Current Electricity - Topics, Formulas, Notes, Books, FAQs

Current Electricity is a significant chapter of the Class 12 Physics syllabus that introduces the next phase of the course, in which the study of electrostatics is replaced by the study of moving charges. Students are taught in the present chapter about the flow of electric charges through conductors and how this flow generates electric current, resistance, and transfer of electrical energy. Physics attempts to explain natural and electrical phenomena in terms of measurement, mathematical relationships and comparison, and this method is fundamental in the intuition of current flow in circuits.

It is necessary to clearly define the electric current as a physical quantity and the measurement of current before analysing electrical circuits. Simply stating that more current flows in one wire than another is incomplete unless the exact magnitude of current is known. In the same manner, the resistance that a conductor presents to current flow cannot be described without the addition of the concept of resistance, which measures the influence of materials on current. The electrical behaviour can be accurately calculated and compared using standard units like the ampere of current and the ohm of resistance, and using basic laws like the Ohm law. Physics is governed by universally accepted units, laws and conventions in order to achieve a consistent understanding worldwide. All electrical quantities are represented by a numerical value with a standard unit so that they can be accurately calculated and the way they are expressed can be easily understood. Therefore, the chapter Current Electricity provides a solid ground in the knowledge of electric circuiting, electrical power, and further chapters on electronics.

Important Topics of Current Electricity

The behaviour of moving electric charges and the laws of electric circuits are described in the important topic of Current Electricity. These topics are concerned with knowledge of electric current, resistance, and energy transfer among conductors. An excellent understanding of these concepts is necessary to solve the numerical and circuit-based problems. They are the basis of higher lessons in electricity and magnetism. These are some of the highly weighted topics in Class 12 boards, JEE, and NEET. Mastery of these areas improves conceptual clarity and accuracy.

1. Electric Current

Electric current is described as the flow rate of electric charge in a conductor. It is a scalar quantity, and the SI unit is ampere (A). Conventional current flows from the positive to the negative terminal. This topic presents the fundamental concept of the movement of charges. It is the foundation of all concepts of circuits. The questions are usually based on a direct definition.

2. Electric Current as a Flow of Charges (Drift Velocity)

This topic explains the microscopic origin of electric current. This subject describes the microscopic genesis of the electric current. It presents the drift velocity, relaxation time and mobility of electrons. The relation between the current and drift velocity is obtained. This assists in the determination of the flow of current within a conductor. The theory relates microscopic movement to macroscopic current. Lots of numerical questions are commonly asked.

3. Electric Potential and Potential Difference

Electric potential difference is the work done per unit charge in moving a charge between two points. It causes a flow of current in an electronic circuit. The volt (V) is the SI unit of the potential difference. This topic links electrostatics with current electricity. It states the fact that current only flows when there is a potential difference.

4. Ohm's Law

Ohm's law states that the current flowing through a conductor is directly proportional to the potential difference applied across its ends, provided temperature and other physical conditions remain constant.

$
V \propto I \quad \text { or } \quad V=I R
$

5. Resistance

Resistance is the opposition offered by a conductor to the flow of electric current. Its SI unit is the ohm (O). This topic also elaborates on factors that influence resistance, which include length, area, and material. The questions on resistance are important to circuit problems. Both theory and numericals are important.

6. Resistivity and Conductivity

Resistivity is a material property that determines the ability of a material to resist the flow of current. It does not depend on the shape and size of the conductor. The opposite of resistivity is conductivity. This topic explains the temperature dependence of resistance. It is used to differentiate conductors, semiconductors and insulators.

7. Combination of Resistors

Resistors can be connected in series or parallel to obtain the desired resistance. This topic describes the equivalent resistance of either combination. It finds extensive use in electric circuits. This is a key concept to the solution of network problems. It is very important to have numerical practice.

8. Electrical Energy and Power

The electrical energy can be defined as the energy used or supplied within an electric circuit. The rate of energy consumption is electric power. This topic introduces Joule's law of heating. It is used in the real-life in electrical appliances. Lots of numerical questions are commonly asked

10. Cells and EMF

A cell is a device that converts chemical energy into electrical energy.

• EMF (Electromotive Force): The total potential difference between the terminals of a cell when no current is drawn. It represents the maximum energy supplied per unit charge. Its SI unit is volt (V).
• Internal Resistance (r): The resistance offered by the electrolyte and electrodes inside the cell.

When a cell of EMF $E$ and internal resistance $r$ supplies current $I$ through an external resistance $R$ :

$
\begin{gathered}
E=I(R+r) \\
V=I R=E-I r
\end{gathered}
$

11. Combination of Cells

• Cells in Series:
  When cells are connected end-to-end (positive to negative), the effective EMF is the sum of individual EMFs, while internal resistances add up.
  $E_{\mathrm{eq}}=E_1+E_2+\ldots, \quad r_{\mathrm{eq}}=r_1+r_2+\ldots$
• Cells in Parallel:
  When cells are connected positive to positive and negative to negative, the effective EMF remains the same as that of a single cell, but internal resistance decreases.
  $E_{\mathrm{eq}}=E, \quad \frac{1}{r_{\mathrm{eq}}}=\frac{1}{r_1}+\frac{1}{r_2}+\ldots$

12. Kirchhoff's Laws

Kirchhoff's rules are used to solve complex electrical circuits:
1. Kirchhoff's Current Rule (KCL):

The total current entering a junction is equal to the total current leaving the junction.

$
\sum I_{\mathrm{in}}=\sum I_{\mathrm{out}}
$

This is based on conservation of charge.
2. Kirchhoff's Voltage Rule (KVL):

The algebraic sum of potential differences around any closed loop of a circuit is zero.

$
\sum V=0
$

This is based on conservation of energy.

13. Wheatstone Bridge

The Wheatstone bridge is an arrangement of four resistors in the form of a quadrilateral, used to measure an unknown resistance accurately. A galvanometer is connected between the two opposite corners.
If resistors are $P, Q, R, S$, then the bridge is balanced (no current through the galvanometer) when:

$
\frac{P}{Q}=\frac{R}{S}
$