TEST OF PRACTICAL KNOWLEDGE QUESTION
You are provided with a resistance box R, voltmeter, key, cell of e.m.f. E, constantan wire, standard resistor, RX, an ammeter, and other necessary apparatus.
(i) Measure and record the e.m.f. E of the cell provided.
(ii) Set up the circuit as shown in the diagram above.
(iii) Set R to 1\(\Omega\). Close the key, read and record the current I and the corresponding voltage V.
(iv) Repeat the procedure for four other values of R= 2\(\Omega\), 4\(\Omega, 6\(\Omega\), and 8\(\Omega\).
(v) In each case, read and record I and V.
(vi) Tabulate the reading.
(vii) Plot a graph of V on the vertical axis and I on the horizontal axis.
(viii) Determine the slope s, of the graph.
(ix) State two precautions are taken to ensure accurate results
(b) )State two advantages of connecting identical cells in parallel.
(ii) State two factors to consider in choosing the material for the design of a resistor
(a) Experiment (variation of V and I with external resistance). First measure the e.m.f. E of the cell with a high-resistance voltmeter on open circuit. Set up the circuit with the resistance box R, the standard resistor \(R_X\), the ammeter (in series) and the voltmeter (across the appropriate element) as shown. Set \(R=1\,\Omega\), close the key and record the current I and voltage V. Repeat for \(R=2,4,6,8\,\Omega\), recording I and V each time. Tabulate the readings, then plot V (vertical) against I (horizontal).
Specimen table
| R /Ω | I /A | V /V |
|---|
| 1 | ... | ... |
| 8 | ... | ... |
The graph is a straight line; its slope s (with the sign and unit of resistance) relates to the resistances in the circuit, and the intercept gives the e.m.f. E, consistent with \(V=E-I r\).
Two precautions: open the key immediately after each reading to avoid heating and cell run-down; avoid parallax and check the meters for zero errors.
(b)(i) Advantages of connecting identical cells in parallel:
- The total internal resistance is reduced, so a larger current can be supplied.
- The cells share the load, so they last longer (greater capacity/longer life) while the e.m.f. stays that of a single cell.
(b)(ii) Factors in choosing a material for a resistor:
- Its resistivity (it should have a suitably high, stable resistivity, e.g. constantan/manganin).
- A low temperature coefficient of resistance (resistance nearly independent of temperature) and a high melting point so it is not damaged by heating.
(a) Experiment (variation of V and I with external resistance). First measure the e.m.f. E of the cell with a high-resistance voltmeter on open circuit. Set up the circuit with the resistance box R, the standard resistor \(R_X\), the ammeter (in series) and the voltmeter (across the appropriate element) as shown. Set \(R=1\,\Omega\), close the key and record the current I and voltage V. Repeat for \(R=2,4,6,8\,\Omega\), recording I and V each time. Tabulate the readings, then plot V (vertical) against I (horizontal).
Specimen table
| R /Ω | I /A | V /V |
|---|
| 1 | ... | ... |
| 8 | ... | ... |
The graph is a straight line; its slope s (with the sign and unit of resistance) relates to the resistances in the circuit, and the intercept gives the e.m.f. E, consistent with \(V=E-I r\).
Two precautions: open the key immediately after each reading to avoid heating and cell run-down; avoid parallax and check the meters for zero errors.
(b)(i) Advantages of connecting identical cells in parallel:
- The total internal resistance is reduced, so a larger current can be supplied.
- The cells share the load, so they last longer (greater capacity/longer life) while the e.m.f. stays that of a single cell.
(b)(ii) Factors in choosing a material for a resistor:
- Its resistivity (it should have a suitably high, stable resistivity, e.g. constantan/manganin).
- A low temperature coefficient of resistance (resistance nearly independent of temperature) and a high melting point so it is not damaged by heating.
