Two parallel, conducting plates with air between them are placed close to one another.

Question 12

Two parallel, conducting plates with air between them are placed close to one another. The top plate is given a negative charge and the bottom one is earthed.

Which diagram best represents the distribution of charges and the field between the plates?

Reference: Past Exam Paper – November 2015 Paper 13 Q30

Solution:

Answer: A.

Electric field lines are drawn from positive charge to negative charge. [C and D are incorrect]

Since the two plates are closer to one another the top plate, which is negative charged, will induce a positive charge on the bottom plate.

It is important thing to understand is that earth (or ground) does not imply that charge = 0. BUT earth is characterised by the fact that the potential = 0.

Also, zero potential does not necessarily imply that the charge is zero. The charge can be anything: zero, positive or negative. What determines the charge at the zero potential is the surrounding. For example, in this case, the top plate being negatively charged induces a positive charge on the lower plate.

 
So, the field lines are drawn upwards.