Assertion A beam of electron can pass undeflected through a region of E and B.
Reason Force on moving charged particle due to magnetic field may be zero in some cases.

Answer

To solve the given problem, we will analyze the assertion and the reason step by step.

### Step 1: Understanding the Assertion
The assertion states that "A beam of electrons can pass undeflected through a region of electric field (E) and magnetic field (B)."

- When a charged particle (like an electron) moves through electric and magnetic fields, it experiences forces due to both fields.
- The electric force (\( F_e \)) acting on the electron is given by:
\[ F_e = qE \]
where \( q \) is the charge of the electron and \( E \) is the electric field.

### Step 2: Understanding the Magnetic Force
The magnetic force (\( F_m \)) acting on a charged particle moving with velocity \( v \) in a magnetic field \( B \) is given by:
\[ F_m = q(v \times B) \]
- The direction of this force is determined by the right-hand rule and depends on the direction of the velocity \( v \) and the magnetic field \( B \).

### Step 3: Condition for Undeflected Motion
For the electron beam to pass undeflected, the net force acting on it must be zero. This means:
\[ F_e + F_m = 0 \]
or
\[ F_e = -F_m \]
- This implies that the electric force must equal the magnetic force in magnitude but be opposite in direction.

### Step 4: Equating Forces
Substituting the expressions for the forces:
\[ qE = qvB \]
- Dividing both sides by \( q \) (assuming \( q \neq 0 \)):
\[ E = vB \]
- Rearranging gives:
\[ v = \frac{E}{B} \]
- This shows that if the speed \( v \) of the electrons is equal to \( \frac{E}{B} \), they will pass through the fields undeflected.

### Step 5: Conclusion on Assertion
Since we have shown that there exists a condition under which the beam of electrons can pass undeflected, the assertion is correct.

### Step 6: Understanding the Reason
The reason states that "Force on a moving charged particle due to magnetic field may be zero in some cases."

- The magnetic force can indeed be zero if the velocity \( v \) of the charged particle is parallel to the magnetic field \( B \) (i.e., \( v \parallel B \)).
- In this case, the cross product \( v \times B = 0 \), leading to \( F_m = 0 \).

### Step 7: Conclusion on Reason
The reason is also correct, but it does not specifically explain why the beam of electrons can pass undeflected through the fields since it does not account for the electric field's influence.

### Final Conclusion
- **Assertion**: Correct
- **Reason**: Correct but not the correct explanation for the assertion.

### Summary:
- The assertion is true because there exists a condition where the forces balance (electric force equals magnetic force).
- The reason is true but does not provide the necessary explanation for the assertion.

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