A student investigates the resistance of three identical filament lamps, L1L_1L1, L2L_2L2, and L3L_3L3, connected in various configurations.
In the first part of the experiment, only lamp L1 L_1\,L1 is connected to a power supply. The student measures the current I1 I_1\,I1 in the circuit and the potential difference V1 V_1\,V1 across L1 L_1\,L1 to be:
I1=0.35 A I_1 = 0.35\text{ A} I1=0.35 A V1=2.4 V V_1 = 2.4\text{ V} V1=2.4 V(i) Calculate the resistance R1 R_1\,R1 of the filament of lamp L1 L_1\,L1 using the formula R1=V1I1\displaystyle R_1 = \frac{V_1}{I_1}R1=I1V1. Include the correct unit in your answer.
In the second part, lamp L2 L_2\,L2 is connected in series with lamp L1L_1L1. The potential difference V2 V_2\,V2 is measured across L2L_2L2, and the new current I2 I_2\,I2 in the circuit is measured:
I2=0.24 A I_2 = 0.24\text{ A} I2=0.24 A V2=1.2 V V_2 = 1.2\text{ V} V2=1.2 VCalculate the resistance R2 R_2\,R2 of the filament of lamp L2 L_2\,L2 using the formula R2=V2I2\displaystyle R_2 = \frac{V_2}{I_2}R2=I2V2.
In the third part, lamp L3 L_3\,L3 is connected in series with lamps L1 L_1\,L1 and L2L_2L2. The potential difference V3 V_3\,V3 across L3 L_3\,L3 and the circuit current I3 I_3\,I3 are measured:
I3=0.18 A I_3 = 0.18\text{ A} I3=0.18 A V3=0.80 V V_3 = 0.80\text{ V} V3=0.80 VCalculate the resistance R3 R_3\,R3 of the filament of lamp L3 L_3\,L3 using the formula R3=V3I3\displaystyle R_3 = \frac{V_3}{I_3}R3=I3V3.
Calculate the sum of the three resistances, R1+R2+R3R_1 + R_2 + R_3R1+R2+R3. Express your answer to a suitable number of significant figures (2 or 3 significant figures).
Some students make the following suggestions about the results of the experiment:
State which suggestion, A, B, or C, agrees with the calculated resistances. Justify your answer using your numerical values.
State a variable that the student could change to determine whether this relationship always holds for these three lamps, and describe how this variable can be changed.
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.