if the solenoid is grasped in the right hand in such that if the fingers curl in the direction of the conventional current, the thumb points to the north pole of the core (same direction as the magnetic field lines in the core).

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Factors affecting the magnetic field of a coil Current in the coil:

Increasing the speed of the current in the coil increases the concentration of magnetic field lines.

Number of loops in the coil: Each loop has its own magnetic field. The magnetic

field of a coil is the sum of the magnetic fields of all its loops.

The more loops, the stronger the magnetic field. Type of core material:

Based on magnetic permeability (ex: Fe, Ni, Co has the most (ferromagnetic) compared to O, Al, Cu, Ag, H2O)

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Applications of Electromagnets

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Practice: Mark the direction of electric current, the direction of the field lines at each end of the coil, and the N-pole and S-pole of the coil.

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Textbook references and HW:

Old textbook: Sections 13.1-13.4 p475 #2,4; p478 #1; p482 #1,2; p489 #1,2,7 +

worksheets

New textbook: Sections 12.1-12.4 p552 #3,6; p556 #1-4; p562 #1-3,5 +

worksheets

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The Motor Principle: Background

Electric motors are used all around us – and they deal with a very important part of EM called the Motor Principle

Michael Faraday set about to prove that the opposite of Oersted & Ampere’s discoveries could also be true a magnet could also cause a current carrying

wire to move

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The Motor Principle

When a current carrying conductor is placed in an external magnetic field, the interaction of the field lines produces a net force perpendicular to both the magnetic field and the direction of the electric current.

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The Motor Principle

We can use the right hand rule for the motor principle to determine the direction of the net force

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The Motor Principle

The magnitude of the force depends on the magnitude of the: Current External magnetic field Angle/orientation of current with external

magnetic field

Applications: analog meters use the force exerted to move the needle

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The Motor Principle

Applications: analog meters use the force exerted to move the needle

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The DC Motor

Allows current flowing in the same direction to induce movement of an armature

Converts electromagnetic energy into kinetic energy

Kinetic energy can be further converted into any application (ex: engine, toys, etc.)

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Parts of a DC Motor

External magnetic field created between 2 external magnets

Loop of wire (or coil of wire) placed in external magnetic field

Brushes maintain contact between current and commutator

Commutator (split-ring) allows direction of current in the loop/coil to change direction every half-turn

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Consider:

Use RHR to verify loop would turn counterclockwise

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Now, use RHR to verify loop would want to turn clockwise

SOLUTION: split-ring commutator changes direction of current to keep loop spinning in the same direction

Steps of a Basic Loop DC Motor

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Step 1:Current flows through A – B – C – D.Use RHR of the motor principle at segments AB and CD to verify the loop would rotate clockwise.

Steps of a Basic Loop DC Motor

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Step 2:The brushes do not make contact with the commutator so no current flows, however, the loop keeps moving due to inertia.

Steps of a Basic Loop DC Motor

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Step 3:The brushes now contact the other side of the commutator.Use RHR of motor principle to verify the loop keeps rotating in the same direction. What would happen without the commutator?

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Video

Design of motor can be improved by replacing the single loop of wire with a coil of wire.

Steps in a DC Motor with a coil

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Use RHR of a coil to verify the polarity of the coil (armature). *Note: commutator alters direction of current in the coil, altering

its polarity

Steps in a DC Motor with a coil

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Use RHR of a coil to verify the polarity of the coil (armature). *Note: armature keeps moving in the same direction.

Improvements to Motor Design

Increase number of loops in solenoid

Increase number of armatures and splits in commutator (for more steady force)

Brushless motors use a permanent magnet that rotates within the electromagnets (brushes wear down, spark, and add weight).

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AC vs. DC

Direct current: charges flow in the same direction

Alternating current: charges reverse direction periodically (sinusoidal) at 60 Hz More effective at

transferring energy across distances

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Practice: Show the labels of the magnetic poles, the magnetic field, andthe direction of force on the conductor

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Practice: Draw the magnetic fields of the permanent magnet and the conductor. Determine the direction of the force on the conductor.

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Ex: Describe which way the loop would tend to turn. Assume + represents the positive terminal.

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Practice: Describe the path of current through the conductor, brushes, commutator, and coil by adding arrows. Identify the magnetic polarity of the armature and the rotation direction of the motor.

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Textbook references and HW:

Old textbook: Sections 13.5-13.6 p493 #1-4; p502 #3-5

New textbook: Sections 12.5-12.6 p566 #1,2; p571 #1-3

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Electromagnetic Induction

If electric current can induce a magnetic field, can a magnetic field induce electric current?

Faraday discovered yes! Electromagnetic induction: the production of

electric current in a changing magnetic field Law of electromagnetic induction: any

change in magnetic field near a conductor induces a voltage in the conductor, causing an induced electric current in the conductor

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Faraday’s Ring

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Galvanometer detects small amounts of current

*Note: once magnetic field is stable, galvanometer would read zero as a changing magnetic field is required to induce current

Electromagnetic Induction

Can also induce an electric current with a moving permanent magnet into a coil

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Factors Affecting EM Induction

Coiled conductor (strengthens field compared to straight conductor)

Number of loops in solenoid Rate of change of magnetic field (movement

of magnet or current in primary coil) Strength of inducing magnetic field

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Applications of EM Induction

Induction cooking: changing magnetic field in stove induces current in pot -> heats up pot quickly

Metal detectors: changing magnetic field induces current in any metal near it, which induces its own magnetic field

Induction chargers (wireless): charger & device have wire coils, charger induces current in device, which charges device

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Generators

Converts mechanical energy into electrical energy Opposite of motor Ex: wind turbine, hydroelectric dam, tides,

steam, etc. Same parts as a motor (commutator, loop of

wire, permanent magnets, etc.) As loop of wire/coil rotates, electric current is

induced due to the changing magnetic field

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DC Generator

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AC Generator

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Textbook References

Old text: sections 14.1 & 14.3 p512 #1-5

New text: sections 13.1 & 13.4 p591 #1-6

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