# Dielectric effect on voltage

1. Jul 24, 2007

### mpswee2

1) When we insert dielectric between plates, plates can "hold" a greater quantity of charge since capacitance is increased.

But when capacitance increases, does it require that voltage decreases? (From the formula C = Q/V, capacitance and charge could increase while voltage remains constant.)

But voltage V = PE/Q. If Q increases due to greater capacitance, then voltage would have to decrease. But in a given circuit, if the voltage source remains the same before and after inserting the dielectric, I don't understand how the voltage could change.

2) Also, why is the "ground," as a generality, considered less resistant than other materials. How are we sure of this? For instance, when using a defibrillator, administrators need to elevate the patient off the ground lest the current "skip" the patients' heart. But why is the ground's resistivity invariably so low compared to human body's, building materials of a house?

Thanks for helping to clarify.

Last edited: Jul 24, 2007
2. Jul 25, 2007

### mgb_phys

1, Correct, adding a dielectric allows you to either a, store the same charge with lower voltage or b, store a greater charge with the same voltage. Snce in proactice the voltage is usually a fixed part of the design a dielectric allows you to store more charge in a smaller space.
If the capacitor is no longer connected to the supply when you insert the dielectric then as the charge cannot change ( charge is conserved) the voltage will drop. If it is still connected to the supply then more charge will flow into the capacitor.

2, The ground is a good conductor because it's cross sectional area is rather large.

3. Feb 3, 2010

### ram456

1. What is the relationship between dielectric and
a. Energy
b. Voltage
as in how the prescence of a dielectric affects the Energy and voltage?? (increases, decreases etc) and please explain

2. Why is it that when a parallel plate capacitor is charged and then a dielectric slab is introduced btween it. Voltage, Capacitance and Energy change but charge remains unchanged?

4. Feb 3, 2010

### Born2bwire

All of this is associated with the phenomenon of polarization in a dielectric. An applied electric field across a dielectric induces dipole moments in the dielectric's molecules. That is, charge separation occurs because the Lorentz force on positive and negative charges act in opposite directions. These dipole moments also create their own secondary fields. The effects of these dipole moments and their associated fields are the cause for the changes in voltage, capacitance, energy and total fields. You can find more about these by looking in any undergraduate electrodynamics text like Griffiths.