Question

When a dielectric is placed between the plates of a charged, isolated capacitor, the electric field inside the capacitor a) increases. b) decreases. c) stays the same. d) increases if the charge on the plates is positive. e) decreases if the charge on the plates is positive.

Short answer

Answer: (b) decreases

Step-by-step solution

Understand the role of a dielectric

Dielectrics are insulating materials that have the property of polarizing when an electric field is applied. The dielectric does not allow the flow of charges but increases the capacitance of the capacitor by polarizing its molecules in the presence of an electric field.

Recall the capacitance formula and electric field inside a capacitor

The capacitance of a capacitor is given by the formula C = Q/V, where C is the capacitance, Q is the charge on the plates, and V is the voltage across the plates. The electric field inside a capacitor is given by the formula E = V/d, where E is the electric field and d is the distance between the plates.

Examine the effect of a dielectric on capacitance

When a dielectric is placed between the plates of a capacitor, the capacitance increases due to the induced polarization of the dielectric molecules. For a given charge Q, an increase in capacitance leads to a decrease in voltage (V) across the plates, as V = Q / C.

Find the effect of a dielectric on the electric field

As we found that the voltage across the plates decreases due to the increased capacitance, it follows that the electric field inside the capacitor should also decrease. Using the formula E = V/d and keeping the distance between the plates constant, a decrease in voltage (V) corresponds to a decrease in the electric field (E).

Choose the correct answer

Putting everything together, we can conclude that - when a dielectric is placed between the plates of a charged, isolated capacitor, the electric field inside the capacitor (b) decreases.