Question #78

The distance between the plates of a parallel plate capacitor is doubled. Which of the following is true?

if other variables are kept constant the strength of the electric field present between the plates will halve

if the capacitance of the capacitor is to be kept constant then the plate area must be doubled

if other variables are kept constant the time constant for the capacitor if placed in a simple series circuit will double

if the plate area is doubled the strength of the electric field present between the plates will be greater than it was originally

if other variables are kept constant the quantity of charge stored by the capacitor when a particular potential difference exists between its plates will double

The capacitance of a parallel plate capacitor is directly proportional to the plate area and indirectly proportional to the distance between the plates. The constant of proportionality is termed the ‘permittivity’ and reflects the degree to which the material between the plates (the ‘dielectric’) is able to shield against an electric field.

If the distance between the plates is doubled while other parameters are kept constant, the capacitance of the capacitor will halve.

Time constant is directly proportional to capacitance and would therefore halve.

The quantity of charge that is stored by a capacitor when a particular potential difference exists between its plates depends on its capacitance. This would therefore halve.

The capacitance of the capacitor could be restored to its previous value by either doubling the plate area or doubling the permittivity of the dielectric.

The field strength between the plates varies linearly with the distance between the plates. Doubling this distance will therefore halve the field strength, i.e. the force that a charge experiences when positioned between the plates. Increasing the plate area will increase the area over which the field acts but will not affect the field strength between the plates.

See A Thinking Approach to Physiology, pages 211 - 216.

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