Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
The direction of the magnetic field is downward.
According to Fleming's left-hand rule, when we stretch the thumb, forefinger, and middle finger of our left hand such that they are mutually perpendicular, the forefinger points in the direction of the magnetic field, the middle finger points in the direction of current, and the thumb points in the direction of force.
In this scenario, the electron beam moves from the back wall towards the front wall. Since the direction of current is opposite to the direction of motion of electrons, the current flows backward. The force deflects the beam to the right. Applying Fleming's left-hand rule, with the middle finger pointing backward and the thumb pointing to the right, the forefinger points downward, indicating the magnetic field direction.
Explanation
This question tests the application of Fleming's left-hand rule. The textbook context clearly states that 'the direction of current is taken opposite to the direction of motion of electrons.' Since electrons move from back to front wall, conventional current flows from front to back. Using Fleming's left-hand rule with current direction (backward) and force direction (right), the magnetic field must be directed downward.
Solution Steps
Step 1: Identify electron motion direction - from back wall to front wall (forward from person's perspective).
Step 2: Determine conventional current direction - opposite to electron motion (backward).
Step 3: Identify force direction - to the right side of the person.
Step 4: Apply Fleming's left-hand rule: middle finger points backward (current), thumb points right (force), forefinger points downward (magnetic field).
Step 5: Conclude that the magnetic field is directed downward.