# (You may use the book and/or notes) 1.Find C T 2. Using V C = V S (1 - e -t/  ), what is the voltage across a capacitor at 10 seconds if R = 5Ω, C=2F,

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(You may use the book and/or notes) 1.Find C T 2. Using V C = V S (1 - e -t/  ), what is the voltage across a capacitor at 10 seconds if R = 5Ω, C=2F, and V S = 8V 3. What is the capacitance in Farads of a capacitor that has a vacuum for a dielectric, where the plates are 1mm x 1mm separated by a distance of 1  m.

What is  ? How many Tau does it take to get exactly charge a capacitor to 75% of being fully charged? How long does it take to charge a capacitor to fully charged if there is no resistor in the circuit? A capacitor acts like a ___________ under DC conditions and an ____________ under high frequencies. Today we will learn how capacitors and inductors act under normal frequencies open short

A long time constant is defined as one in which the RC time constant (τ) is at least five times longer, in time, than the pulse width of the applied waveform. As a result, the capacitor of a series RC circuit accumulates very little charge, and V C remains small. Draw V C

A medium time constant is defined as one in which the RC time constant (τ), in time, is equal to the pulse width of the applied waveform. As a result, the voltage across the capacitor of a series RC circuit falls between that of a long and short time constant circuit. Draw V C An RC circuit can be used to convert a Square wave signal into a Triangle signal. Also known as an Integrator circuit.

A short time constant is defined as one in which the time constant is no more than one-fifth the pulse width, in time, of the applied voltage. Here, the capacitor quickly charges to the applied voltage and remains there until the input drops to zero. Then the capacitor quickly discharges to zero. Draw V C

Capacitive and Inductive Reactance

Keeping in mind: Inductors act like shorts under DC and opens at instant change. Capacitors act like opens under DC and shorts at instant change. An AC signal is voltage level that is constantly changing. Both inductors and capacitors resist this mid level voltage change. (Not 0 Hz but not high freq either). Since Sine waves are continuous, this resistance is constant. In this sense, you can say capacitors are like constant resistors under steady AC waveforms.

The resistance or opposition that inductors and capacitors have towards an AC signal is not called resistance, it is called REACTANCE. Reactance for a capacitor is labeled X C. Reactance for an inductor is labeled X L. If both resistance and reactance exist in a circuit, then this total opposition is called IMPEDANCE: IMPEDANCE - THE TOTAL OPPOSITION TO CURRENT FLOW. Impedance is labeled Z

Because the plates of a capacitor are changing polarity at the same rate as the AC voltage, the capacitor seems to pass AC. Suppose you put capacitor in series with your scope probes when you were measuring voltage signal that contained both AC and DC parts. What would your oscope show?

Capacitors have LESS reactance under 2 conditions: (What do you think they are?) 1. Increase in frequency 2. Increase in Capacitance

= 15.9Ω = 39.8kΩ

= 1061 Hz = 3nF

What is the current flowing in the circuit below? However, this is not the whole story. Capacitors don’t act purely like a resistor. Why? There is a lag, as we will see in a few slides.

What is the current flowing in the circuit below? Yes you could have combined the capacitors first. How would you do this? Once you’ve converted Capacitors into resistances, treat them like resistors when doing the math.

You can also combine all capacitors first. What would be the total capacitance in this circuit?

Why is X C measure in ohms? What is the proportionality between f and X C ?

Capacitors have MORE reactance under 2 conditions: (What do you think they are?) 1. Increase in frequency 2. Increase in Capacitance

= 62.8Ω = 154Ω

= 1432 Hz = 3.2mH

What is the current flowing in the circuit below? However, this is not the whole story. Inductors don’t act purely resistive either. Why? There is a lag, as we will see in a few slides. Can you graph this current

The answer for current in the previous problem was 332mA. This current would look sinusoidal if you graphed it. (Similar to voltage) It is not a flat signal. Would this current be graphed as peak-to-peak? Peak? RMS? Average?

http://www.wimp.com/meltsrock/

The difference between reactance and resistance is with reactance there is a delay of some sort. Its called reactance because inductors and capacitors react to voltage changes, and it takes time to react, while resistors resist instantly as voltage changes.

Does capacitor current lead or lag capacitor voltage? ICE Current in a capacitor leads voltage in a capacitor

Reactance of an Inductor

Does inductor voltage lead or lag inductor current? ELI Voltage in an inductor leads current in an inductor

ELI the ICEman A pneumonic for remembering the leading and lagging characteristics for inductors and capacitors is:

http://www.sweethaven.com/sweethaven/modelec/dc ac/basicequs/default.asp http://www.sweethaven.com/sweethaven/modelec/dc ac/basicequs/default.asp

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