A-level Physics/Forces, Fields and Energy/Electromagnetism
Magnetic Force on a Current
F = B I L
F is force measured in newtons (N)
B is flux density (the strength of a magnetic field I.e No. of magnetic lines of force from magnet per unit area) measured in teslas (T, named after Nikola Tesla)
I is current measured in amperes (A)
L is the length of conductor in the magnetic field measured in metres (m)
B = F/IL This defines flux density, B.
We can measure F using the current balance.
If the current cuts across the magnetic field at the angle θ then the component the current across the field is ISineθ and therefore -
F = B I L sinθ
θ is the angle the current makes with the magnetic field.
F (force) is at its maximum when θ = 90 degrees, F = 0 when the current is parallel to the field lines. i.e., θ = 0 degrees.
Use Fleming's left hand rule for the direction of motion.
Magnetic Force on a moving charge
= BQL/T= BQV
The SI unit for charge, Q, is coulombs - C.
If charge moving at right angles to the field
F = -Bev for an electron ( e = charge on an electron)
Remember that the direction of conventional current is opposite to that of the electron.
The magnitude of an electron charge is -1.6Ε-19
F is always perpendicular to the path of the charged particle, so the particle moves in a circular path.
Therefore, centripetal force = mv²/R = BeV
Radius of path = R = mv/Be
Parallel Current Carrying Conductors
F/l=k (I1 I2)/d
Where F = Force; Newtons N
l = Length of the current carrying conductors parallel to each other.
k = constant; mu0/2p = 2 x 10-7 SI Units
I1 and I2 are current carrying conductors respectively
d = distance between the current carrying conductors (mm)
Radius is large for more massive, faster particles. Radius is smaller when the magnetic field strength is large
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