Theory - 4 :- Concept of Current, Voltage, and Power

 

Current and Voltage

Voltage (V) is the electrical potential difference between two points in a circuit. It's the "pressure" or electromotive force that pushes charged electrons to flow. Think of it like the pressure in a water hose; the higher the voltage, the greater the pressure. It's measured in volts (V).

Current (I) is the flow of electric charge (electrons) through a conductor. It's the "volume" of electrons moving per unit of time. Using the water hose analogy, current is the amount of water flowing out of the hose. It's measured in amperes (A).

Direct Current (DC)

• Definition: DC is a unidirectional flow of electric charge. The current flows in a single, constant direction.

• Voltage: The voltage is stable over time and does not change polarity.

• Waveform: The graph of voltage vs. time for DC is a straight, horizontal line, indicating a constant value.

• Sources: Batteries, solar cells, and rectified AC power supplies produce DC.

• Applications: Most electronic devices like computers, phones, and LEDs operate on DC power.

Alternating Current (AC)

• Definition: AC is a current that periodically reverses its direction.

• Voltage: The voltage's polarity also reverses, changing from positive to negative at a specific frequency. This creates a sinusoidal (sine wave) pattern.

• Waveform: The graph of voltage vs. time for AC is a sine wave, oscillating above and below zero.

• Sources: Electrical generators at power plants produce AC. It's the standard for power transmission and distribution to homes and businesses.

• Applications: It's used to power large motors and most household appliances.

Why AC is Used for Power Transmission: AC can be easily "stepped up" or "stepped down" to different voltage levels using a transformer. This allows electricity to be transmitted over long distances at very high voltages (which minimizes power loss) and then reduced to a safer, lower voltage for home use. DC transmission is also possible but requires more complex and expensive technology.

---

Power

Power (P) is the rate at which electrical energy is consumed or dissipated in an electric circuit. It's a measure of how much work an electrical circuit can do per unit of time.

• Unit: The unit of power is the watt (W).

Power in DC Circuits

In a simple DC circuit, power is calculated as the product of voltage and current.

$P=V\times I$

This formula is a direct application of Ohm's Law, which states $V=IR$ (where R is resistance). By substituting for V or I, you can derive two other useful power equations:

$P=I^{2}R$ $P=V^2/R$

Power in AC Circuits

Calculating power in AC circuits is more complex due to the constantly changing voltage and current and the potential for a phase shift between them. AC power is divided into three types:

• Real Power (P): This is the power that actually performs work, such as running a motor or lighting a bulb. It's measured in watts (W).

• Reactive Power (Q): This power does no useful work. It's needed to establish and maintain magnetic and electric fields in components like inductors and capacitors. It's measured in volt-amperes reactive (VAR).

• Apparent Power (S): This is the total power supplied by the source. It is the vector sum of real and reactive power. It's measured in volt-amperes (VA).

The relationship between these three is often represented by the power triangle and the following equation:

$S^2=P^2+Q^2$