Direct Current (DC) Electricity

Direct current or DC electricity is the continuous movement of electrons from an area of negative (−) charges to an area of positive (+) charges through a conducting material such as a metal wire. Whereas static electricity sparks consist of the sudden movement of electrons from a negative to positive surface, DC electricity is the continuous movement of the electrons through a wire.

A DC circuit is necessary to allow the current or steam of electrons to flow. Such a circuit consists of a source of electrical energy (such as a battery) and a conducting wire running from the positive end of the source to the negative terminal. Electrical devices may be included in the circuit. DC electricity in a circuit consists of voltage, current and resistance. The flow of DC electricity is similar to the flow of water through a hose.

Questions you may have include:

• What is DC electricity?

• What are voltage, current and resistance?

• How do we create DC electricity?

Continuous movement of electrons

DC electricity is the continuous movement of electrons through a conducting material such as a metal wire. The electrons move toward a positive (+) potential in the wire.


In reality, there are millions of electrons weaving their way among the atoms in the wire. This is just an illustration of the movement.

Electrical circuit
An electrical circuit consisting of a source of DC power and a wire making a complete circuit is required for DC electricity to flow. (See DC circuits for more information.)





Current shown opposite
Although the negative charged electrons move through the wire toward the positive (+) terminal of the source of electricity, the current is indicated as going from positive to negative. This is an unfortunate and confusing convention.

Ben Franklin originally named charges positive (+) and negative (−) when he was studying static electricity. Later, when scientists were experimenting with electrical currents, they said that electricity travels from (+) to (−), and that became the convention.

This was before electrons were discovered. In reality, the negative charged electrons move toward the positive, which is the opposite direction that people show current moving. It is confusing, but once a convention is made, it is difficult to correct it.

Voltage, current and resistance
The electricity moving through a wire or other conductor consists of its voltage (V), current (I) and resistance (R). Voltage is potential energy, current is the amount of electrons flowing through the wire, and resistance is the friction force on the electron flow.

A good way to picture DC electricity and to understand the relationship between voltage, current and resistance is to think of the flow of water through a hose, as explained below.

Electrical voltage
A potential or pressure builds up at one end of the wire, due to an excess of negatively charged electrons. It is like water pressure building up in a hose. The pressure causes the electrons to move through the wire to the area of positive charge. This potential energy is called Voltage, its unit of measurement is the Volt.

Electrical current
The number of electrons is called current and its unit of measurement is the Ampere or Amp. Electrical current is like the rate that water flows through a hose.

Resistance
An Ohm is the unit of measurement of the electrical resistance. A conductor like a piece of metal has its atoms so arranged that electrons can readily pass around the atoms with little friction or resistance. In a nonconductor or poor conductor, the atoms are so arranged as to greatly resist or impede the travel of the electrons. This resistance is similar to the friction of the hose against the water moving through it.



Creating DC electricity
Although static electricity can be discharged through a metal wire, it is not a continuous source of DC electricity. Instead, batteries and DC generators are used to create DC.

Batteries
Batteries rely on chemical reactions to create DC electricity.

Car battery
The automobile battery consists of lead plates in a sulfuric acid solution. When the plates are given a change from the car's generator or alternator, they change chemically and hold the charge. That source of DC electricity can then be used to power the car's lights and such. The biggest problem with this type of battery is that sulfuric acid is very caustic and dangerous.

Lemon battery
Another battery that you can make yourself is a lemon battery. This one needs no charging but depends on the acidic reaction of different metals.

Copper and zinc work the best. You can use a copper penny or copper piece of wire. A zinc-coated or galvanized nail can be used as the other terminal. A standard iron nail will work, but not as good.

Push the copper wire and galvanized nail into an ordinary lemon and measure the voltage across the metals with a voltmeter. Some people have been able to dimly light a flashlight bulb with this battery.

DC generator
Another reliable source of DC electricity is the DC generator, which consists of coils of wire spinning between North and South magnets. (See Generating Electrical Current for more information.)

Summary
Direct current or DC electricity is the continuous movement of electrons from negative to positive through a conducting material such as a metal wire. A DC circuit is necessary to allow the current or steam of electrons to flow. In a circuit, the direction of the current is opposite the flow of electrons. DC electricity in a circuit consists of voltage, current and resistance. The flow of DC electricity is similar to the flow of water through a hose. Batteries and DC generators are the sources to create DC electricity.



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faryal shah

Direct Current (DC) Electrical Circuits

A direct current (DC) electrical circuit consists of a source of DC electricity with a conducting wire going from one of the source terminals to a set of electrical devices and then back to the other terminal, in a complete circuit. A DC circuit is necessary for DC electricity to exist. DC circuits may be in series, parallel or a combination. Understanding DC circuits is important for learning about the more complex AC circuits, like those used in the home.

Questions you may have include:

• What does an electric circuit consist of?

• What is a series circuit?

• What is a parallel circuit?

Simple circuit
If you take a continuous source of DC electricity, such as a battery, and connect conducting wires from the positive and negative poles of the battery to an electrical device such as a light bulb, you have formed an electric circuit.


In other words, the electricity flows in a loop from one end of the battery (or source of electricity) to the other end in a circuit. The concept of electric circuits is the basis for our use of electricity.

One nice feature of an electrical circuit is that you can install a switch in the circuit to turn the power on or off when you want.

Note: Although electrons move from a negative (−) area toward the positive (+), the convention was established that electricity is designation as moving from (+) to (−).



