Magnetism and Electromagnetism

Basic Principles of Magnetism and Magnetic Poles

• Magnets possess two distinct poles: a North pole and a South pole.

• Interactions between magnetic poles follow fundamental rules of attraction and repulsion:

  • Same (like) poles will repel each other.

  • Opposite poles will attract each other.

Permanent and Induced Magnets

• Permanent Magnets:

  • These materials are always magnetic and consistently exhibit North and South poles.

• Induced Magnets:

  • These consist of materials that are considered "magnetic" but do not naturally have fixed poles.

  • They can be transformed into temporary magnets through a process of 'stroking' them with a permanent magnet.

  • The mechanism of induction involves aligning the internal domains of the material so they all point in the same direction, thereby creating a temporary magnetic effect.

  • Primary examples of magnetic materials include Iron, Nickel, and Cobalt.

Characteristics of Magnetic Fields

• Magnetic field lines indicate the direction and strength of a magnetic field:

  • They always point from the North pole toward the South pole.

  • The strength of the magnetic field decreases as the distance from the magnet increases.

  • At any point in space, the field direction points away from the North pole and toward the South pole.

• Plotting Compasses:

  • These are small compasses used to visualize and map magnetic fields.

  • They show the specific direction of the magnetic field at any given point.

Earth’s Magnetic Field and Core

• The Earth's core is magnetic, which generates a massive magnetic field surrounding the entire planet.

• Evidence for this field is seen in freely suspended magnetic compasses, which align themselves with the Earth's field lines and point toward the North.

• Geographic vs. Magnetic Poles:

  • A compass does not point exactly to the Geographic North Pole; rather, it points to a location over North Canada.

  • Because a compass is effectively a suspended bar magnet, its North pole is attracted to Earth's magnetic North.

  • However, based on the laws of magnetism (like poles repel), the magnetic pole located near the geographic North must actually be a Magnetic South Pole.

  • Conversely, the geographic South Pole is located near the Earth's Magnetic North Pole.

Magnetic Effects of Electric Current

• When an electric current flows through a wire, it produces a magnetic field around that wire.

• Right-Hand Grip Rule: This rule is used to dictate the direction of the magnetic field relative to the current flow.

• Visualization can be achieved by placing plotting compasses on a sheet of paper that has been pierced by a current-carrying wire.

• Strength of the Field:

  • A greater current results in a stronger magnetic field.

  • A greater distance from the wire results in a weaker magnetic field.

Solenoids

• A solenoid is a coil of wire that creates a magnetic field shape similar to that of a bar magnet.

• Enhancement Mechanism: Coiling the wire enhances the magnetic effect because it causes the individual fields around the wire to align into a single, giant field, rather than being multiple fields perpendicular to the current direction.

• Iron Core: Inserting an iron core into the center of a solenoid increases its strength because magnetic field lines pass through iron much more easily than through air.

• Factors Affecting Solenoid Strength:

  • The size of the electric current.

  • The total length of the solenoid.

  • The cross-sectional area of the coil.

  • The number of turns (coils) in the wire.

  • The use of a soft iron core.

The Motor Effect and Fleming's Left Hand Rule

• The Motor Effect: When a current-carrying wire is placed within a magnetic field produced by permanent magnets, the two magnetic fields interact. This interaction results in a physical force of attraction or repulsion.

  • The magnetic field around a wire is circular, while the field between two permanent magnets is straight.

  • Interaction causes the wire to be pushed away from the field between the poles at right angles to both the wire's direction and the field's direction.

• Three-Axis Visualization:

  • If fixed permanent magnets have field lines along the x-axis,

  • and the wire is placed along the y-axis with current moving from points C to D,

  • the force exerted on the wire will be along the z-axis (perpendicular to both).

• Fleming's Left Hand Rule:

  • This rule is used to determine an unknown factor (typically the direction of the force) when each of the three factors is at 90° to the others.

  • It is critical to remember that "current" refers to conventional current, which flows in the opposite direction to electrons.

• Mathematical Model for Force (Higher Tier):

  • The force is the product of magnetic flux density, current, and length.

  • $F = BIl$

  • Magnetic Flux Density ($B$): Measured in Tesla ($T$). It represents the number of magnetic flux lines per square meter ($m^2$).

Electric Motors

• Structure: Electric motors consist of permanent magnets in fixed positions with a coil of current-carrying wire situated on an axis between them.

• Operation:

  • The force acting on one side of the coil moves that side up, while the force on the opposing side (where current flows in the opposite direction) moves that side down.

  • These directions can be verified using Fleming's Left Hand Rule.

  • This opposing pair of forces causes the coil to rotate.

Electromagnetic Induction (Physics Only)

• Electromagnetic induction occurs when there is relative movement between a conductor and a magnetic field, or when a magnetic field changes.

• This process induces a potential difference across the conductor.

• If the conductor is part of a complete circuit, a current will flow.

• This induced current generates its own magnetic field, which acts to oppose the specific change that induced it.

Electric Generators: Alternators and Dynamos (Physics Only)

• Basic Concept: Generators use the same setup as a motor (a coil of wire between two magnets) but provide mechanical energy to generate electricity.

• Mechanism:

  • A turbine spins the coil of wire, causing the wire to cut through the magnetic field lines.

  • The wire experiences a change in the magnetic field, which creates a potential difference.

• Alternators (AC):

  • If the coil is connected to a complete circuit, an alternating current ($AC$) flows.

• Dynamos (DC):

  • To produce direct current ($DC$), the ends of the coil are connected to a split-ring commutator.

  • The commutator reverses the current every half-rotation, ensuring the current remains positive.

Transformers (Physics Only)

• Principle of Operation:

  • An alternating current ($AC$) in the primary coil creates a constantly changing magnetic field.

  • This changing field cuts through the secondary coil, inducing a current in it (which is also $AC$).

  • If the primary used direct current ($DC$), the resulting magnetic field would be constant and would not induce anything in the secondary coil.

• Types of Transformers:

  • Step-up Transformer: Has more coils on the secondary than the primary. It increases voltage because the changing field cuts through more wire, inducing a larger potential difference.

  • Step-down Transformer: Has fewer coils on the secondary, resulting in a smaller potential difference.

• Transformer Equation:

  • $\frac{\text{number of coils on primary}}{\text{number of coils on secondary}} = \frac{\text{pd of primary}}{\text{pd of secondary}}$

  • $\frac{n_p}{n_s} = \frac{V_p}{V_s}$

  • This relationship applies to current only if the transformer is 100% efficient. Unless explicitly stated, assume it is not and focus on voltage calculations.

Dynamic Microphones and Loudspeakers (Physics Only)

• Dynamic Microphones:

  • They generate a current proportional to a sound signal.

  • A fixed magnet is centered within a coil of wire that is free to move.

  • Sound wave pressure variations cause the coil to vibrate/move.

  • As the coil moves and cuts the magnetic field, a current is induced and sent to a loudspeaker.

• Loudspeakers:

  • Functioning in reverse of the microphone, current flows into the coil.

  • The magnetic fields from the permanent magnet and the current interact, forcing the coil to move.

  • This movement drives a cone, which creates pressure variations in the air, producing sound waves.