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4.5 Kirchhoff’s Junction Rule and the Conservation of Electric Charge

5 min readmarch 15, 2023

Riya Patel

Riya Patel


AP Physics 2 🧲

61 resources
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Introduction

Kirchhoff's junction rule and the conservation of electric charge are two fundamental principles of electrical circuits that play an essential role in understanding and analyzing complex electrical systems. In this article, we will explore Kirchhoff's junction rule and the conservation of electric charge, their significance in electrical engineering, and how they are applied in real-world circuits.

Key Vocabulary

  • Kirchhoff's junction rule: Also known as Kirchhoff's first law, it states that the total current entering a junction in an electrical circuit must equal the total current leaving the junction.
  • Conservation of electric charge: A fundamental law of physics that states that electric charge cannot be created or destroyed, only transferred from one object to another.
  • Electrical circuit: A system of electrical components (such as resistors, capacitors, and inductors) connected together to form a closed loop.
  • Current: The flow of electric charge through a conductor, measured in amperes (A).
  • Voltage: The potential difference between two points in an electrical circuit, measured in volts (V).
  • Resistance: The measure of how much a material opposes the flow of electric current, measured in ohms (Ω).
  • Ohm's law: A fundamental law of physics that states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them.
  • Capacitance: The ability of a capacitor to store electrical charge, measured in farads (F).
  • Inductance: The property of a conductor by which a change in current induces an electromotive force (EMF) in itself and in nearby conductors, measured in henries (H).
  • Circuit analysis: The process of studying and analyzing the behavior of electrical circuits using mathematical techniques and principles.

Kirchhoff's Junction Rule

Kirchhoff's junction rule, also known as Kirchhoff's first 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 the conservation of electric charge, which states that the total electric charge in a closed system remains constant.
The junction rule can be expressed mathematically as ΣI_in = ΣI_out, where ΣI_in is the sum of all currents entering the junction, and ΣI_out is the sum of all currents leaving the junction. The junction rule is essential for analyzing complex circuits with multiple branches and junctions, as it helps to ensure that the conservation of electric charge is upheld at every point in the circuit.

The Conservation of Electric Charge

The conservation of electric charge is a fundamental law of physics that states that electric charge cannot be created or destroyed, only transferred from one object to another. This law applies to all electrical circuits, and it is the basis for many of the principles used in electrical engineering.
In an electrical circuit, the total electric charge must be conserved at all times. This means that the total charge entering a circuit must equal the total charge leaving the circuit. In addition, the charge on each component of the circuit must be conserved. This principle is essential for ensuring that electrical circuits function correctly and that the components do not become damaged or overloaded.

Application of Kirchhoff's Junction Rule and the Conservation of Electric Charge

Kirchhoff's junction rule and the conservation of electric charge are applied in a wide range of electrical circuits, from simple circuits with just a few components to complex circuits used in industrial applications.
For example, in a simple circuit consisting of a battery, a resistor, and a light bulb, Kirchhoff's junction rule can be used to ensure that the current entering and leaving the junction is conserved. By applying the conservation of electric charge, we can ensure that the total electric charge in the circuit is conserved, and the light bulb will continue to function correctly.
In more complex circuits, Kirchhoff's junction rule and the conservation of electric charge are used to analyze the behavior of the circuit and predict its performance. This allows engineers to design circuits that are efficient, reliable, and safe for use in a range of applications.

Practice Problems

Here are a few practice problems related to Kirchhoff's junction rule and the conservation of electric charge:

  1. In a circuit with three junctions, if the current entering the first junction is 4A, and the current leaving the third junction is 2A, what is the current leaving the second junction?
  2. In a circuit consisting of a battery, a resistor, and a capacitor, the charge on the capacitor is 10mC. If the capacitance of the capacitor is 5μF, what is the voltage across the capacitor?
  3. In a circuit consisting of a battery, two resistors, and a switch, the switch is closed for 5 seconds, during which time a total charge of 200μC flows through the circuit. If the resistance of one of the resistors is 10Ω and the resistance of the other resistor is 5Ω, what is the voltage across each resistor?

Answers:

  1. By applying Kirchhoff's junction rule, we know that the total current entering the three junctions must equal the total current leaving the three junctions. Therefore, the current leaving the second junction is 4A - 2A = 2A.
  2. The formula for the charge on a capacitor is Q = CV, where Q is the charge, C is the capacitance, and V is the voltage. Rearranging this formula gives V = Q/C. Substituting the given values, we get V = 10mC / 5μF = 2V.
  3. The total charge that flows through the circuit is equal to the product of the current and the time the switch is closed, which is 200μC / 5s = 40μA. By applying Ohm's law, we can find the voltage across each resistor. The voltage across the 10Ω resistor is V = IR = 40μA * 10Ω = 0.4V. The voltage across the 5Ω resistor is V = IR = 40μA * 5Ω = 0.2V.

Conclusion

Kirchhoff's junction rule and the conservation of electric charge are two fundamental principles that are essential for understanding and analyzing electrical circuits. These principles ensure that the total electric charge in a circuit is conserved, and that the circuit functions correctly and safely. By applying these principles, engineers can design circuits that are reliable, efficient, and meet the needs of a wide range of applications.
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