German physicist Gustav Kirchhoff's laws are fundamental to understanding how electronic circuits function. Kirchhoff's laws, also known as Kirchhoff's Voltage and Current Laws, are a set of rules that deal with the conservation of current and energy within electrical circuits. These laws can be applied to both AC and DC circuits. When applied to AC circuits, Kirchhoff's laws state that the phasor sum of the currents at any point in the circuit and the phasor sum of the voltages around any closed loop are both zero.
Characteristics | Values |
---|---|
Name of the laws | Kirchhoff's Circuit Laws |
Number of laws | 2 |
First law | Kirchhoff's Current Law (KCL) |
First law statement | The total current entering a junction or node is equal to the charge leaving the node as no charge is lost. |
Second law | Kirchhoff's Voltage Law (KVL) |
Second law statement | The sum of the voltages around the closed loop is equal to zero. |
Application | AC and DC circuits |
What You'll Learn
Kirchhoff's First Law or Kirchhoff's Current Law
Kirchhoff's First Law, also known as Kirchhoff's Current Law (KCL), Kirchhoff's Junction Rule, Kirchhoff's nodal rule, or Kirchhoff's point rule, is fundamental to circuit theory. The law, an application of the principle of conservation of electric charge, was first described in 1845 by German physicist Gustav Kirchhoff.
Kirchhoff's First Law states that at any circuit junction, the sum of the currents flowing into and out of that junction is equal. In other words, the algebraic sum of the currents at a junction is zero in an electrical network. This can be understood as the total current entering a node being equal to the total current leaving the node.
Kirchhoff's Current Law is expressed mathematically as:
`sum_("i" = 1)^"n" "I"_"i" = 0`
Where Ii is the current in the ith conductor at a junction having n conductors.
Kirchhoff's laws for AC circuits are as follows:
- The phasor sum of the currents at any point in the circuit is zero.
- The phasor sum of the voltages around any closed loop is zero.
The current and voltage equations for AC circuits are derived in the same way as for DC circuits. Kirchhoff's laws are applied to AC circuits in the same way as DC circuits, with the algebraic manipulation of phasor quantities being identical until numerical quantities are introduced.
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Kirchhoff's Second Law or Kirchhoff's Voltage Law
Kirchhoff's Second Law, also known as Kirchhoff's Voltage Law (KVL), is one of the two fundamental laws formulated by German physicist Gustav Kirchhoff in 1845 (or 1847, according to one source) that deal with resistance, current, and voltage in the lumped element model of electrical circuits.
Kirchhoff's Voltage Law states that the algebraic sum of all voltages in a closed loop of a circuit is equal to zero. This is because a circuit loop is a closed conducting path, so no energy is lost. In other words, the algebraic sum of all the potential differences around the loop must be equal to zero, which can be expressed as ΣV = 0. Here, the term "algebraic sum" means taking into account the polarities and signs of the sources and voltage drops around the loop.
Kirchhoff's Voltage Law is based on the principle of conservation of energy, which states that if a charge moves around a closed loop in a circuit, it must gain as much energy as it loses. This can be summarised as the gain in energy by the charge being equal to the corresponding losses through resistances.
Kirchhoff's Voltage Law is applicable to any circuit configuration, including series and parallel circuits. It is important to note that the "loop" traced for KVL doesn't have to be a real current path. As long as we begin and end at the same point in the circuit, tallying voltage drops and polarities as we move between points, KVL will hold true.
When applying Kirchhoff's Voltage Law to a specific circuit element, it is crucial to pay attention to the algebraic signs (+ and -) of the voltage drops across elements and the electromotive forces (EMFs) of sources, or our calculations may be incorrect. The direction of current flow around a closed circuit can be assumed to be either clockwise or anticlockwise, and even if the chosen direction differs from the actual direction of current flow, the result will still be valid but will have a negative sign.
Kirchhoff's Voltage Law is particularly useful when dealing with series circuits, as they also act as voltage dividers, making the voltage divider circuit an important application of many series circuits.
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Kirchhoff's Law and AC Circuit Analysis
Kirchhoff's Circuit Laws are fundamental to circuit analysis, providing a set of basic rules and theorems for voltages and currents in a circuit. These laws were formulated by German physicist Gustav Kirchhoff in 1845 and are applicable to all electric circuits, including AC circuits.
Kirchhoff's Current Law (KCL)
KCL, also known as Kirchhoff's First Law, states that the total current entering a junction or node is equal to the charge leaving the node, as no charge is lost. Mathematically, this is expressed as:
> I(exit) + I(enter) = 0
In other words, the sum of all currents entering and leaving a node must be zero, a property known as the Conservation of Charge. This law can be applied to analyse parallel circuits.
