Electric current can be directly measured with a galvanometer, but this method involves breaking the electrical circuit, which is sometimes inconvenient. Current can also be measured without breaking the circuit by detecting the magnetic field associated with the current. What is the symbol for direct current? There actually are simple D. C is the unidirectional flow of electric charge. Why is AC preferred over DC?
Answer: Ac is more preferred than dc because it is easy to maintain and change the voltage of ac for transmission and distribution purpose. Plant cost of ac transmission is much lower compared to dc transimission. When fault occurs it is easy to interrupt ac supply. What do you mean by resistance? Resistance is the opposition that a substance offers to the flow of electric current. When an electric current of one ampere passes through a component across which a potential difference voltage of one volt exists, then the resistance of that component is one ohm.
How do you do alternating current? Alternating current describes the flow of charge that changes direction periodically. An alternating emf can be developed in a coil of wire in one of the three ways: By changing the flux through a coil.
By moving the coil through a magnetic field. In the late s, a variety of inventions across the United States and Europe led to a full-scale battle between alternating current and direct current distribution.
Thomas Edison, on the other hand, had constructed DC power stations in the United States by A turning point in the battle came when George Westinghouse, a famous industrialist from Pittsburgh, purchased Nikola Tesla's patents for AC motors and transmission the next year.
Thomas Edison Image courtesy of biography. In the late s, DC could not be easily converted to high voltages. As a result, Edison proposed a system of small, local power plants that would power individual neighborhoods or city sections.
Even though the voltage drop across the power lines was accounted for, power plants needed to be located within 1 mile of the end user. This limitation made power distribution in rural areas extremely difficult, if not impossible. With Tesla's patents, Westinghouse worked to perfect the AC distribution system. Transformers provided an inexpensive method to step up the voltage of AC to several thousand volts and back down to usable levels. At higher voltages, the same power could be transmitted at much lower current, which meant less power lost due to resistance in the wires.
As a result, large power plants could be located many miles away and service a greater number of people and buildings. Over the next few years, Edison ran a campaign to highly discourage the use of AC in the United States, which included lobbying state legislatures and spreading disinformation about AC. Edison also directed several technicians to publicly electrocute animals with AC in an attempt to show that AC was more dangerous than DC. In attempt to display these dangers, Harold P. In , the International Electro-Technical Exhibition was held in Frankfurt, Germany and displayed the first long distance transmission of three-phase AC, which powered lights and motors at the exhibition.
Several representatives from what would become General Electric were present and were subsequently impressed by the display. The following year, General Electric formed and began to invest in AC technology. Westinghouse won a contract in to build a hydroelectric dam to harness the power of Niagara falls and transmit AC to Buffalo, NY. The project was completed on November 16, and AC power began to power industries in Buffalo.
This milestone marked the decline of DC in the United States. However, due to the high cost and maintenance of the Thury systems, HVDC was never adopted for almost a century. With the invention of semiconductor electronics in the s, economically transforming between AC and DC became possible. Specialized equipment could be used to generate high voltage DC power some reaching kV. In the end, Edison, Tesla, and Westinghouse may have their wishes come true. AC and DC can coexist and each serve a purpose.
You should now have a good understanding of the differences between AC and DC. AC is easier to transform between voltage levels, which makes high-voltage transmission more feasible. DC, on the other hand, is found in almost all electronics. You should know that the two do not mix very well, and you will need to transform AC to DC if you wish to plug in most electronics into a wall outlet.
With this understanding, you should be ready to tackle some more complex circuitry and concepts, even if they contain AC. Take a look at the following tutorials when you are ready to dive deeper into the world of electronics:. See our Engineering Essentials page for a full list of cornerstone topics surrounding electrical engineering. Effective resistance convert the electrical energy completely into heat energy; They are independent of frequency and do not cause phase shifts between current and voltage.
Storage elements such as coils and capacitors have a reactance. There are inductive and capacitive reactances. In inductive reactances, the current lags behind the voltage, and in capacitive reactances, the current is in advance of the voltage. Impedances are interconnections of effective resistances and reactances.
They are dependent on frequency because of the reactance included in the system. The magnitude of the impedance can be found by diagrams or by calculation by geometric addition.
Depending on the preponderance of the inductive component or the capacitive component, either the voltage is ahead of the current or vice versa. If, in one circuit, inductive and capacitive components are present, they neutralise each other partly or completely. The special case where the inductive reactance is equal to the capacitive reactance is called resonance. The frequency in the presence of which resonance occurs is called resonant frequency or frequency of resonance.
When resonance is present, a circuit has the behaviour of an effective resistance. What is the essence of effective or acktive resistances, reactances and impedances? A coil has a reactance of W at a frequency of 50 Hz. What is the size of the inductance? Represent graphically the curve of the reactance in dependence of the frequency from 0 to 10 kHz for a coil having an inductance of 5 H.
At a frequency of 50 Hz, a capacitor has an impedance of about 65 W. What is the size of its capacity? Determine the magnitude of the inductive reactance and the inductance by graph and by calculation! What is the frequency t which resonance occurs in a circuit? Power of Alternating Current When loads carry current, a voltage drop is caused.
The product of the instantaneous values of current and voltage is called instantaneous power. Normally, the instantaneous power changing from time to time is of less interest than its mean value.
In an effective resistance, current and voltage are in phase. The electrical power becomes completely, i. It is called effective power P e active power. It can be determined on the basis of the effective values according to the relations derived in Section 4. The electrical power is required for the duration of a quarter of a cycle for the building up of the magnetic field in a coil or of the electrical field in a capacitor and delivered in the subsequent quarter of a cycle.
There is no power conversion in a temporal mean. This power is called reactive power P r. The electrical power is converted partly as effective power and partly as reactive power. This power is called apparent power P a.
Consequently, effective and reactive powers are the legs and the apparent power is the hypotenuse of a right-angled triangle having the phase angle Fig.
From equation 7. Since reactive power unnecessarily loads the generators of the power stations and the distribution network, the reactive power must be kept as small as possible for economical reasons; in other words, cos j must be as high as possible. In networks of power electrical engineering, an inductive phase shift occurs always because of the necessary transformers and connected motors; this phase shift always worsens the power factor. In accordance with the various types of power, there are effective work, reactive work and apparent work.
An enterprise is connected to a V network 50 Hz. With alternating current, there are three types of powers, namely, apparent power, effective power and reactive power. Effective power and reactive power are made up at right angles. The apparent power is always greater than effective power or reactive power but it is always smaller than the algebraic sum of the latter two. Efforts are always made to achieve a high effective power and a reactive power which is as small as possible.
The ratio of effective power to apparent power is called power factor. An alternating current will flip the direction of charge flow 60 times a second in North America 60 Hz and 50 times a second in Europe 50 Hz.
This is usually caused by a sinusoidally varying current and voltage that reverses directions, creating a periodic back and forth motion for the current see Figure 1. Despite this current flowing back and forth many times a second, the energy still essentially flows continuously from the power plant to the electronic devices.
A major advantage of alternating current is that its voltage can be modified relatively easily using a transformer , which allows power to be transmitted at very high voltages before being taken down to safer voltages for commercial and residential use.
As seen in the first equation, the power lost through transmission is proportional to the square of the current through the wire. Therefore, it is preferable to minimize the current through the wire so that the energy loss is reduced.
Of course, minimizing the resistance would reduce the energy lost as well, but the current has a much larger impact on the amount of energy lost due to its value being squared. The second equation shows that if voltage is increased, the current is decreased equivalently to transmit the same power.
Hence, the voltage through transmission lines is very high, which reduces the current, which in turn minimizes the energy lost through transmission.
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