Moving Charges and Magnetism

Magnetic Force and Magnetic Field

As children, I am sure all of us have played with magnets. Magnets have always been a mystery to us. They are a fun manipulative.  At some orientation, they would pull each other towards themselves and at some, they would move away from each other. Over the years we learnt that the force that works behind this behaviour of magnets is its Magnetic Force which is attractive or repulsive in nature depending on its orientation with other magnets.

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Magnetic Force

Magnetic Force can be defined as the attractive or repulsive force that is exerted between the poles of a magnet and electrically charged moving particles. Hence, it is a consequence of the electromagnetic forces.

We have seen that the interaction between two charges can be considered in two stages. The charge Q, the source of the field, produces an electric field E, wherev\(\vec{E}\) = Q \(\vec{r}\) / (4πε0 ) r2, \(\vec{r}\) is unit vector along r, and the field E is a vector field. A charge q interacts with this field and experiences a force F given by

 \(\vec{F}\) = q\(\vec{E}\) = q Q \(\vec{r}\) / ( 4 π ε0 ) r 2

Magnetic Force

(source: flikr)

Magnetic Field

The Magnetic Field is the space around a magnet or current carrying conductor around which magnetic effects can be experienced. Furthermore, it is a vector quantity and its SI unit is Tesla (T) or Wbm‒2

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Magnetic Force and Magnetic Field

Magnetic Lines of Force

It can be defined as curved lines used to represent a magnetic field. In fact, the number of lines relates to the magnetic field’s strength at a given point. Furthermore, the tangent of any curve at a particular point is along the direction of the magnetic force at that point.

Magnetic Force

[source: qsstudy]


  1. Magnetic lines of force start from the North Pole and end at the South Pole.
  2. They are continuous through the body of a magnet.
  3. Magnetic lines of force can pass through iron more easily than air.
  4. Two magnetic lines of force can not intersect each other.
  5. They tend to contract longitudinally.
  6. They tend to expand laterally.

Magnetic Force on Current-Carrying Conductor

A current-carrying conductor experiences magnetic forces in a magnetic field. Fleming’s Left-Hand Rule predicts the direction of the magnetic forces,

F = IlBsinθ

where F is the magnetic force, I is current, l is the length of a straight conductor in a uniform magnetic field B and θ is the angle between I and B.

Magnetic Force

Magnetic Force on Current-Carrying Conductor

Solved Examples For You

Question: If a charged particle projected in a gravity-free room deflects, then

A) There must be an electric field          B) There must be a magnetic field.

C) Both fields cannot be zero                  D) None of these

Solution: Since there must be some external force which will cause the deflection of charged particle and it can be both magnetic force or electric force. Therefore, simultaneously both the fields cannot be zero, therefore, option (C) is the answer. Also, option (A) and (B) are saying that there should be electric field compulsory or magnetic field compulsory for a deflection which is not true, therefore, the only option is (C).

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One response to “Magnetic Field Due to a Current Element, Biot-Savart Law”

  1. Wasique Azmi says:

    What about 2 m magnetic feild? They hv not asked at centre!!!!!

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