Current is the flow of electric charge in a conductor. It is measured in amperes (A) and can be classified into direct current (DC) and alternating current (AC). The behavior of current in three-phase systems is also crucial for industrial applications.

In DC circuits, current flows in one direction. The relationship between voltage (V), current (I), and resistance (R) is given by Ohm's Law:

**Formula:** *I = V / R*

**I:**Current (amperes, A)**V:**Voltage (volts, V)**R:**Resistance (ohms, Ω)

**Example Calculation:**

**Task:**Calculate the current in a DC circuit with a voltage of 12 V and a resistance of 6 Ω.**Solution:**Use the formula*I = V / R*.-
*I = 12 V / 6 Ω = 2 A*

In AC circuits, current alternates in direction and varies in magnitude with time. The relationship between voltage (V), current (I), and impedance (Z) is given by Ohm's Law for AC circuits:

**Formula:** *I = V / Z*

**I:**Current (amperes, A)**V:**Voltage (volts, V)**Z:**Impedance (ohms, Ω)

Impedance (Z) combines resistance (R) and reactance (X) and is given by:

**Formula:** *Z = √(R² + X²)*

**Z:**Impedance (ohms, Ω)**R:**Resistance (ohms, Ω)**X:**Reactance (ohms, Ω)

**Example Calculation:**

**Task:**Calculate the current in an AC circuit with a voltage of 120 V and an impedance of 10 Ω.**Solution:**Use the formula*I = V / Z*.-
*I = 120 V / 10 Ω = 12 A*

In three-phase systems, current can be calculated for both balanced and unbalanced loads. For balanced loads, the line current (I_{L}) and phase current (I_{ph}) are related by:

**Formula for Star (Wye) Connection:** *I _{L} = I_{ph}*

**Formula for Delta Connection:** *I _{L} = √3 × I_{ph}*

**I**Line current (amperes, A)_{L}:**I**Phase current (amperes, A)_{ph}:

**Example Calculation for Star Connection:**

**Task:**Calculate the line current for a star-connected system with a phase current of 10 A.**Solution:**Use the formula*I*._{L}= I_{ph}*I*_{L}= 10 A

**Example Calculation for Delta Connection:**

**Task:**Calculate the line current for a delta-connected system with a phase current of 10 A.**Solution:**Use the formula*I*._{L}= √3 × I_{ph}*I*_{L}= √3 × 10 A ≈ 17.32 A

Kirchhoff's Current Law states that the total current entering a junction in an electrical circuit must equal the total current leaving the junction. This law is based on the principle of conservation of charge.

**Formula:** *ΣI _{in} = ΣI_{out}*

**ΣI**Sum of currents entering the junction_{in}:**ΣI**Sum of currents leaving the junction_{out}:

**Example Calculation:**

**Task:**Determine the unknown current I_{x}at a junction where I_{1}= 3 A, I_{2}= 2 A, and I_{3}= 4 A are entering, and I_{4}= 7 A is leaving.**Solution:**Apply KCL:*ΣI*_{in}= ΣI_{out}-
*3 A + 2 A + 4 A = I*_{4}+ I_{x}*9 A = 7 A + I*_{x}*I*_{x}= 2 A