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Solved exercise · level 4 of 5

Not-so-perfect voltmeter

Skills: Analyse

Statement

R1 = 22 kΩ and R2 = 10 kΩ in series at 12 V. Voltmeter V1 across R2 has an internal resistance of only 100 kΩ (adjustable in the inspector).

What voltage does it display? (It disturbs the circuit: R_V is in parallel with R2.)

Parametric exercise: the values change with every draw; these are one example.

2 hints from Bastaya available in the lab
Solution

R2 ∥ R_V = 10000 × 100000 / (10000 + 100000); U_meas = 12 × R2' / (R1 + R2') = 3.51 V. With a 1 GΩ voltmeter you would read 12 × R2 / (R1 + R2).

Course sheet of the chapter : Measurement & instruments

This chapter is covered by the paths: General electricity, Measurement prep

Other exercises of the chapter

Simulated time00:00:00
Q = I·t-
W = P·t-

Circuit diagram CEI 60617

How does it work?

Each big hole is a circuit node: you push one leg of a component into it. The 4 small holes around it are connected to it: chips and leads plug in there.

  1. Place a component Drag it from the palette: it snaps onto the grid. R rotates it.
  2. Bridge neighbouring holes Pick the jumper chip (or C) and drag on the board: bridged holes become a single node.
  3. Draw a lead For long links and the power supply: from one big hole to another, or to a terminal.
  4. Close the loop The circuit is solved live (Ohm, Kirchhoff) and the diagram follows.

Shortcuts

Del
delete
R
rotate
C
chip brush
Esc
cancel
wheel
zoom
0
fit the board
drag background
pan

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