DPP Science Module

TE Unit 4-1: Magnetism

Introduction to Magnetism

  • Magnetism plays a major role in the operation of a diverse array of electrical systems including:

    • Motors

    • Generators

    • Transformers

    • Contactors

    • Solenoids

    • Relays

    • Measuring instruments

  • To design and maintain these systems, it is necessary to:

    • Understand the laws relating to magnetic fields.

    • Apply these laws to resolve magnetic circuit problems.

Magnetism in Everyday Life

  • Examples of devices utilizing magnetism include:

    • Induction Motors

    • Lifts

    • Escalators

    • Fans

    • Speakers

    • Phones

    • Earpieces

Magnetic Field Lines

  • A compass points in the direction of magnetic field lines, aligning itself accordingly.

  • Key characteristics of magnetic field lines:

    • They point from North to South.

    • They form continuous loops.

Rules of Magnetic Field Lines

  1. The direction of the magnetic field is tangent to the field line at any given point.

    • Tangent = Direction of the magnetic field line.

  2. Magnetic field lines do not cross.

  3. Field lines that run in the same direction repel each other.

  4. Field lines that run in opposite directions attract each other.

  5. Field strength is proportional to the density of the field lines, known as magnetic flux density.

    • More magnetic field lines intersecting a surface area indicates a higher magnetic flux density.

    • Lower field line density results in lower magnetic flux density.

Magnetic Flux

  • Definition: The total number of magnetic field lines that pass through a specified area.

    • Symbol: ᶲ (phi)

    • Unit: Weber (Wb)

    • Graphical relationship:

    • Higher magnetic flux corresponds to denser field lines.

    • Lower magnetic flux corresponds to sparser field lines.

Magnetic Flux Density

  • Definition: The total number of magnetic field lines per unit area. It is vital for determining the strength of the magnetic field.

    • Formula: B=FAB = \frac{F}{A} where:

    • B = magnetic flux density

    • F = total magnetic flux

    • A = area

    • Symbol: B

    • Unit: Weber/m² (Wb/m²) or Tesla (T)

Practical Exercises

  • Determine the following for given examples of magnetic fields:

    • More flux

    • Higher flux density

Production of Magnetic Fields

  • Solenoids: A single conductor carrying current produces a small magnetic field. The field strength is significantly increased when wires are coiled, creating a solenoid.

Electromagnetism

  • Definition: It refers to the physical interactions among electrically charged particles, producing both electric and magnetic fields.

    • The movement of charged particles creates magnetic fields.

    • A change in magnetic fields can induce movement of charged particles, leading to current generation.

Right Hand Grip Rule

  • To determine the direction of the magnetic field produced when a current flows through a conductor, apply the right hand grip rule.

Representing 3D Direction on Paper

  • An arrow can represent direction. If it moves away from you, feathers form a cross, while if it moves towards you, you see the tip of the arrow.

  • Use a cross for movement going into the paper and a dot for movement coming out.

Exercise Using the Right Hand Grip Rule

  • Apply the right hand grip rule to determine magnetic field interactions for pairs of wires:

    • Exercise 1: Wires carrying current in the same direction.

    • Exercise 2: Wires carrying current in opposite directions.

Forces Due to Current in a Magnetic Field

  • When a current-carrying conductor is positioned perpendicular to a magnetic field, a force is generated.

    • Depicted Directions:

    • N (North) and S (South)

    • Different interactions lead to repulsion or attraction of the current flow.

Fleming’s Left Hand Rule (Motor)

  • This rule is utilized to determine the direction of the force on a current-carrying conductor within a magnetic field:

    • First Finger: Represents the magnetic field direction.

    • Second Finger: Represents the current direction.

    • Thumb: Represents motion.

  • Practice: Determine the resultant force on a current-carrying conductor using various orientations.

Calculating Force on a Conductor

  • The force (F) on a current-carrying conductor situated at right angles to a magnetic field is calculated by:

    • F=BILF = BIL where:

    • B = flux density (T)

    • I = current (A)

    • L = length of the conductor (m)

Example Calculations

  • Example 1: For a magnetic field with a flux in an area of 0.1 m² measuring 800 μWb:

    • Given: A = 0.1 m², F = 800 x 10⁻⁶ Wb

    • Calculation:

    • B=FA=800×10−60.1=8×10−3T=8mTB = \frac{F}{A} = \frac{800 \times 10^{-6}}{0.1} = 8 \times 10^{-3} T = 8 mT

  • Example 2: Calculating average flux density at a pole face where magnetic flux per pole in a DC machine is 2 mWb and pole dimensions are 10 x 20 cm:

    • Calculation reveals flux density as 0.1 T.

  • Example 3: The total flux from a bar magnet is 2 x 10⁻⁴ Wb with a 1 cm² cross-sectional area:

    • Conduct flux density calculation.

  • Additional examples follow similar structure demonstrating calculations for different setups, including air gaps in contactors and forces in various configurations.

Summary of Key Concepts in Magnetism

  • Key formulas:

    • Φ=FAΦ = \frac{F}{A} (Flux)

    • Right Hand Grip Rule helps determine the magnetic field direction due to current.

    • Fleming’s Left Hand Rule determines the resultant force direction in a magnetic field.

  • Understanding interactions within magnetic fields and current flowing through conductors is critical for applications in electrical engineering.