ELECTRIC CURRENTS IN CONDUCTORS: A Deep Dive into Class 12 Physics
If you're a Class 12 student gearing up for your board exams or preparing for competitive tests like NEET, mastering the basics of electric currents in conductors is crucial. In this blog post, we'll explore the fundamental concepts outlined in Chapter 2 of the NCERT Physics textbook, specifically tailored for students from the Rajasthan Board and those aiming to excel in national-level competitions.
We'll define electric current, explore its behavior in conductors, discuss key concepts, units, and dimensions, and offer MCQs and short-answer questions to enhance your grasp of the fundamental physics topic.
ELECTRIC CURRENT
Definition of Electric Current
“Electric current is defined as the rate of flow of electric electric charge (electrons or ions) through a conductor or medium, driven by a potential difference or an electric field.”
(or)
“ The time rate of flow of electric charge through any cross-section is called electric current.”
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| ELECTRIC CURRENTS IN CONDUCTORS |
If `\delta Q` electric charge flows from the cross-sectional area in `\delta t` time interval then the average current is
`I_{av} = \frac{\Delta Q}{\Delta t}`
For very small time intervals we consider the instantaneous current
`I = \text{limit}_{t \to \infty}\frac{Delta Q}{\Delta t}`
`I = \frac {dQ}{dt}`
Where I is instantaneous current which is measured when electric current is not steady and at a particular time point like t = 2 second etc.
* Electric current is a scalar quantity.
* It's S.I. unit is A (Ampere) and 1 A = 1 C/s = 1 Coulomb/second.
* Ampere is the fundamental unit of electric current.
* It is measured by Ammeter.
Important Points
* Electric charge produces only electric field when stationary )in rest position).
* If electric charge flows with constant speed it produces constant current and constant magnetic field.
* If electric charge flows with variable speed, it produces variable current and variable magnetic field.
* Every conductor contains a large amount of free electrons as current carriers.
* In electrolyte positive positive and negative ions are current carriers.
* In semiconductors electrons and holes are current carriers.
* Electric current is zero in any conductor without a potential difference or electric field.
* When an electric potential is applied to a conductor, an electric current is produced due to the flow of free electrons.
* The direction of flow of electric current is in the direction of high potential to low potential, in the positive terminal to the negative terminal in an electric circuit, and in the direction of the electric field.
Types of Electric Current
Alternating Current (AC)
Alternating current (AC) is an electric current in which the flow of electric charge periodically reverses direction.
Direct Current (DC)
Direct current (DC) is an electric current that flows in a single, constant direction, typically from the positive terminal to the negative terminal of a power source.
ELECTRIC CURRENTS IN CONDUCTORS
Free Electrons in Conductors
Electric Field and Force
Free Charged Particles in the Ionosphere
Bound Electrons in Atoms
In every atom, negatively charged electrons are bound (attached) to positively charged nuclei which do not contribute to electric current. These electrons are known as bound electrons.
Electrons in Bulk Matter
Bulk Matter is made up of many molecules, a gram of water contains approximately `10^{22}` molecules. These molecules are so closely packed that the electrons are no longer attached to individual nuclei.
In some materials, electrons remain bound, meaning they won't accelerate even when an electric field is applied.
Current in Solid Conductors
In solid conductors, atoms are tightly bound together, so that the current is carried by the negatively charged free electrons in the background of fixed positive ions in solid conductors.
Electrolytic Solutions as a Conductor
Electrolytic solutions are conductors in which both positive and negative electric charges can move and both can flow electric current.
Thermal Motion of Electrons in Conductors
Without an electric field, electrons in a solid conductor move randomly (in no fixed direction) due to thermal motion, frequently colliding with fixed ions. After a collision, an electron retains its speed but changes direction randomly. This randomness leads to an equal distribution of electron velocities in all directions, resulting in no net electric current.
Therefore, we can conclude an electric current is zero in solid conductors due to the thermal motion of electrons.
Effect of Electric Current in Conductors
Consider a cylindrical conductor with two thin dielectric discs attached to its ends. One disc carries a positive charge (+ Q), and the other a negative charge (- Q).
This arrangement creates an electric field and it is directed from the positive towards the negative charge which accelerates electrons from negative to positive disc which means - Q to + Q. This flow of electrons creates electric current for a very short time and after that, there is no current.
To maintain a continuous current, the cylinder's ends must be continuously supplied with fresh electric charges to replace those neutralized by moving electrons. This creates a steady electric field within the conductor, allowing a constant flow of electric current. Mechanisms like batteries or cells can maintain this steady state by providing a consistent source of electric charge.
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