Force on a moving charge - AS & A Level

Force on a moving charge - AS & A Level

by Azizah Mutiarani

​Learning Goals

​• Predict the direction of the force on a charge moving in a magnetic field

• Recall and solve problems using F = BQv sin θ

​Fleming’s left-hand rule can be used again to find the direction of the force, magnetic field and velocity

• The key difference is that the second finger representing current I (direction of positive charge) is now the direction of velocity v of the positive charge

​if particles is an electron the direction of the current is in the opposite direction to the direction of electron's velocity

​positive charge

negative charge

you don't need to adjust the direction by the clockwise. Just focus on the page

​and the charge (+) or (-)

​positive charge

negative charge

you don't need to adjust the direction by the clockwise. Just focus on the page

​and the charge (+) or (-)

​positive charge

negative charge

you don't need to adjust the direction by the clockwise. Just focus on the page

​and the charge (+) or (-)

• Equivalent to the force on a wire, if the magnetic field B is perpendicular to the direction of the charge’s velocity, the equation simplifies to:

F = BQv

​The magnetic force on an isolating moving charge, such an electron, is given by the equation:

F = BQv sinθ

​F = BIL is for a current carrying conductor

F = Bqv is for an isolated moving charge (which may be inside a conductor)

​NOTE

​An electron is moving at 5.3 × 10⁷ m s^-1 in a uniform magnetic field of flux density 0.2 T. Calculate the force on the electron when it is moving at 30°