An alpha particle (a positively charged particle) projected towards the West is deflected towards the North by a magnetic field. What is the direction of the magnetic field?
Answer
Vertically upwards (out of the page).
The correct answer is Vertically upwards (out of the page). — Observe the forefinger, which represents the magnetic field. It points vertically upwards, perpendicular to the plane containing North and West.
Step-by-step solution
- Determine the direction of current: Electric current is taken in the direction of motion of positive charge. Since the alpha particle moves towards the West, the direction of current is towards the West.
- Determine the direction of force: The deflection of the particle towards the North means the magnetic force acts towards the North.
- Apply Fleming's Left-Hand Rule: Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular to each other.
- Align the middle finger (representing current) towards the West and the thumb (representing motion/force) towards the North.
- Observe the forefinger, which represents the magnetic field. It points vertically upwards, perpendicular to the plane containing North and West.
Common mistake: Confusing the direction of motion of positive charge with negative charge (electrons) or using the right hand instead of the left hand.
Concept tested: Fleming's left-hand rule
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