Question

Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally towards the front wall from the back wall, is deflected by a strong magnetic field to your right side. Find the direction of the magnetic field.

Hint:

For electron beam: Current direction is opposite to electron flow. Use Fleming's Left Hand Rule with current direction = opposite to electron motion. Force on electron = to right \(\Rightarrow\) magnetic field upward.

Answer 1

Finding the Direction of the Magnetic Field:

We are given:

– An electron beam is moving horizontally from the back wall toward the front wall.
– The electron beam is deflected toward your right side.
– We have to find the direction of the magnetic field.

Step 1: Understand the Given Directions

Let us define directions clearly:

– The electron beam is moving forward (from back wall to front wall).
– The deflection is toward your right side.
– We need to determine the direction of the magnetic field that causes this deflection.

Step 2: Apply Fleming’s Left-Hand Rule

According to Fleming’s Left-Hand Rule:

– Forefinger → Magnetic Field (B)
– Middle finger → Current (I)
– Thumb → Force (F)

Important point: An electron has negative charge, so the direction of current is opposite to the direction of electron motion.

Since the electron beam moves forward, the direction of current is backward (toward the back wall).

Step 3: Determine Magnetic Field Direction

We are told:

– Current (due to electrons) is backward.
– Force is toward your right side.

Using Fleming’s Left-Hand Rule:

If:

– Middle finger (current) → backward
– Thumb (force) → right side

Then the forefinger (magnetic field) must point downward.

Final Answer:
The direction of the magnetic field is vertically downward.

Conclusion:
Since electrons move opposite to current direction and the force is toward the right, applying Fleming’s Left-Hand Rule shows that the magnetic field must be directed downward.