Power source
A DC circuit requires a source of power. Typically, a battery is used to provide continuous DC electricity. A DC generator is another source of energy. Alternating current (AC) electricity can be modified through a rectifier or adapter to create DC electricity. The common adapter used for some of your small DC-powered devices will transform 110V AC house current into 12V DC current for your device.

Voltage, current and resistance
The electricity moving through a wire or other conductor consists of its voltage (V), current (I) and resistance (R). The voltage or potential energy of a source of electricity is measured in Volts. The current of amount of electrons flowing through the wire is measured in Amperes or Amps. The resistance or electrical friction is measured in Ohms.

Conductors
The wire and electrical devices must be able to conduct electricity. Metal such as copper is a good conductor of electricity and has a low resistance. The tungsten filament in a light bulb conducts electricity, but it has high resistance that causes it to heat up and glow.

Series DC circuit
In an electrical circuit, several electrical devices such as light bulbs can be placed in a line or in series in the circuit between the positive and negative poles of the battery. This is called a series circuit.




One problem with such an arrangement is if one light bulb burns out, then it acts like a switch and turns off the whole circuit.

Schematic
Every device in a DC circuit--whether a light bulb or electrical motor--can be represented by an electrical resistance or resistor Usually, when drawing a circuit diagram or schematic, you use certain symbols for the battery and resistors.



Parallel DC circuit
Devices can also arranged in a parallel configuration, such that if any bulbs go out, the circuit is still intact. Not only is a parallel circuit useful for holiday lighting, the electrical wiring in homes is also in parallel. In this way lights and appliances can be turned on and off at will. Otherwise if you turned one light off--or one burned out--all the other lights in the house would go off too.





If either light bulb would go out, the other would still shine. You could add other bulbs or even appliances such as electric motors in parallel to this circuit, and they would remain independent of each other.




You could also replace a bulb with a series circuit of bulbs or add bulbs or devices in series between parallel items. There are many combinations possible.

Summary
DC electrical circuits consist of a source of DC electricity with a conducting wire going from one of the terminals to a set of electrical devices and then back to the other terminal, in a complete circuit. DC circuits may be in series, parallel or some complex combination.

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Ohm's Law for Electrical Circuits

The most fundamental equation in electrical circuits is called Ohm's Law. While doing experiments on how well metals conducted electricity, German physicist Georg Ohm discovered the law in 1827. Ohm's Law is the equation V = I R and is used in both AC and DC circuits. Knowing two items in this equation allows you to calculate the third.

Questions you may have include:

• What do the parameters in the equation mean?

• What are AC and DC circuits?

• How can you use Ohm's Law?

Parameters
Ohm's Law states that in a simple electrical circuit, the voltage equals the electrical current times the resistance.

V = IR

where:

V is the voltage in volts
I is the current in amperes or amps
R is the resistance in ohms
IR is I times R


Voltage
Voltage is the electrical potential energy and is measured in volts.

A good analogy is to think of a water hose. There is water pressure or potential energy on the other side of the faucet or outlet valve. Once you open the faucet, the pressure causes the water to rush through the hose.

The unit symbol for volts is V, as in 110V.

Current
Current indicates the amount of electrons passing through the wire and is measured in amperes or amps for short. For some reason, they use I to indicate current instead of a different letter. The unit symbol for amps is A, as in 2.0A.

Electrical current is similar to the rate of water flowing through a hose.

Resistance
Electrical resistance can be thought of as the "friction" on the movement of electrons in a wire. Resistance is measured in ohms, and the unit symbol for it is the Greek letter omega, Ω. Thus 3 ohms is often written as 3 Ω.

Most devices in an electrical circuit can be considered resistors, including light bulbs and electric motors. Even the wire itself provides some resistance. Just as you get some heat from friction, electrical resistance also results in heat. That is why the light bulb filament gets hot and glows.

Following the water hose analogy, resistance is similar to the friction inside the hose. But also, the resistance increases with a narrower hose, just like a thin copper wire has more electrical resistance than a thick wire.

Circuits
Ohm's Law applies to electrical circuits. The equation V = IR says that in a simple electrical circuit with a source of resistance—such as a resistor or a light bulb—the voltage equals the current times the resistance. This is true in a DC circuit and in an AC circuit.

Ohm's Law applies whether the current flows in one direction or alternates.

AC circuit
A simple AC circuit is illustrated below. A circle with the sine wave symbolizes an AC generator with some given voltage.




The resistor could be a single light bulb or an electrical motor. Or it could be a combination of resistive devices, such as several bulbs. Complex AC sub-circuits that include capacitors and inductors, besides resistors, are not being discussed in this lesson.

DC circuit
The power source for a DC circuit could be a battery or DC generator. The (+) and (−) indicate the direction of the current.




Using equation
The importance of Ohm's Law is that if you know the value two of the variables in the equation, you can then determine the third.

You can measure any of the parameters with a voltmeter. Most voltmeters or multi-meters measure voltage, current and resistance for both AC and DC.

Find voltage
If you know current and resistance, you can find voltage from V = I R. For example, if the current I = 0.2A and the resistance R = 1000 ohms, then

V = 0.2A * 1000 Ω = 200V

Find current
If you know voltage and resistance, you can use algebra to change the equation to I = V / R to find the current. For example, if V = 110V and R = 22000 ohms, then

I = 110V / 22000 Ω = 0.005A

Find resistance
If you know voltage and current, you can use algebra to change the equation to R = V / I to find the resistance. If V = 220V and I = 5A, then

R = 220V / 5A = 44 Ω

Summary
Ohm's Law is the equation V = I R that shows the relationship between voltage, current and resistance in a simple electrical circuit. It applies both the AC and DC circuits.

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