15. Electromagnetism

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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.

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The following are the vector symbols used in diagrams to represent the direction of vectors in 3 dimensional space:
- $\to$ : on the plane of the page
- $\otimes$ : into of the page
- $⊙$ : 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.

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[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 $θ$ , the magnitude of the force $F$ on the conductor is given by $F = B I l s i n θ$ .
The directions of the vectors can be recalled by using the Fleming’s Left-Hand Rule.

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

A charge $q$ travelling at constant speed $v$ at an angle $t h e t a$ to a magnetic field of flux density $B$ experiences a force $F = B q v s i n θ$ .