An electromagnetic wave is transposing energy in the negative y direction . At one point and one instant the magnetic field is in the positive x direction . The electric field at that point and instant is

Options

A. Positive y direction
B. Negative y direction
C. Positive z direction
D. Negative z direction

Answer

To solve the problem, we need to determine the direction of the electric field in an electromagnetic wave given the direction of energy propagation and the magnetic field.

### Step-by-Step Solution:

1. **Identify the Direction of Energy Propagation:**
- The problem states that the electromagnetic wave is propagating in the negative y direction. This means that the energy flow is also in the negative y direction.

2. **Identify the Direction of the Magnetic Field:**
- The magnetic field at the given point and instant is in the positive x direction.

3. **Use the Right-Hand Rule:**
- In electromagnetic waves, the electric field (E), magnetic field (B), and the direction of wave propagation (k) are mutually perpendicular to each other.
- According to the right-hand rule, if you point your thumb in the direction of the wave propagation (negative y), and your index finger in the direction of the magnetic field (positive x), your middle finger will point in the direction of the electric field.

4. **Determine the Direction of the Electric Field:**
- Since the wave is propagating in the negative y direction (thumb pointing down), and the magnetic field is in the positive x direction (index finger pointing to the right), the middle finger will point out of the page towards you, which corresponds to the positive z direction.
- However, since we are looking for the direction of the electric field vector, we need to consider the orientation. The electric field is perpendicular to both the direction of propagation and the magnetic field, which means it will actually be in the negative z direction.

5. **Conclusion:**
- Therefore, the electric field at that point and instant is in the negative z direction.

### Final Answer:
The electric field at that point and instant is in the **negative z direction**.