Current electricity is a branch of physics that deals with the flow of electric charge through a conductor. Understanding current electricity is crucial for comprehending how electrical circuits work, which is fundamental in both physics and engineering domains.
Electric current is the rate at which electric charge flows through a conductor. It is denoted by the symbol $I$.
The electric current $I$ is given by the formula: $$I = \frac{Q}{t}$$ where:
The SI unit of electric current is the ampere (A).
Example
If a charge of 5 coulombs passes through a conductor in 2 seconds, the current is: $$I = \frac{5 , \text{C}}{2 , \text{s}} = 2.5 , \text{A}$$
The potential difference (or voltage) between two points in an electric field is the work done to move a unit charge from one point to another. It is denoted by $V$.
The potential difference $V$ is given by: $$V = \frac{W}{Q}$$ where:
The SI unit of potential difference is the volt (V).
Note
1 volt is equivalent to 1 joule per coulomb.
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature remains constant.
$$V = IR$$ where:
Example
If a resistor has a resistance of $10 , \Omega$ and the voltage across it is $5 , V$, the current is: $$I = \frac{V}{R} = \frac{5 , V}{10 , \Omega} = 0.5 , A$$
Common Mistake
A common mistake is to assume that Ohm's Law applies to all materials. It only applies to ohmic materials, which have a constant resistance.
Resistance is a measure of the opposition to the flow of electric current through a conductor. It is denoted by $R$.
The resistance $R$ is given by: $$R = \rho \frac{L}{A}$$ where:
Tip
To reduce resistance in electrical wiring, use materials with low resistivity and larger cross-sectional areas.
Resistivity is a material-specific property that quantifies how strongly a material opposes the flow of electric current.
The SI unit of resistivity is ohm-meter (Ω·m).
Note
Copper and aluminum are commonly used in electrical wiring due to their low resistivity.
Electrical power is the rate at which electrical energy is consumed or produced by a device.
The electrical power $P$ is given by: $$P = VI$$ where:
Using Ohm's Law, power can also be expressed as: $$P = I^2R$$ $$P = \frac{V^2}{R}$$
Example
If a device operates at $10 , V$ and draws a current of $2 , A$, the power consumed is: $$P = VI = 10 , V \times 2 , A = 20 , W$$
Example
For two resistors $R_1 = 6 , \Omega$ and $R_2 = 3 , \Omega$ in parallel: $$\frac{1}{R_{\text{total}}} = \frac{1}{6} + \frac{1}{3} = \frac{1}{6} + \frac{2}{6} = \frac{3}{6} = \frac{1}{2}$$ Thus, $R_{\text{total}} = 2 , \Omega$.
Understanding the principles of current electricity is fundamental for analyzing and designing electrical circuits. Mastery of concepts like electric current, potential difference, Ohm's Law, resistance, and power is essential for both academic and practical applications.
This concludes our study note on current electricity. Make sure to practice solving problems and conducting experiments to reinforce these concepts.