9702 · 20.2
Force on a current-carrying conductor — FAQ
Frequently asked questions for 9702 Force on a current-carrying conductor. Direct answers first, then deeper explanation — then practise with marking.
Why is 'magnetic flux density' also called 'magnetic field strength'?
While subtly different in advanced physics, for A-Level Cambridge Physics, 'magnetic flux density' (B) is often interchangeably referred to as 'magnetic field strength' because it directly quantifies the strength of the magnetic field's effect on charges or currents. The unit, Tesla, represents this strength.
What's the main difference between F = BILsinθ and F = BQvsinθ?
F = BILsinθ calculates the force on a macroscopic current-carrying conductor (a wire), where 'I' is the total current and 'L' is the length. F = BQvsinθ calculates the force on a single, individual charged particle moving through the field, where 'Q' is the particle's charge and 'v' its velocity. The latter is the fundamental force, and the former is its cumulative effect.
Does Fleming's Left-Hand Rule work for negative charges?
Yes, but with a modification. If you use the middle finger for the direction of conventional current (positive charge flow), then for a negative charge moving, you must either point your middle finger in the opposite direction of the negative charge's motion, or keep the middle finger with the negative charge's motion and then reverse the final direction of the force (thumb). It's generally safer to consider the conventional current direction, which is opposite to electron flow.