Difference between AC and DC Current

Electricity arises due to the motion of electric charges, in general the motion of electrons. The direction of current in a circuit is the opposite to the direction of flow of electrons and since the electrons travel from the negative terminal to the positive terminal of a circuit, the direction of current is taken from the positive terminal to the negative terminal of the circuit. The difference between AC and DC arises with the direction of flow of current. In direct current, the electrons flow steadily in a single direction, whereas in alternating current, the direction of flow of the electrons is switched in regular time intervals. The type of current released from cells and batteries are always direct current, whereas, the current going through the electricity cables outside homes is alternating current.


Alternating current produces a varying or as the name suggests, an `Alternating voltage pattern as time progresses. The general variation of the voltage with respect to time represents a sinusoidal wave (whose graphical plot resembles a sine wave). The number of times the AC voltage pulse oscillates (i.e. the direction changes from forward to backward and backward to forward) in each second is called its frequency and its units is `Hertz’ (Hz). The Indian standard AC voltage and frequency readings that come through the cables is 240 volts at 50Hz.

 AC Signal

Origin of DC and AC:

In presence of a magnetic field the electrons are attracted towards the North Pole and repelled by the South Pole, therefore when a conductor was placed in presence of a magnetic field there was a steady flow of electrons in a single direction, giving rise to direct current, which was used to power DC batteries. This marvel was an outcome of the experiments carried out by Thomas Edison in the early 19th century.


However, there were downsides to direct current like loss of energy as the current travelled through longer distances and lack of variation in the voltage. So, in the later part of the 19th century a Serbian-American physicist named Nikola Tesla placed the conductor in the presence of a rotating magnet, which constantly induced a change in the direction of flow of current, producing alternating current.


Gradually Edison’s battery system was replaced by AC generators, invented by George Westinghouse in the early 20th century. As a result of this, electricity was popularized as it had attained a wider reach from the power plants. People had just realized the greatest invention of the 19th century.


Advantages of AC and DC:

Although many of our household appliances like the television sets, computers, laptops, refrigerators use direct current, the main electricity received from the electric cables is alternating current. The main reason for this is the long transmission range of AC. It can cover a large radius from the main power plants without loss of energy (ideally speaking). To transmit to long distances the current carried must be as less as possible since transmission line power loss is proportional to the square of current; therefore to keep the current at minimal level while energy transmission is at a high enough level, transformers are required. Transformers can either raise or decrease the voltages which are accordingly called step-up or step-down transformers. As the voltage of DC for a steady current would have a constant voltage, it would not be possible to transform direct current. But in the case of alternating currents, the voltages can be stepped up and stepped down accordingly while operating on low current. At the power plants the current produced is stepped up by transformers at the power plant which are stepped down at local areas by step down transformers to a lower voltage which is safer for customer usage. In view of this we can comprehend that current in the AC form is relatively safer as it can operate on lower current.


So why use DC at all? The answer is that, most micro-electronic components of our appliances consist of ‘transistors’. Transistors are semiconductor devices which are small switches which use voltage signals to change between the switches. So if an alternating current is used, the voltage would constantly change its orientation because of which control over the switch would be lost. So although it is not impossible, it would be rather hard to make a transistor work with an alternating current.


AC-DC Inter Conversions:

The AC that comes to our homes is converted to DC by adaptors that come with the electronics to its specified voltage. On the other hand a DC is converted to AC by motor generators.

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