Question

A beam of $\alpha$-particles projected along $X$-axis, experiences a force due to a magnetic field along the $Y$-axis. What is the direction of the magnetic field?

Answer 1

Hint: Use Fleming’s left-hand rule to find the direction magnetic field of alpha particles from the magnetic force and current directions. Magnetic field will be perpendicular to the magnetic force and current directions.

Complete step by step answer:
Moving charges like alpha, beta particles will experience a force in the magnetic field. Fleming’s left-hand rule can be used to find the direction of this force if you know the direction of the magnetic field and the direction of the electric current. According to the rule, we have to keep our fingers of our left hand in following directions;

Thumb towards direction of magnetic force, middle finger towards the direction of current and forefinger in the direction of magnetic field.

Since the $\alpha$-particles are projected along the X-axis, the direction of the current will be along the X-axis. Magnetic force direction is along the Y-axis. Hence the magnetic field will be in the negative Z-axis direction according to Fleming's left-hand rule.

Additional information:
Alpha and beta particles normally move in circular paths in a magnetic field. This is because the magnetic force always tries to act at right angles to the direction of particle movement. So, this path will curve and form a circular motion. Here the force points towards the centre of the circle. Alpha particles travel in the same direction of convection current and they are positively charged. Beta particles move in the opposite direction of conventional current. Hence, they are negatively charged.

Note: You should remember alpha particles are positive. Therefore, they travel in the same direction of conventional current. i.e. The direction of positive charge movement). If it is beta particles, the direction of the magnetic field will change since they are negative particles. They travel in the opposite direction of current.