Firstly let's consider that current (alpha particle)is coming towards us, So we got the above result. Now if you consider the alpha particle is going away from us, then put your middle finger away from your body, and your index finger will now be pointing to your left,ok? Now if that is the direction of motion of the alpha particle ( current)is away from our body, and the magnetic field direction remains the same (to the left). Now if an electron moves in that direction (away from our body) , current direction (alpha particle)will be opposite the direction of the direction in which the electron is moving. So, now if we realign our fingers keeping the magnetic field direction to the left (as per question) middle finger (current) will point towards us (opposite to electron movement) Now you will see that the thumb again points downwards, meaning force will act in downward direction. I hope you are clear, beta. 😊
