Basic Laws of Electric Circuits Nodes Branches Loops

Basic Laws of Electric Circuits Nodes, Branches, Loops and Current Division Lesson 4

Basic Laws of Electric Circuits Nodes, Branches, and Loops: Before going further in circuit theory, we consider the structure of electric circuits and the names given to various member that make up the structure. We define an electric circuit as a connection of electrical devices that form one or more closed paths. Electrical devices can include, but are not limited to, resistors capacitors inductors 1 transistors logic devices switches transformers light bulbs batteries

Basic Laws of Electric Circuits Nodes, Branches, and Loops: A branch is a single electrical element or device. Figure 4. 1: A circuit with 5 branches. A node: A node can be defined as a connection point between two or more branches. 2 Figure 4. 2: A circuit with 3 nodes.

Basic Laws of Electric Circuits Nodes, Branches, and Loops: 3 If we start at any point in a circuit (node), proceed through connected electric devices back to the point (node) from which we started, without crossing a node more than one time, we form a closed-path. A loop is a closed-path. An independent loop is one that contains at least one element not contained in another loop.

Basic Laws of Electric Circuits Nodes, Branches, and Loops: The relationship between nodes, branches and loops can be expressed as follows: # branches = # loops + # nodes - 1 or B=L + N - 1 4 Eq. 4. 1 In using the above equation, the number of loops are restricted to be those that are independent. In solving most of the circuits in this course, we will not need to resort to Eq. 4. 1. However, there are times when it is helpful to use this equation to check our analysis.

Basic Laws of Electric Circuits Nodes, Branches, and Loops: Consider the circuit shown in Figure 4. 3: A multi-loop circuit 5 give the number of nodes give the number of independent loops give the number of branches verify Eq. 4. 1

Basic Laws of Electric Circuits Single Node Pair Circuits: Current division. A single node pair circuit is shown in Figure 4. 4: A circuit with a single node pair. We would like to determine how the current divides (splits) in the circuit. 6

Basic Laws of Electric Circuits Single Node Pair Circuits: Current division. Eq. 4. 2 Eq. 4. 3 Therefore; Eq. 4. 4 7

Basic Laws of Electric Circuits Single Node Pair Circuits: Current division. From Eq. 4. 4 we can write, Eq. 4. 5 Equation 4. 5 is a very important expression. In words it says that the equivalent of two resistors in parallel equals to the product of the two resistors divided by the sum. The equivalent resistance of two resistors in parallel is always less than the smallest resistor. 8

Basic Laws of Electric Circuits Single Node Pair Circuits: Current division. In general, if we have N resistors in parallel as in Figure 4. 5: Resistors in parallel. Eq. 4. 6 9

Basic Laws of Electric Circuits Single Node Pair Circuits: Current division. Back to current division: We can write from Figure 4. 4; In summary form; Eq. 4. 7 10 The above tells us how a current I divides when fed into two resistors in parallel. Important

Basic Laws of Electric Circuits Single Node Pair Circuits: Current division. In general, if we have N resistors in parallel and we want to find the current in, say, the jth resistor, as shown in Figure 4. 6, Figure 4. 6: General case for current division. Eq. 4. 8 11

Basic Laws of Electric Circuits Current Division: Example 4. 1 Given the circuit of Figure 4. 7. Find the currents I 1 and I 2 using the current division. Fig 4. 7: Circuit for Ex. 4. 1. By direct application of current division: 12

Basic Laws of Electric Circuits Current Division: Example 4. 2 Given the circuit of Figure 4. 8. Find the currents I 1 and I 2 using the current division. Figure 4. 8: Circuit for Ex. 4. 2. The 7 resistor does not change that the current toward the 4 and 12 ohm resistors in parallel is 10 A. Therefore the values of I 1 and I 2 are the same as in Example 4. 1. 13

Basic Laws of Electric Circuits Current Division: Example 4. 3 Find the currents I 1 and I 2 in the circuit of Figure 4. 9 using current division. Also, find the voltage Vx Figure 4. 9: Circuit for Ex. 4. 3. We first find the equivalent resistance seen by the 20 V source. 14

Basic Laws of Electric Circuits Current Division: Example 4. 3 We can now find current I by, We now find I 1 and I 2 directly from the current division rule: 15

Basic Laws of Electric Circuits Current Division: Example 4. 3 We can find Vx from I 1 x 12 or I 2 x 4. In either case we get Vx = 6 V. We can also find Vx from the voltage division rule: 16

Basic Laws of Electric Circuits Current Division: Example 4. 4 For the circuit of Figure 4. 10, find the currents I 1, I 2, and I 3 using the current division rule. Figure 4. 10: Circuit for Example 4. 4. 17