In air, two horizontal parallel plate conductors are 5.0\,\text{mm} apart. The lower plate is earthed, while the upper plate is at a potential of +50\,\text{V}. What is the electric field strength $E$ at the point halfway between the plates?
- A$1.0 \times 10^4\,\text{V m}^{-1}$ downwards
- B$1.0 \times 10^4\,\text{V m}^{-1}$ upwards
- C$2.0 \times 10^4\,\text{V m}^{-1}$ downwards
- D$2.0 \times 10^4\,\text{V m}^{-1}$ upwards