Kirchhoff's Voltage Law (KVL)
KVL, or Kirchhoff's Second Law, states that in any closed-loop network driven by a voltage source, the total voltage around the loop is equal to the sum of all voltage drops within the same loop, which is equal to zero. This is expressed as:
> Voltage around the loop = Sum of voltage drops in the loop
This property is known as the Conservation of Energy, and it is important to maintain the same direction (clockwise or anticlockwise) when considering voltage drops to ensure the final voltage sum is zero. KVL is used when analysing series circuits.
Application to AC Circuits
Kirchhoff's laws can be applied to AC circuits in a similar manner as DC circuits. The current and voltage equations are derived in the same way as for DC circuits, and the algebraic manipulation of phasor quantities is identical until numerical quantities are introduced. When there are multiple AC voltage sources in the circuit, their relative polarities must be specified, denoted by "+" and "-" signs or an arrow indicating direction.
Kirchhoff's laws provide a powerful tool for analysing complex AC circuits, allowing for the calculation of currents and voltages within the circuit.
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Kirchhoff's Law and Junction Rule
Kirchhoff's laws, developed by German physicist Gustav Kirchhoff in 1845, are a set of rules that deal with the conservation of current and energy within electrical circuits. These laws are fundamental to understanding how electronic circuits function and can be applied to both AC and DC circuits.
Kirchhoff's Current Law (KCL)
Kirchhoff's First Law, also known as Kirchhoff's Current Law (KCL) or the Junction Rule, states that the total current entering a junction or node is equal to the charge leaving the node. In other words, the algebraic sum of all the currents entering and leaving a node must be equal to zero, as no charge is lost within the node. This is commonly known as the Conservation of Charge and can be expressed as:
I(exiting) + I(entering) = 0
For example, if three currents I1, I2, and I3 are entering the node and two currents I4 and I5 are exiting, the equation becomes:
I1 + I2 + I3 – I4 – I5 = 0
A node refers to a junction connecting two or more current-carrying routes, such as cables and other components. Kirchhoff's Current Law can be applied to analyse parallel circuits.
Kirchhoff's Voltage Law (KVL)
Kirchhoff's Second Law, also known as Kirchhoff's Voltage Law (KVL) or the Loop Rule, states that the sum of the voltages around a closed loop is equal to the sum of all the voltage drops within the same loop, which is equal to zero. This is known as the Conservation of Energy and can be expressed as:
V(total) = V(drops) = 0
When calculating the voltage drops, it is important to maintain the same direction, either clockwise or anticlockwise, to ensure the final voltage sum is zero. Kirchhoff's Voltage Law can be applied to analyse series circuits.
Application to AC Circuits
When applying Kirchhoff's laws to AC circuits, the relative polarities of the sources must be considered. The relative polarity, or sense, of a source is given with respect to its phasor values, denoted by "+" and "-" signs or an arrow pointing from "+" to "-".
The current and voltage equations for AC circuits derived using Kirchhoff's laws are similar to those for DC circuits, with the algebraic manipulation of phasor quantities being the same until numerical quantities are introduced.
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Kirchhoff's Law and Closed Loop Rule
Kirchhoff's laws, developed by German physicist Gustav Kirchhoff in 1845, are a set of rules that deal with the conservation of current and energy within electrical circuits. These laws are applied to all electric circuits, including AC circuits, and are fundamental to understanding how electronic circuits function.
Kirchhoff's laws consist of two rules: the first is Kirchhoff's Current Law (KCL), and the second is Kirchhoff's Voltage Law (KVL). These laws allow us to calculate and find the currents and voltages around any closed circuit, also known as the closed-loop rule, as long as we know the values of the electrical components within it.
Kirchhoff's Current Law (KCL)
KCL, also known as the Junction Rule, states that the total current or charge entering a junction or node is equal to the charge leaving the node. In other words, the algebraic sum of all the currents entering and leaving a node must be zero, as no charge is lost within the node. This is expressed as:
> I(exiting) + I(entering) = 0
Kirchhoff's Voltage Law (KVL)
KVL, also known as the Loop Rule, states that in any closed-loop network driven by a voltage source, the total voltage around the loop is equal to the sum of all the voltage drops within the same loop, which is equal to zero. In other words, the algebraic sum of all voltage sources and voltage drops within a closed loop must be zero, as this represents the conservation of energy.
When applying KVL, it is important to maintain the same direction, either clockwise or anticlockwise, when noting the direction of all the voltage drops and returning to the starting point. Otherwise, the final voltage sum will not be zero.
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