Circuits
There are two types of circuits that we dealt with in this lab. One type is a RC circuit, which has a power supply, resistor and a capacitor. The other type is a LR circuit, which has a power supply, inductor and a resistor.
The first one was a RC circuit. The capacitance (C) of a capacitor is equal to the charge (Q) on either plate divided by the potential difference (VC) between the plates.
C º Q / VC
The capacitance has the units of farads (F). Using Kirchoff's Loop Rule, we know that The voltage drop across the resistor (VR) plus the voltage drop across the capacitor (VC) equals the voltage rise across the battery (x). This equation looks like:
VR + VC = x
Using Ohm's law and the definition of current we get:
VR = IR I = DQ / Dt
Therefore: VR = (DQ / Dt )R
Using the above information, we can rewrite Kirchoff's Loop Rule, which looks like: R(DQ / Dt) + (Q / C) = x
Substituting the following variables x and t, we can look at the equation a different way. Here are the definitions of these variables:
x º Q - xC t º RC
To get the charge as a function of time, we use this equation
Dx / Dt = - x / t
By graphing x versus time, we get the following equation:
x = x0 e(-t / t)
x0 is the value of x at t = 0. If we replace the x in the above equation with the definition of x, we get:
Q = Q¥ (1 - e(-t / t))
Substituting the above equation into the equation for capacitance and resistance, we get: VC = x(1 - e(-t / t)) VR = xe(-t / t)
Since current (I) is VR / R, we can get the equation:
I = I0e(-t / t)
Discharging a capacitor in a RC circuit can be related to time also. This is seen in the following equation:
DQ / Dt = - Q / t
Relating this equation to a similar equation that we defined earlier, we can get:
Q = Q0e(-t / t)
Using the definition of capacitance and Kirchoff's Loop Rule, we can expand the above equation to these equations:
VC = xe(-t / t) VR = -xe(-t / t)
Using the definition of current, we get:
I = -I0e(-t / t)
The other type of circuit was a LR circuit. The back emf (xL) is equal to the rate of change of the current times the inductance of the coil (L).
Ewald Georg von Kleist is a German scientist who created the capacitor in November of 1745. Regrettably, Kleist did not have the proper paper work to claim in the records that the design of the capacitor was his idea. Many months later, a Dutch professor named Pieter van Musschenbroek created the Leyden jar, the world’s first capacitor (on record). It was a simple jar that was half filled with water and metal above it. A metal wire was connected to it and that wire released charges. Benjamin Franklin created his own version of the Leyden jar, the flat capacitor. This was the same experiment for the more part, but it had a flat piece of glass inside of the jar. Michael Faraday was the first scientist to apply this concept to transport electric power over a large distance. Faraday created the unit of measurement for a capacitor, called Farad.
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