A beam of positively charged particles, say A, is going from north to south. On the other hand, a beam of negatively charged particles, say B, is going from south to north. Which one of the following in this respect is correct?
- (a)B is deflected towards A.
- (b)B is deflected away from A.
- (c)B is deflected upwards.
- (d)B is deflected downwards.
Correct — A, B is deflected towards A. Convert both beams into conventional currents. Beam A is positive charge moving north to south, so its conventional current points north to south. Beam B is negative charge moving south to north, and for negative carriers the conventional current points opposite to the motion — so it too points north to south. The two beams are therefore parallel currents flowing in the same sense, and parallel currents in the same direction attract one another. Beam B is pulled towards beam A, and beam A is equally pulled towards B.
- (b)B is deflected away from A. — This would be right if the two conventional currents were antiparallel. They are not: reversing both the sign of the charge and the direction of travel reverses the current direction twice, which leaves it unchanged.
- (c)B is deflected upwards. — Nothing in the stem defines an up direction relative to the two beams, and the magnetic force here lies along the line joining them. A vertical deflection would require a field component that the problem does not supply.
- (d)B is deflected downwards. — Wrong for the same reason as the previous option. The force between two parallel current-carrying beams acts along the line joining them, towards or away, not perpendicular to that line.
A moving charge is a current, and the direction of conventional current is the direction in which positive charge moves. For negative carriers the conventional current therefore points opposite to the particles' velocity — the fact that makes this question work. Each current sets up a magnetic field circling it, and the other current, sitting in that field, feels a force given by the motor rule. Working it through for two long straight parallel currents gives the standard result: like directions attract, opposite directions repel. This is the very effect used to define the ampere in the older system of units.
The double reversal is the whole difficulty. Beam B has its charge sign flipped and its direction of travel flipped, and two flips cancel, so B behaves electrically like a positive beam travelling north to south — the same as A. A candidate who applies only one of the two reversals will conclude that the currents are opposed and will pick repulsion. The other two options invite a deflection perpendicular to the beams, but no such direction is defined by the geometry given. This card marks a different letter from the answer we hold on file.
- Conventional current points in the direction of positive charge flow, and opposite to the flow of negative charges.
- Two parallel currents in the same direction attract; in opposite directions they repel.
- The force per unit length between two long parallel currents is proportional to the product of the currents and inversely proportional to their separation.
- The force on a current-carrying conductor in a magnetic field is given by Fleming's left-hand rule.
- This attraction between parallel currents was the basis of the earlier definition of the ampere.
Two reversals cancel: flipping the charge sign and flipping the direction of travel leaves the current direction where it started.
- Applying only one of the two reversals and concluding that the currents oppose each other.
- Treating a beam of negative particles as a current in the direction the particles travel.
- Choosing a vertical deflection when the problem defines no vertical reference at all.
A conceptual electromagnetism item disguised as a direction puzzle; it tests one rule about conventional current and one standard result.
A charged particle moves through a magnetic field B with a velocity v. Which one of the following statements is true for the force (F) experienced by the particle?
- (a) F is maximum when v and B are parallel to each other.
- (b) F is maximum when v and B are anti-parallel to each other.
- (c) F is maximum when v and B are perpendicular to each other.
- (d) The force F is independent of the angle between v and B.
Answer(c) F is maximum when v and B are perpendicular to each other.
The force law underneath this question. Each beam sits in the other's magnetic field with its velocity perpendicular to that field, which is exactly the arrangement that gives the maximum force.
The rule that determines the direction of a magnetic field produced around a straight conductor carrying current is:
- (a) Right-hand thumb rule
- (b) Fleming's left-hand rule
- (c) Fleming's right-hand rule
- (d) Hund's rule
Answer(a) Right-hand thumb rule
Gives the field each beam creates around itself, which is the first half of working out the force between them.
- practice — not a real PYQ
Two long straight parallel wires carry currents in opposite directions. The wires will
- (a)attract each other
- (b)repel each other
- (c)exert no force on each other
- (d)rotate about their midpoint
Answer(b) repel each other — antiparallel currents repel, while currents in the same direction attract.
- practice — not a real PYQ
A beam of electrons travels from east to west. The direction of the conventional current associated with this beam is
- (a)east to west
- (b)west to east
- (c)north to south
- (d)there is no current
Answer(b) west to east — conventional current runs opposite to the direction of electron flow.