Apparatus:
- Battery
- Voltmeter
- Resistance box
- Key and connecting wires
Theory:
Consider a battery of electromotive force E connected in series with a resistance R provided by a resistance box R.B. a key K and a voltmeter V whose resistance Rv Is to be determined. Usually a voltmeter is connected in parallel with a circuit, but here the voltmeter is connected in series and it measures the potential difference V across its own resistance Rv. The current I flowing through the circuit is given by
I =
The potential difference V across the resistance Rv of the voltmeter is given by
| V = IRV = |
Or
| V (R+RV) = ERV |
Or
| VR = ERV |
Or
| R = ERV |
If a graph between R and 1/vis drawn, it will be a straight line as shown in fig.3 (b). Whose intercept with Y – axis gives the resistance RV‑of the voltmeter.
Precautions:
- Draw a neat circuit diagram as shown in fig 3. (a). Clean the ends of connecting wires and arrange the apparatus as shown in fig 3. (a) Where a battery B, Key K, resistance R.B and a voltmeter V, whose resistance Rv is to be determined, are connected in series. See that the positive terminal of the voltmeter is connected to the positive pole of the battery B and all the connections are clean and tight.
- Apply some suitable resistance R with the help of the resistance box R.B. Insert the plug in the key K and note the reading V of the voltmeter.
- Increase the resistance R and again note the reading of the voltmeter. Similarly take at least six observations by changing the resistance R in equal steps. Calculate 1/V for each observation.
- Plot a graph between R and 1/V taking R along the x-axis and 1/V along y-axis. Take the origin as (0,0). The graph will be straight line. Produce this straight line to meet the x-axis. Find the value of the intercept of the graph with the x-axis which gives the resistance RV of the voltmeter.
Observation and Calculation:
| No. of obs. | Voltmeter Reading V (volts) | Resistance R (ohms) | |
| 1 2 3 4 5 6 | 0 500 1000 1500 2000 2500 | 2.6 1.8 1.4 1.1 1.0 0.8 | 0.38 0.55 0.71 0.90 1.0 1.25 |
Intercept of the graph on the x-axis: = ______ ohm.
Resistance of the voltmeter: = intercept of graph
= RV = 1200 ohm.
Precautions:
- All the connections should be clean and tight.
- Terminal of the voltmeter marked + should be connected to the positive pole and the other terminal to the negative pole of the battery.
- Change the resistance R in equal steps.
- The origin of the graph should be taken as (0,0).
Viva Voice
Q.1. What is a voltmeter?
Ans. Voltmeter is an instrument used for measuring the potential difference between two points. It is simply a galvanometer with a suitable high resistance connected in series with it.
Q.2. Why should a voltmeter have a high resistance?
Ans. A voltmeter should have a very high resistance so that it should not draw any appreciable current from the circuit across which it is connected. Thus the potential difference which is to be measured, is not changed by connecting such a voltmeter for measuring it.
Q.3. How can an ordinary galvanometer be converted into a voltmeter?
Ans. This can be done by connecting a suitable high resistance in series with the galvanometer.
Q.4. Can an AC voltmeter be used in a DC circuit?
Ans. An AC voltmeter cannot be used in a DC circuit.
Q.5. Define unit of potential difference.
Ans. The unit of potential difference is volt.
The potential difference between two points is said to be one volt, if the work done in moving a positive charge of one coulomb from one point to other point is on joule keeping electrostatic equilibrium.(i.e., it moves with uniform velocity).
Q.6. Define what if potential difference.
Ans. It is the amount of work done in moving unit positive charge from one point to other in an electric field keeping the charge in electrostatic equilibrium.
Q.7. What is voltmeter?
Ans. Voltmeter is a device used to measure potential difference difference between two points In a circuit.
Q.8. How is a voltmeter connected in a circuit?
Ans. A voltmeter is connected in parallel between the points across which potential difference is to be measured.
Q.9. Why is a voltmeter connected in parallel?
Ans. Potential difference across all resistances connected in parallel difference across voltmeter and resistance across which it is connected would be equal.
Q.10. How a galvanometer is converted into voltmeter?
Ans. A galvanometer is converted into voltmeter by connecting a suitable high resistance in series with the galvanometer.
Q 11. What is an ideal voltmeter?
Ans. A ideal voltmeter is one whose resistance is infinite. I t should not draw any current from the circuit.
Q.12. How does the potential difference vary across the series resistance?
Ans. Potential difference increases with the value of resistance as the current through all of them is the same (i.e., V = IR).
Q.13. Which resistance is called a low resistance?
Ans. A resistance whose value is loss than ohms (fractional values) is called a low resistance.
Q.14. Can an ordinary voltmeter measure the potential difference accurately?
Ans. No, a voltmeter cannot measure the potential difference accurately, because it draws some current from the circuit.
Q.15. Which instrument other than voltmeter can be used to measure the voltage accurately?
Ans. A cathode ray oscilloscope (CRO) and digital voltmeter can measure the voltage accurately.
Q.16. How can you calculate the resistance of voltmeter from the graph?
Ans. It is equal to negative x-intercept of straight line.
