Question

Consider a parallel plate capacitor created by sandwiching a 2 mm thick wire between two metal plates.The central metal core of the wire is 1 mm, covered by 0.5 mm of plastic (hence the total 2 mm distance between the plates).

Now 'd' here will be 2mm, area=> 2 cm2.

What is the type of plastic used often(P.V.C- dielec. const.=>3)?

What is the capacitance: 1. When there is no potential in the wire( i.e. the wire is not connected).
2. When the wire is connected to 220 V supply only on the +ve side (i.e. -ve terminal is not connected).

Answer 1

Continuation...
3. When the wire is connected to 220 V supply and carrying 5A current.

The 1st question is easy; Please answer Question 2 and 3.

Please explain whether the capacitance will change in questions 1 and 2 and 3.

Answer 2

I was thinking that the capacitance will change in question 2 as the +ve potential in the wire would attract the electrons in the plastic (hence changing the capacitance). Is it so?

+ve => Positive
-ve => Negative

Answer 3

It sounds reasonable, but it is my (unreasonable) opinion that the capacitance would not change. Capacitance being determined by the physical configuration. If not so, then the capacitance would be determined by the amount of current. another if the central wire was grounded, then the capacitance would appear as two capacitors in series and thus the capacitance would be reduced. Thinking out aloud. lol

Answer 4

Yes it does work.

I have attached a circuit using ic555.
9 V battery.
R1 => 1Kohm
R2 => variable (maybe 4 Kohm)
C => is NOT a capacitor; actually, use two insulated wires (i used solid core) and connect ECG(electrocardiogram) electrodes (mine was AgCl), to the 2 terminals)-since they are very sensitive.

Connect headphones to the output 3 (other end goes to ground).

TWIST the ECG wires and move the variable resistance till you get a very high pitch (3 KHz) sound.

Now take the ECG wires to a live wire (you need to touch only the wire connected to output 3).

If the wire is ON (has any potential; it need not be carrying any current), you will hear a very distinctly audible disturbance (the smooth sound will become a thumping low pitch Hum).

This circuit can be used to detect underground live wires without cutting them open, or a break in an insulated wire.

The ECG wires are TWISTed to increase their capacitance(15-20 twists).

Answer 5

Some older transmitters used twisted insulated wires to form the neutralization capacitor, similar to what you have done with the 555 oscillator.