magnetism

Introduction to Magnetism

• Magnetism principles, behaviors, and applications discussed.

Basic Principles of Magnetism

• Pole Interaction

  • Like poles repel each other (North-North, South-South).
  • Opposite poles attract (North-South).

• Magnetic Materials

  • Common ferromagnetic materials include:
  • Cobalt
  • Steel
  • Iron
  • Nickel

Types of Magnets

• Permanent Magnets

  • Definition: Always magnetic and possess fixed poles.
  • Applications: Used in devices such as speakers, compasses, and electric generators.

• Induced Magnets

  • Definition: Materials that can become magnetic only when magnetism is induced; they do not have fixed poles.
  • Process of Induction:
  • Induced by ‘stroking’ the magnetic material with a permanent magnet.
    • This process aligns the magnetic domains in the material in the same direction, temporarily magnetizing it.
  • Characteristics:
  • Can lose magnetism over time or after being knocked, as the domains return to a random arrangement.

Magnetic Fields

• Properties of Magnetic Fields

  • Field lines originate from North poles and terminate at South poles.
  • The strength of the magnetic field decreases with distance from the magnet.
  • At any given point, the magnetic field direction points away from the North pole and towards the South pole.

• Plotting Compasses

  • Definition: Small compasses used to indicate the direction and pattern of the magnetic field at a specific point.

Earth's Magnetism

• Earth’s Core
  • The Earth itself has a magnetic core, producing a large magnetic field around the planet.
  • Observations:
  • A freely suspended magnetic compass aligns itself with Earth’s magnetic field lines, directing North.
  • However, this leads to a misconception as the compass’ North pole (a magnetic South pole) aligns with the geographic North pole, indicating:
    • The Earth’s North magnetic pole is essentially a magnetic South pole.
    • The geographic South pole is near the magnetic North pole.

Electric Current and Magnetism

Current Effects

• Magnetic Field Generation

  • An electric current flowing through a wire creates a magnetic field around the wire.
  • Directionality of the magnetic field is determined by the "Right Hand Rule."

• Demonstration with Plotting Compasses

  • When a wire pierced through a piece of paper has current flowing, plotting compasses can visualize the magnetic field around it.

Characteristics of Magnetic Fields

• Dependence on Current Size
  • The strength of the magnetic field produced is directly proportional to the amount of current flowing through the wire; greater current equates to a stronger magnetic field.
  • The magnetic field strength also diminishes with increased distance away from the conductor; further distance means weaker magnetic fields.

Solenoids

• Structure and Functionality

  • A solenoid is a coiled wire that produces a fashioned magnetic field similar to that of a bar magnet.
  • Purpose of Coiling:
  • By coiling wire, the magnetic fields can align and create a single, uniform magnetic field, especially along the center of the solenoid.

• Effect of Iron Cores

  • Incorporating an iron core within the solenoid enhances its magnetic field strength because iron permits magnetic field lines to traverse through more effortlessly compared to air.
  • Inside the solenoid, fields from individual turns of wire cancel each other out, resulting in a weaker field externally.

• Factors Affecting Magnetic Field Strength

  • Size of current
  • Length of the solenoid
  • Cross-sectional area of the solenoid
  • Number of turns (coils) of the wire
  • Use of soft iron cores

Interactions between Wires and Magnets

Magnetic Field Interactions

• Wire and Magnet Interaction

  • When a wire with current flows in the proximity of a magnet, the magnetic field generated by the current interacts with the magnet’s own magnetic field.
  • Force Principles
  • The force acting on the conductor is equal and opposite to the force exerted on the magnet by the interaction of the respective fields.
  • Magnetic forces are resultant of interactions between any two magnetic fields.

• Geometrical Orientation

  • The magnetic field around a current-carrying wire is circular, whereas the magnetic field between two magnets is linear.
  • Upon interaction, the current-carrying wire will be pushed away from the linear magnetic field established between the two poles, at right angles to both the wire's direction and the established magnetic field.

• Visualization Scenario

  • Fixed permanent magnets (A and B) generate field lines along the x-axis.
  • The wire is oriented along the y-axis, with current moving from point C to point D.
  • The resulting force acting on the wire is perpendicular to both the current direction and the magnetic field, aligned along the z-axis.

Fleming’s Left Hand Rule

• Rule Application

  • Each component (Field, Current, Force) is perpendicular (90°) to each other.
  • The rule can be utilized to determine the unknown variable among the three, frequently the direction of the force felt.
  • Current is considered conventional, suggesting the motion of positive charge in the opposite direction to electron flow.

• Force Equation

  • Force generated can be calculated using:

F = BIL

• Where:
  • F = Force (in Newtons)
  • B = Magnetic Flux Density (measured in Tesla [T])
  • I = Current (in Amperes)
  • L = Length of the conductor within the magnetic field (in meters)
  • Note: Magnetic flux density represents the number of flux lines per square meter.

Motors

• Operational Mechanics
  • A motor comprises a coil of wire positioned between two permanent magnets.
  • Current flows through the wire, generating a magnetic field that interacts with the permanent magnets.
  • This interaction results in one side of the coil being pushed down and the other side moving up, leading to rotation of the coil.
  • Employ Fleming’s Left Hand Rule to ascertain which side of the coil experiences upward or downward force during its operation.