Question 22M.2.SL.TZ1.b
| Date | May 2022 | Marks available | [Maximum mark: 3] | Reference code | 22M.2.SL.TZ1.b |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Determine | Question number | b | Adapted from | N/A |
A power supply is connected to three resistors P, Q and R of fixed value and to an ideal voltmeter. A variable resistor S, formed from a solid cylinder of conducting putty, is also connected in the circuit. Conducting putty is a material that can be moulded so that the resistance of S can be changed by re-shaping it.
The resistance values of P, Q and R are 40 Ω, 16 Ω and 60 Ω respectively. The emf of the power supply is 6.0 V and its internal resistance is negligible.
The voltmeter reads zero. Determine the resistance of S.
[3]
ALTERNATIVE 1
Pd across Q = 2.4 V so I = 0.15 « A » ✓
and pd across S is 6.0 – 2.4 = 3.6 « V » ✓
«Ω» ✓
ALTERNATIVE 2
pd at PR junction = pd at QS junction ✓
so OR ✓
24 «Ω» ✓
ALTERNATIVE 3
«A» ✓
OR ✓
R = 24 «Ω» ✓
Allow ECF for MP3 from incorrect MP1 or MP2.
Many candidates struggled with this question. A very common mistake was to assume the current was the same in each branch, leading to a resistance of 84 Ohms. The placement of the voltmeter may have caused some confusion for candidates, and they may not have understood what the zero reading was indicating. It is important that candidates understand what voltmeters are actually reading and are familiar with different placements in circuits.
