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مقدمه ای بر الکتریسیته بخش پنجم

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DC Circuits

Series Circuits A series circuit is formed when any number of devices are

connected end-to-end so that there is only one path for current

to flow. The following illustration shows five resistors connected

in series. There is one path for current flow from the negative

terminal of the voltage source through all five resistors and

returning to the positive terminal.

The total resistance (Rt) in a series circuit can be determined by

adding all the resistor values. Although the unit for resistance

is the ohm, different metric unit prefixes, such as kilo (k) or

mega (M) are often used. Therefore, it is important to convert all

resistance values to the same units before adding.

Current in a series circuit can be determined using Ohm’s law.

First, total the resistance and then divide the source voltage by

the total resistance. This current flows through each resistor in

the circuit.

The voltage measured across each resistor can also be

calculated using Ohm’s law. The voltage across a resistor is

often referred to as a voltage drop. The sum of the voltage

drops across each resistor is equal to the source voltage.

The following illustration shows two voltmeters, one measuring

total voltage and one measuring the voltage across R3.

Parallel Circuits A parallel circuit is formed when two or more devices are

placed in a circuit side-by-side so that current can flow through

more than one path.

The following illustration shows the simplest parallel circuit,

two parallel resistors. There are two paths of current flow.

One path is from the negative terminal of the battery through

R1 returning to the positive terminal. The second path is from

the negative terminal of the battery through R2 returning to

the positive terminal of the battery. The current through either

resistor can be determined by dividing the circuit voltage by the

resistance of that resistor.

The total resistance for a parallel circuit with any number of

resistors can be calculated using the formula shown in the

following illustration.

In the unique example where all resistors have the same

resistance, the total resistance is equal to the resistance of one

resistor divided by the number of resistors.

The following example shows a total resistance calculation for a

circuit with three parallel resistors.

Current in each of the branches of a parallel circuit can be

calculated by dividing the circuit voltage, which is the same for

all branches, by the resistance of the branch. The total circuit

current can be calculated by adding the current for all branches

or by dividing the circuit voltage by the total resistance.

Series-Parallel Circuits Series-parallel circuits

 are also known as compound circuits.

At least three components are required to form a series-parallel

circuit. The following illustration shows the two simplest seriesparallel

circuits. The circuit on the left has two parallel resistors

in series with another resistor. The circuit on the right has two

series resistors in parallel with another resistor.

Series-parallel circuits are usually more complex than

the circuits shown here, but by using the circuit formulas

discussed earlier in this course, you can easily determine circuit

characteristics.

The following illustration shows how total resistance can be

determined for two series-parallel circuits in two easy steps

for each circuit. More complex circuits require more steps, but

each step is relatively simple. In addition, if the source voltage

is known, by using Ohm’s law you can also solve for current and

voltage throughout each circuit, .

Using the same two series-parallel circuits as in the previous

example, but with source voltages included, the following

illustration shows how Ohm’s law can be used to calculate

other circuit values.

Power in a DC Circuit Whenever a force of any kind causes motion, work is

accomplished. If a force is exerted without causing motion,

then no work is done.

In an electrical circuit, voltage applied to a conductor causes

electrons to flow. Voltage is the force and electron flow is

the motion. Power is the rate at which work is done and is

represented by the symbol P. The unit of measure for power

is the watt, represented by the symbol W. In a direct current

circuit, one watt is the rate at which work is done when 1 volt

causes a current of 1 amp. You will learn later that there are

other types of power that apply to alternating current circuits.

From the basic formula power = current times voltage, other

formulas for power can be derived using Ohm’s law.

The following example shows how power can be calculated

using any of the power formulas.

Review 3

1. The total current in a circuit that has a voltage of 12 V

and a resistance of 24 W is ________ A.

2. The total resistance of a series circuit with resistors

of the following values: R1 = 10 W, R2 = 15 W, and

R3 = 20 W is _______ W.

3. The voltage for a series circuit that has a current of

0.5 A and a resistance of 60 W is ________ V.

4. The total resistance of a parallel circuit that has four

20 W resistors is ________ W.

5. In a parallel circuit with two resistors of equal value and

a total current flow of 12 A, the current through each

resistor is ________ A.

6. For a DC circuit with a voltage of 24 V and a current of

5 A, the power is ________ W.


نوشته شده در : دوشنبه 20 آذر 1391  توسط : جواد خانی.    نظرات() .

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