Paper 1 Magnetic Force Answers

Q	Answer	Reason
1	B	$\sin \theta$ is maximum at $90^{\circ }$.
2	A	Motor-effect magnitude for a straight wire.
3	B	$\sin 0=0$.
4	C	$q\vec{v}\times \vec{B}$ is perpendicular to both.
5	B	Radial force changes direction of velocity.
6	A	Equate $Bqv=m{v}^2/r$.
7	A	No deflection means electric and magnetic forces balance, so $v=E/B$.
8	B	Same-direction parallel currents attract.
9	B	Fleming left-hand rule gives force direction.
10	C	Perpendicular force has no component along displacement.
11	B	Charge sign reverses force direction.
12	B	$F\propto B$.
13	B	$\sin 0=0$.
14	A	From $F=BIL$.
15	A	Known force, current and length give $B$.
16	A	Radial force changes direction but not speed.
17	B	Parallel component gives no magnetic force.
18	C	Parallel component continues while perpendicular component circles.
19	A	Definition of motor effect.
20	A	$F/L\propto I_1{I}_2/d$.