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A \(224 - \Omega \) resistor and a \(589\Omega \)resistor are connected in series across a \(90.0 - V\) line. (a) What is the voltage across each resistor? (b) A voltmeter connected across the \(224 - \Omega \) resistor reads \(23.8 V\). Find the voltmeter resistance. (c) Find the reading of the same voltmeter if it is connected across the \(589\Omega \) resistor. (d) The readings on this voltmeter are lower than the 鈥渢rue鈥 voltages (that is, without the voltmeter present). Would it be possible to design a voltmeter that gave readings higher than the 鈥渢rue鈥 voltages? Explain

Short Answer

Expert verified

(a) The voltage across 1st and 2nd resistor will be respectively \({V_1} = 24.8{\rm{V}}\)and \({V_2} = 65.2{\rm{V}}\).

(b) The voltmeter resistance is \({R_V} = 3840\Omega \).

(c)\({\rm{62}}{\rm{.6V}}\)

(d) No, any real voltmeter will draw some current and thereby reduce the current through the resistance whose voltage is being measured. \(\)

Step by step solution

01

Given information

\(\)

Resistor one =\(224 - \Omega \)

Resistor two = \(589 - \Omega \)

Both the resistor connected by = \({\rm{90}}{\rm{.0 - V}}\)

Reading by resistor \(224 - \Omega \)= \({\rm{23}}{\rm{.8 V}}\)

\(\)

02

Resistor

The passive two terminal electrical component by which electrical resistance get implemented as a circuit element is known as resistor R.

\(\begin{aligned}{}{R_{Total}} = {R_1} + {R_2} + {R_3} + ...(series)\\{R_{Total}} &= \frac{1}{{{R_1}}} + \frac{1}{{{R_2}}} + \frac{1}{{{R_3}}} + ...(parallel)\\R &= \frac{V}{I}\\V &= voltage\\I &= current\end{aligned}\)

03

(a) Voltage of the resistors

The current of the resistor will be,

\(\begin{aligned}{}I &= {I_1} &= {I_2}\\I &= \frac{{90.0{\rm{V}}}}{{{R_1} + {R_2}}}\\I &= \frac{{90.0{\rm{V}}}}{{224\Omega + 589\Omega }}\\I &= 0.1107{\rm{A}}\end{aligned}\)

So the voltage in the resistor will be,

\(\begin{aligned}{}{V_1} &= {I_1}{R_1}\\{V_1} &= \left( {0.1107{\rm{A}}} \right)\left( {224\Omega } \right)\\{V_1} &= 24.8{\rm{V}}\end{aligned}\)

And,

\(\begin{aligned}{}{V_2} &= {I_1}{R_1}\\{V_2} &= \left( {0.1107{\rm{A}}} \right)\left( {589\Omega } \right)\\{V_2} &= 65.2{\rm{V}}\end{aligned}\)

So the voltage across 1st and 2nd resistor will be respectively \({V_1} = 24.8{\rm{V}}\)and \({V_2} = 65.2{\rm{V}}\). \(\)

04

(b) Voltmeter resistance

The following figure is describing the resistor network,

As given in the question the potential difference across the voltmeter terminals is \({\rm{23}}{\rm{.8 V}}\), so to find out the current flow in the system we have to apply the following ohm鈥檚 law,

The voltage drop across the \(589 - \Omega \) resistor is \(90.0{\rm{V}} - 23.8{\rm{V = 66}}{\rm{.2V}}\), so now,

\(\)

\(\begin{aligned}I &= \frac{V}{R}\\I &= \frac{{{\rm{66}}{\rm{.2V}}}}{{589\Omega }}\\I &= 0.1124{\rm{A}}\end{aligned}\)

The voltage drop across resistor \(224 - \Omega \)= \({\rm{23}}{\rm{.8 V}}\), so now,

\(\begin{aligned}{I_2} = \frac{{{\rm{23}}{\rm{.8V}}}}{{224\Omega }}\\{I_2} = 0.1062{\rm{A}}\\{I_1} = I - {I_2}\\{I_1} = 0.1124{\rm{A}} - 0.1062{\rm{A = }}0.0062{\rm{A}}\end{aligned}\)

The resistor will be,

\(\begin{aligned}{R_V} = \frac{V}{{{I_1}}}\\{R_V} = \frac{{{\rm{23}}{\rm{.8V}}}}{{0.0062{\rm{A}}}}\\{R_V} = 3840\Omega \end{aligned}\) \(\)

The voltmeter resistance is \({R_V} = 3840\Omega \) \(\)

\(\)

05

(c) Voltmeter reading if connected to another resistor

The circuit that is connected with the voltmeter is shows as below,

\(\begin{aligned}{l}\\\end{aligned}\)

\(\begin{aligned}\frac{1}{{{R_{eq}}}} = \frac{1}{{3840\Omega }} + \frac{1}{{589\Omega }}\\{R_{eq}} = 510.7\Omega \end{aligned}\)

The voltmeter current,

\(\)

\(\begin{aligned}I &= \frac{V}{R}\\I &= \frac{{{\rm{90}}{\rm{.0V}}}}{{224\Omega + 510.7\Omega }}\\I &= 0.1225{\rm{A}}\end{aligned}\)

The potential drop across the first resistor is

\(\)

\(\begin{aligned}{V_1} &= {I_1}{R_1}\\{V_1} &= \left( {0.1225{\rm{A}}} \right)\left( {224\Omega } \right)\\{V_1} &= 27.4{\rm{V}}\end{aligned}\)

So on the second one it will be,

\(\begin{aligned}{}\\\end{aligned}\)

\(90{\rm{V}} - 27.4{\rm{V = 62}}{\rm{.6V}}\)

\(\)

\(\)

06

(d) Reading of voltmeter

No, any real voltmeter will draw some current and thereby reduce the current through the resistance whose voltage is being measured. Thus the presence of the voltmeter connected in parallel with the resistance lowers the voltage drop across that resistance. The resistance of the voltmeter in this problem is only about a factor of ten larger than the resistances in the circuit, so the voltmeter has a noticeable effect on the circuit.

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