Physics Lens

15. Electromagnetism

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[accordion title=”1. Definitions”]

  • The magnetic flux density at a point is defined as the force acting per unit current per unit length of the conductor when the conductor is placed at right angles to the field.
  • One tesla is the uniform magnetic flux density which, acting normally to a long straight wire carrying a current of 1 ampere, causes a force per unit length of 1 N m–1 on the conductor.


[accordion title=”2. Magnetic Fields”]

  • The following are the vector symbols used in diagrams to represent the direction of vectors in 3 dimensional space:
    • $$\rightarrow$$ : on the plane of the page
    • $$\otimes$$ : into of the page
    • $$\odot$$ : out of the page
  • The following are some important points to take note when representing a magnetic field by magnetic field lines:
    • Magnetic field lines appear to originate from the north pole and end on the south pole.
    • Magnetic field lines are smooth curves.
    • Magnetic field lines never touch or cross.
    • The strength of the magnetic field is indicated by the distance between the lines – closer lines mean a stronger field.


[accordion title=”3. Force on a Current-Carrying Conductor in a Magnetic Field”]

  • When a wire of length $$l$$ carrying a current $$I$$ lies in a magnetic field of flux density $$B$$ and the angle between the current $$I$$ and the field lines $$B$$ is $$\theta$$, the magnitude of the force $$F$$ on the conductor is given by $$F = BIl sin \theta$$.
    magnetic force
  • The directions of the vectors can be recalled by using the Fleming’s Left-Hand Rule.
    Fleming's Left-Hand Rule


[accordion title=”4. Force on a Moving Charge in a Magnetic Field”]

  • A charge $$q$$ travelling at constant speed $$v$$ at an angle $$theta$$ to a magnetic field of flux density $$B$$ experiences a force $$F = Bqv sin\theta$$.


[accordion title=”5. Magnetic fields of current-carrying conductors”]

  • Long straight wire
    Right-Hand Grip Rule
  • Flat circular coil
  • Solenoid


[accordion title=”6. Ferromagnetic Materials”]


[accordion title=”7. Force between Two Parallel Current-Carrying Conductors”]

  •  Like currents attract and unlike currents repel.