A2 Daily A Level Physics question
In a lab e/m experiment, an electron beam of fixed speed enters a region of uniform magnetic field at right angles and traces a circular arc on a fluorescent screen. The coil current is increased so the magnetic flux density becomes 1.5 times larger, while the accelerating voltage (and hence electron speed) is unchanged. What happens to the radius of the arc?
Answer
The correct answer is C.
Correct: C — It decreases to two-thirds of its original value. A assumes the radius grows with field strength, but a stronger field gives a larger sideways force so the path bends more tightly. B ignores that increasing B at the same speed increases the centripetal force requirement, so the radius must fall. C states the inverse change: with speed fixed, radius is inversely proportional to B, so multiplying B by 1.5 divides r by 1.5 (r_new = 2/3 r_old). D would match doubling the field; a 50% increase reduces the radius by a factor of 1.5, not 2.