A student investigates the resistance of two different wire types, wire X and wire Y.
She measures the potential difference VVV and the current III for two different lengths of each wire. Some of her measurements and calculations are recorded in the table below.
| Wire | Length l/cml / \text{cm}l/cm | Potential difference V/VV / \text{V}V/V | Current I/AI / \text{A}I/A | Resistance R/…R / \dots R/… |
|---|---|---|---|---|
| X | 80.0 | 3.2 | 0.40 | |
| Y | 80.0 | 2.5 | 0.50 | |
| X | 40.0 | 2.4 | 0.60 | |
| Y | 40.0 | 1.8 | 0.75 |
(i) State the unit of resistance that should complete the column heading in the table.
(ii) Calculate and record the resistance RRR of each wire section to 2 or 3 significant figures using the equation:
R=VI R = \frac{V}{I} R=IV(i) Calculate a value PPP using your results from the table and the equation:
P=R for 80.0 cm of wire XR for 80.0 cm of wire Y P = \frac{R \text{ for } 80.0\text{ cm of wire X}}{R \text{ for } 80.0\text{ cm of wire Y}} P=R for 80.0 cm of wire YR for 80.0 cm of wire X(ii) Calculate a value QQQ using your results from the table and the equation:
Q=R for 40.0 cm of wire XR for 40.0 cm of wire Y Q = \frac{R \text{ for } 40.0\text{ cm of wire X}}{R \text{ for } 40.0\text{ cm of wire Y}} Q=R for 40.0 cm of wire YR for 40.0 cm of wire XA student suggests that the ratio of the resistances of the two types of wire should be independent of length, meaning PPP and QQQ should be equal. State whether your calculated values support this suggestion. Justify your answer by reference to the values of PPP and QQQ.
Suggest one practical reason why different students carrying out this experiment carefully with the same apparatus might not obtain identical results.
452 exam-style questions on CIE IGCSE Physics 4.2 Electrical quantities, covering 4.2.1a Positive and negative charge, 4.2.1b Charging by friction and detecting charge, 4.2.1c Electrical conductors and insulators, 4.2.1d Electric fields and electric field patterns, 4.2.2a Electric current, charge and measuring current, 4.2.2b Conduction in metals and the direction of current, 4.2.2c Direct current and alternating current, 4.2.3a Electromotive force, 4.2.3b Potential difference and voltmeters, 4.2.4a Resistance, 4.2.4b Resistance of a metallic wire, 4.2.4c Current–voltage graphs, and 4.2.5 Electrical energy and electrical power. Each one has a worked solution and a mark scheme showing where the marks go.