Why Do Voltmeter Readings Differ in a Series Circuit?

  • Thread starter Thread starter swaha
  • Start date Start date
  • Tags Tags
    Voltmeter
AI Thread Summary
In a series circuit with a battery and two resistors, the voltmeter readings can differ due to the finite resistance of the voltmeter itself. When measuring voltages across resistors, if the voltmeter's impedance is not significantly higher than the circuit's impedance, it can load the circuit, affecting the voltage readings. This loading effect can lead to discrepancies, as the voltages across the resistors may not sum to the battery voltage. Modern digital multimeters have high input impedances, typically around 10M ohms, minimizing this effect compared to older analog voltmeters. Understanding the impact of voltmeter impedance is crucial for accurate voltage measurements in circuit analysis.
swaha
Messages
5
Reaction score
0

Homework Statement


A person measures 9volts across the battery, 3 volts across the resistor Ra and 4.5volts across the resistor Rb connected in series (all three battery, Ra, Rb are connected in series) with a voltmeter of finite resistance. The resistor Ra=2M, and the battery has negligible internal resistance. Find- 1.The resistor Rb and 2. The internal resistance of the voltmeter.

[M=mega ohms]

Homework Equations

- Ohm's law and Kirchoff's laws are only possible relevant equations.



The Attempt at a Solution

- I really didnot get this. Shouldnt the voltages at the two resistors add up to give battery voltage?

[source of the question- tifr gs 2010]
 
Physics news on Phys.org
The voltages across each resistor should add up to the battery voltage BUT only if the impedance of the voltmeter is very high. If the impedance of the voltmeter isn't very high (compared to the circuit impedance) then the volt meter will behave like an additional resistor that loads up the circuit and messes with the voltages. Note that it's not just the displayed voltage, it's the actual voltage that's affected. In some cases a circuit might work fine until you attach a voltmeter and then it stops working! Same applies to scope probes.

Pretend the voltmeter in the problem is an ideal meter in parallel with an unknown resistor. The resistor represents the input impedance of the real world meter.
 
For info...

Modern digital multimeters typically use an input amplifier based on a FET transistor or similar to ensure the input impedance is very high. I have one with an input impedance of 10MOhms.

When my father started out an engineer such things didn't exist. Voltmeters were just moving coils of wire and they might have an imput impedance of just 1,000 ohms. Every time you tried to measure a voltage you had to correct for the loading effect of the meter exactly as per this problem.

See also..

http://en.wikipedia.org/wiki/Multimeter#Analog_multimeters
 
okay thanks.
 
Thread 'Collision of a bullet on a rod-string system: query'

Thread 'Collision of a bullet on a rod-string system: query'

In this question, I have a question. I am NOT trying to solve it, but it is just a conceptual question. Consider the point on the rod, which connects the string and the rod. My question: just before and after the collision, is ANGULAR momentum CONSERVED about this point? Lets call the point which connects the string and rod as P. Why am I asking this? : it is clear from the scenario that the point of concern, which connects the string and the rod, moves in a circular path due to the string...
View full post »
Thread 'A cylinder connected to a hanged mass'

Thread 'A cylinder connected to a hanged mass'

Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
View full post »

Similar threads

Ammeter and Voltmeter in Series
Replies
11
Views
5K
Voltmeter problem from (OCR paper) multiple choice
Replies
7
Views
2K
Ohm's Law - Finding Source Voltage
Replies
3
Views
1K
High voltage circuit with voltmeter in between
Replies
3
Views
2K
Calculating voltmeter resistance in DC circuit
Replies
4
Views
4K
Why do the headlights on the car become dim when the car is starting?
Replies
4
Views
2K
Resistance of the resistor and voltmeter's readings
Replies
25
Views
3K
Finding the internal resistance of battery
Replies
10
Views
3K
Circuits: What does the voltmeter read?
Replies
3
Views
12K
Calculating Voltages Across Resistors Using Ohm's Law
Replies
4
Views
2K

Hot Threads

Recent Insights

Back
Top