Alternating Current (AC) Electricity
by Ron Kurtus (revised 13 February 2016)
Alternating current (AC) electricity is the type of electricity commonly used in homes and businesses throughout the world.
While direct current (DC) electricity flows in one direction through a wire, AC electricity alternates its direction in a back-and-forth motion. The direction alternates between 50 and 60 times per second, depending on the electrical system of the country.
AC electricity is created by an AC electric generator, which determines the frequency.
What is special about AC electricity is that the voltage can be readily changed, thus making it more suitable for long-distance transmission than DC electricity. But also, AC can employ capacitors and inductors in electronic circuitry, allowing for a wide range of applications.
Note: We usually say AC electricity instead of simply saying AC, since that is also the abbreviation for air conditioning. You need to be exact in science to avoid any misunderstandings.
Questions you may have include:
- What is the difference between AC and DC electricity?
- Why do we use AC instead of DC?
- What are advantages of AC electricity?
This lesson will answer those questions. Useful tool: Units Conversion
Difference between AC and DC electricity
Electrons have negative (−) electrical charges. Since opposite charges attract, they will move toward an area consisting of positive (+) charges. This movement is made easier in an electrical conductor, such as a metal wire.
Electrons move direct with DC electricity
With DC electricity, connecting a wire from the negative (−) terminal of a battery to the positive (+) terminal will cause the negative charged electrons to rush through the wire toward the positive charged side. The same thing happens with a DC generator, where the motion of coiled wire through a magnetic field pushes electrons out of one terminal and attracts electrons to the other terminal.
Electrons alternate directions in AC electricity
With an AC generator, a slightly different configuration alternates the push and pull of each generator terminal. Thus the electricity in the wire moves in one direction for a short while and then reverses its direction when the generator armature is in a different position.
This illustration gives an idea of how the electrons move through a wire in AC electricity. Of course, both ends of the wire extend to the AC generator or source of power.
AC movement of electrons in wire
The charge at the ends of the wire alternates between negative (−) and positive (+). If the charge is negative (−), that pushes the negatively charged electrons away from that terminal. If the charge is positive (+), the electrons are attracted in that direction.
Rate of change
AC electricity alternates back-and-forth in direction 50 or 60 times per second, according to the electrical system in the country. This is called the frequency and is designated as either 50 Hertz (50Hz) or 60 Hertz (60Hz).
(See Worldwide AC Voltages and Frequencies for more information.)
Many electrical devices—like light bulbs—only require that the electrons move. They don't care if the electrons flow through the wire or simply move back-and-forth. Thus a light bulb can be used with either AC or DC electricity.
AC is periodic motion
The regular back-and-forth motion of the electrons in a wire when powered by AC electricity is periodic motion, similar to that of a pendulum.
Because of this periodic motion of the electrons, the voltage and current follow a sine waveform, alternating between positive (+) and negative (−), as measured with a voltmeter or multimeter.
Waveform varies between positive and negative as it travels in time
The rate that the voltage or current peaks pass a given point is the frequency of the AC electricity.
Advantages of AC electricity
There are distinct advantages of AC over DC electricity. The ability to readily transform voltages is the main reason we use AC instead of DC in our homes.
The major advantage that AC electricity has over DC electricity is that AC voltages can be readily transformed to higher or lower voltage levels, while it is difficult to do that with DC voltages.
Since high voltages are more efficient for sending electricity great distances, AC electricity has an advantage over DC. This is because the high voltages from the power station can be easily reduced to a safer voltage for use in the house.
Changing voltages is done by the use of a transformer. This device uses properties of AC electromagnets to change the voltages.
(See AC Transformers for more information.)
AC electricity also allows for the use of a capacitor and inductor within an electrical or electronic circuit. These devices can affect the way the alternating current passes through a circuit. They are only effective with AC electricity.
A combination of a capacitor, inductor and resistor is used as a tuner in radios and televisions. Without those devices, tuning to different stations would be very difficult.
We commonly use AC electricity to power our television, lights and computers. In AC electricity, the current alternates in direction. AC electricity was proven to be better for supplying electricity than DC, primarily because the voltages can be transformed. AC also allows for other devices to be used, opening a wide range of applications.
Electrify society by applying your knowledge of science
Resources and references
Elements of AC Electricity - Basic electronics tutorial site
Alternating Current - Overview of AC
This lesson selected by the SciLinks program, a service of National Science Teachers Association.
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