Assertion Maganetic field (B) and electric field (E) are present in a this region. Net force on a charged particle in this region is zero , if
`E=Bxx v`
Reason E/B has the dimensions of velocity.

Answer

To solve the given question, we need to analyze both the assertion and the reason provided.

### Step 1: Understanding the Assertion
The assertion states that in a region where both a magnetic field (B) and an electric field (E) are present, the net force on a charged particle is zero if the relationship \( E = B \times v \) holds true.

**Explanation**:
The force on a charged particle in an electric field is given by:
\[ F_E = qE \]
where \( q \) is the charge of the particle.

The force on a charged particle in a magnetic field is given by:
\[ F_B = q(v \times B) \]
where \( v \) is the velocity of the particle.

For the net force to be zero:
\[ F_E + F_B = 0 \]
This implies:
\[ qE + q(v \times B) = 0 \]
Dividing through by \( q \) (assuming \( q \neq 0 \)):
\[ E + (v \times B) = 0 \]
This can be rearranged to:
\[ E = - (v \times B) \]
If we assume that \( E = B \times v \), it indicates that the electric force and the magnetic force are equal in magnitude but opposite in direction, leading to a net force of zero.

### Step 2: Understanding the Reason
The reason states that the ratio \( \frac{E}{B} \) has the dimensions of velocity.

**Explanation**:
The dimensions of electric field \( E \) are:
\[ [E] = \frac{ML^2}{T^3I} \]
The dimensions of magnetic field \( B \) are:
\[ [B] = \frac{ML}{T^2I} \]
Now, calculating the dimensions of \( \frac{E}{B} \):
\[ \frac{E}{B} = \frac{\frac{ML^2}{T^3I}}{\frac{ML}{T^2I}} = \frac{L^2}{T^3} \cdot \frac{T^2I}{L} = \frac{L}{T} \]
This is indeed the dimension of velocity.

### Conclusion
Both the assertion and the reason are correct. However, the reason does not directly explain the assertion; it merely provides a dimensional analysis.

### Final Answer
- **Assertion**: True
- **Reason**: True
- **Explanation**: The reason is not a correct explanation of the assertion.