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Direct Current (DC) Electricity
by Ron Kurtus (revised 11 January 2004)
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?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
Useful tools: Metric-English Conversion | Scientific Calculator.
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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.
DC movement of electrons in 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.)
A flashlight is a good example of a DC circuit
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.
Comparison with hose
The following chart compares water running in a hose and DC electricity flowing in a wire:
Water in a Hose |
DC in a Wire |
Electrical Units |
| pressure | potential (V) | Volts |
| rate of flow | current (I) | Amps |
| friction | resistance (R) | Ohms |
Analogy between a Hose and Electricity in a Wire
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 charge 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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Resources
The following resources provide information on this subject:
Websites
Direct Current description - Simple illustrations of DC
Electricity - Difference between AC and DC
DC and AC Electricity Resources
Books
Top-rated
books on DC Electricity
Mini-quiz to check your understanding
If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.
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Direct Current (DC) Electricity