The diagram below shows a circuit with one battery and 10 resistors; 5 on
the left and 5 on the right. Determine…the current through
the voltage drop across
the power dissipated by each resistor
Answers
In Current and Resistance, we described the term ‘resistance’ and explained the basic design of a resistor. Basically, a resistor limits the flow of charge in a circuit and is an ohmic device where V=IR . Most circuits have more than one resistor. If several resistors are connected together and connected to a battery, the current supplied by the battery depends on the equivalent resistance of the circuit.
The equivalent resistance of a combination of resistors depends on both their individual values and how they are connected. The simplest combinations of resistors are series and parallel connections (Figure 10.3.1 ). In a series circuit, the output current of the first resistor flows into the input of the second resistor; therefore, the current is the same in each resistor. In a parallel circuit, all of the resistor leads on one side of the resistors are connected together and all the leads on the other side are connected together. In the case of a parallel configuration, each resistor has the same potential drop across it, and the currents through each resistor may be different, depending on the resistor. The sum of the individual currents equals the current that flows into the parallel connections.
Par a shows four resistors connected in series and part b shows four resistors connected in parallel.
Figure 10.3.1 : (a) For a series connection of resistors, the current is the same in each resistor. (b) For a parallel connection of resistors, the voltage is the same across each resistor.
Resistors in Series
Resistors are said to be in series whenever the current flows through the resistors sequentially. Consider Figure 10.3.2 , which shows three resistors in series with an applied voltage equal to Vab . Since there is only one path for the charges to flow through, the current is the same through each resistor. The equivalent resistance of a set of resistors in a series connection is equal to the algebraic sum of the individual resistances.
Part a shows original circuit with three resistors connected in series to a voltage source and part b shows the equivalent circuit with one equivalent resistor connected to the voltage source.
Figure 10.3.2 : (a) Three resistors connected in series to a voltage source. (b) The original circuit is reduced to an equivalent resistance and a voltage source.
In Figure 10.3.2 , the current coming from the voltage source flows through each resistor, so the current through each resistor is the same. The current through the circuit depends on the voltage supplied by the voltage source and the resistance of the resistors. For each resistor, a potential drop occurs that is equal to the loss of electric potential energy as a current travels through each resistor. According to Ohm’s law, the potential drop V across a resistor when a current flows through it is calculated using the equation V=IR , where I is the current in amps ( A ) and R is the resistance in ohms (Ω) . Since energy is conserved, and the voltage is equal to the potential energy per charge, the sum of the voltage applied to the circuit by the source and the potential drops across the individual resistors around a loop should be equal to zero:
∑i=1NVi=0.(10.3.1)
This equation is often referred to as Kirchhoff’s loop law, which we will look at in more detail later in this chapter. For Figure 10.3.2 , the sum of the potential drop of each resistor and the voltage supplied by the voltage source should equal zero:
V−V1−V2−V3V=0,=V1+V2+V3,=IR1+IR2+IR3,
Solving for I
I=VR1+R2+R3=VRS.
Since the current through each component is the same, the equality can be simplified to an equivalent resistance ( RS ), which is just the sum of the resistances of the individual resistors.
Equivalent Resistance in Series Circuits
Any number of resistors can be connected in series. If N resistors are connected in series, the equivalent resistance is
RS=R1+R2+R3+...+RN−1+RN=∑i=1NRi.(10.3.2)
One result of components connected in a series circuit is that if something happens to one component, it affects all the other components. For example, if several lamps are connected in series and one bulb burns out, all the other lamps go dark.
Example 10.3.1 : Equivalent Resistance, Current, and Power in a Series Circuit
A battery with a terminal voltage of 9 V is connected to a circuit consisting of four 20Ω and one 10Ω resistors all in series (Figure 10.3.3 ). Assume the battery has negligible internal resistance.
Calculate the equivalent resistance of the circuit.
Calculate the current through each resistor.
Calculate the potential drop across each resistor.
Determine the total power dissipated by the resistors and the power supplied by the battery.
The figure shows four resistors of 20 Ω and one resistor of 10 Ω connected in series to a 9 V voltage source.
Figure 10.3.3 : A simple series circuit with five resistors.