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Voltage vs. Current in a Resistor, Capacito r or Inductor

http://electrical- engineering- portal.com/voltage- vs- current- resistor- capacitor- inductor April 18, 2013

Voltage vs. Current in a Resistor, Capacitor or Inductor

Edvard

Elements in an electrical system behave dif ferently if they are exposed to direct current ascompared to alternating current. For ease of explanation, the devices have often beencompared to similar every day items.

1. Resistors in electrical systems are similar to rocks in a stream of water.

2. A capacito ris comparable to a boat paddle inserted into the stream.

3. The action of inductor is similar to a coiled spring.

DC Circuit Components

Let us first look at t he simple case of a dc circuit composed of a constant EMF (batt ery) andthe three basic elements and two conf igurations (series/parallel).

Resistors

http://electrical-engineering-portal.com/capacitor-typeshttp://electrical-engineering-portal.com/a-look-at-the-power-rating-of-resistorshttp://electrical-engineering-portal.com/voltage-vs-current-resistor-capacitor-inductor

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Figure 1 - DC Circuit Resistance

As the current f lows through the resistors, in the same way that water flows over rocks, itexpends some of its energy. If the rocks in a stream were in the form of rapids, the streamwould have considerable resistance. However, if the same amount of rocks were placed in arow across the stream, the overall resistance to current f low would be less.

The diagrams below illustrate the basic but underlying principle in the majority of electricalsystems. The amount of pot ential required to f orce 1 Amp through 1 Ohm of resistance is 1Volt (Ohms Law). This is of ten writ ten as V=IR.

In the series circuit (on left), the same current f lows through every resistor, but the appliedvoltage isdivided between them. In the parallel circuit (on right), the same voltage is applied toall resistors but t he current divides between them.

For a series circuit the tot alresistance is stated asRTotal = R1 + R2 + (et c.). In

this example RTotal = 6R.

For a parallel circuit the totalresistance is stated as1/RTotal = 1/R1 + 1/R2 +

(etc.). In this example1/RTotal = 6/R orRTotal =

R/6. Circuits containing acombinat ion of series andparallel portions apply thesame basic theory with more

lengthy calculat ions.

Capacitors

A capacito r, as previously described, is physicallymade of two conduct ing surfaces separatedby an insulator. In an electrical circuit capacitors have bot h a steady stat e and transient ef fecton the circuit .

As the electrical conductors are not in physical contact, it will not, in the long-term pass direct

current. The action is the same as placing a boat paddle against a stream of water it blockscurrent f low. However when voltage is f irst applied to a capacitor current will f low unt il thecapacitor is charged. This is a transient ef fect.

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Figure 2 - DC Circuit CapacitanceSteady State Ef f ect

Figure 3 - DC Circuit Capacitance Transient Ef f ect

Figure 4 - DC Circuit Inductance SteadyState Ef f ect

Inductors

The inductor as illustrated in Figure 4 is similar to acoiled spring and in the steady, has no resistingcapability. If a steady force is exerted on it, it can passhuge amounts of energy limited only by the supply

capability or heaviness of the inductor. An inductor in aDC system has to be used with caut ion as itallowsunrestricted f low of energy and will drain theenergy source or damage the inducto r itself .

However, when voltage is first applied the inductorimpedes current f low in the circuit .

Transient Effects

The t ransient ef fects of capacitors andinductors are the result of stored energyin the electrical circuit . Energyis stored intwo forms, as stored electrical charge ina capacitor and in the magneticfield inan inductor.

The amount of energy stored in an

inductor depends on the current in thecircuit ; the amount of energy inacapacitor depends on t he voltageacross it. If the circuit conditions changeso will the amount of energy stored inthe component and a transient willoccur.

One way of looking at t he transienteff ects of these components is that inductors resistchanges in current f low in a circuit and capacitors

resist changes in voltage. In basic dccircuits inductorsand capacitors are for the most part curiosit ies.

However, in an ac circuit the voltage and current arecont inuously changingso capacitor and inductors havea continuous effect on the circuit .

Resource: Science and Reactor Fundamentals -CNSC Technical Training Group

http://electrical-engineering-portal.com/220v-dc-system-at-thermal-power-station

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Figure 5 - DC Circuit Inducto r Transient Ef f ect