#31Q4) Electricity and Electronics

Determination of the internal resistance and the electromotive force of a dry cell

DifficultyMedium
Est. Time45 mins

Required Apparatus

A dry cell, a milliammeter, a digital voltmeter, a rheostat (0-100 Ω), a tap key, connecting wires, and a 10 Ω resistor.

Scientific Theory

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In the above circuit if EE is the E. M. F. of the cell, rr its internal resistance, II the current in the circuit and VV the potential difference between the terminals of the cell,

E=V+IrE=V+Ir V=−Ir+EV=-Ir+E V=−rI+EV=-rI+E

When VV is plotted against II,

Gradient of the graph =−r= -r

Intercept =E= E

Experimental Method

  1. Set up the circuit as shown in the Figure 31.1 and adjust the rheostat for its maximum resistance.
  2. Observe the reading of the voltmeter with key K open and record the value.
  3. Next close the key K, and while decreasing the resistance of the rheostat and varying the value of II by 0.025 A (25 mA) at a time obtain relevant voltmeter readings along with milliampere readings of II (from the milliammeter). Record this readings.

Important Points

1
  • Plot VV against II. Calculate the gradient and the intercept of the graph. Obtain the initial resistance of the cell and its electromotive force from them.
2
  • Since the E.M.F. of a dry cell is about 1.5 V and internal resistance is about 1 Ω1~\Omega, a current of about 1.5 A is drawn from the cell if it is short circuited. Therefore to protect the cell and the milliammeter a suitable protective resistor has to be connected in series with the cell.
3
  • A current drawn from the cell for a long time causes polarization and decreases its electromotive force. To prevent this happening the key K should be kept closed only when taking readings.
4
  • Discuss the advantages of using a digital voltmeter for this experiment.

Past Paper Questions

Q4

2020 Question 4

2020 • GCE Advanced Level • medium