# Electric Circuits – Electric Current, OHM’S Law

## INTRODUCTION:

What are the best Electric Circuits in the world? Basically, we considered all the charges whether free or bound to be at rest in our last topic. Charges in motion constitute an electric current. Such currents occur naturally as well as artificially. Lightning is one of the natural phenomena in which the charges flow from clouds to earth through the atmosphere. The current produced during the lightening is not steady. Artificially the current is generated by dynamo, inverters, cells, etc. the currents through the devices like cell-driven clocks, torches, etc are steady.

## ELECTRIC CURRENT:

The quotient of q/t=l is defined as the current across the area in the direction left to right. If the quotient is negative then the current is in the direction of right to left. But the current is not always steady. In that case, we define the current into 2 ways:

AVERAGE CURRENT

INSTANTANEOUS CURRENT

Blood flow in a human body can be simulated by current flow in electrical circuits, and hence this is the way in which we can find out the direct and indirect applications like:

In the technique of impedance plethysmography, the electrical resistance of a part of the body is measured to diagnose deep venous thrombosis.

Heart pacemakers incorporate RC circuits to control the timing of the voltage pulses that are delivered to a manufacturing heart to regulate its beating cycle.

## ELECTRIC CURRENT IN CONDUCTORS:

If an electric field is applied to an electric charge, it will experience a force. If it is free to move, then it will contribute to the current. In atoms and molecules, negatively charged electrons and positively charged nuclei are bound to each other and are thus not free to move in the electric field. In some materials, the electrons will still be bound so when an electric field is applied, they will not accelerate to develop current. These materials are generally called insulators. In electric solutions, both positive and negative ions move to develop current.

In bulk matters, these molecules are so closely packed that the electrons no longer are attached to individual nuclei. If an electric field is applied some of the electrons are practically free to move within the bulk material to develop currents in them. These materials are generally called conductors and these electrons are known as the free electrons.

In absence of the electric field, the electrons move with their thermal motion. During their motion, they collide with the fixed ions such that their speed before the collision is equal to the speed after the collision but the direction of the velocity after the collision is completely random. Therefore, the number of electrons in any direction will be equal to the number of electrons traveling in the opposite direction, so there is no net electric current.

Now the electric field e is produced along the conductor ab by using 2 charged insulating discs carrying _ and _ q charges. The electrons will be accelerated due to this field e from end b a to neutralize the charges on the discs gradually. This motion constitutes an electric current. The current will stop after a short time. If the charges in the disc are maintained to keep e constant by some mechanism then a steady continuous current can be established in the conductor. To maintain the steady electric field, cells, batteries are used across the conductor.

## OHM’S LAW:

It states that the potential difference between the ends of the conductor through which a current I then v=ri, where r is the constant known as the resistance of the conductor. The resistance of the conductor depends on :

The dimension of the conductor

The material of the conductor

## CURRENT DENSITY:

It is defined as the current per unit area.

## DRIFT OF ELECTRONS AND THE ORIGIN OF RESISTIVITY:

In the absence of an electric field, a free collision with heavy fixed ions and after the collision it emerges with the same speed but in a random direction. If we consider n free electrons with the velocity of the ith electron at the given cross-section, then the average velocity can easily be calculated.

## LIMITATIONS OF OHM’S LAW:

The proportionality of v and I does not hold for certain materials and devices used in electric circuits.

V ceases to be pr4oportional to me for a good conductor

Value of current is different for the same potential difference on reversing the direction of v

The value of potential is different for the same current.

## CONCLUSION:

Metals have a resistivity in the range of 10 to the power of -8 to 10 to the power of -6-ohm m. insulators have resistivities 10 to the power of 18 times greater than normal metals or more. Semiconductors have resistivities in between conductors and insulators. Resistivities of semiconductors decrease with the rise in temperature. Their resistivity is affected by the presence of a small number of impurities.

### Dewakar Pandey

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