A pair of large horizontal metal plates has a separation of $4\,\text{mm}$. The lower plate is at a potential of $-80\,\text{V}$. What potential should be applied to the upper plate to produce an upward electric field of strength $60\,000\,\text{V m}^{-1}$ in the region between the plates?
- A$-320\,\text{V}$
- B$-160\,\text{V}$
- C$+160\,\text{V}$
- D$+320\,\text{V}$