Determine the uncertainty in the experimental value of the e

Determine the uncertainty in the experimental value of the emf for the unknown dry cell (see Appendix B) 2. POST LAB QUESTIONS 1. When a Wheatstone Bridge is balanced, there is essentially no current in the meter. Does this mean 2. The terminal voltage of the unknown cell as indicated by the voltmeter should be less than that 3. The voltage of the potentiometer power supply, 6, was not measured or used in the calculations. 4. How can a potentiometer be made to read directly in volts on a slide wire bridge, without having that there is essentially no current in the resistor whose resistance is being determined? Explain. found by the potentiometer. Explain why Why is the value of this voltage not needed? to do calculations based on the null reading from the meter stick.?

Solution

1. There is no current in the meter because the potential across the ends of the meter are same in balanced wheatstone-bridge. This does not require that the potential across the unknown resistor to be same. hence current still flows through the unknown resistor. Therefore current from meter zero at ballanced situation does not mean the current to be zero across unknown resistor.

2. When we use potentiometer to measure the terminal voltage of an unknown cell, it uses patential balance condition. In this condition there is no flow of current through the cell hence no voltage drop across the terminals of the cell. Hence this gives the emf of the cell.

But when we measure terminal potential using voltmeter there is current flow through the battery and hence there is potential drop through internal resistance of the cell. The measured terminal potential is V = emf - i*r ( i = current and r = internal resistance of the cell). Hence, V (measured by voltmeter) is lesser than that of emf ( found by potentiometer).

3. The potentiometer power supply emf works as sorce of a potetial divider. Potential divider circuit is a simple circuit where large potential is turned to small potential through resistance ( here the wire of the potentiometer ). We compare the external voltage source with this small potential by achieving a balance point. Hence the value of the sorce emf is not required rather the balace point or the length of the wire balancing the external potential is needed for calculation.

4. Since wire of the potentiometer is uniform, the length of wire is directly proportional to the potential drop. Therefore we can calibrate length wire as potential drop per mm using a standard cell. Now during each measurement we can directly note down the voltage marked at particular length.