Physics Lesson 15.8.1 - Measurement of Current and Voltage

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Welcome to our Physics lesson on Measurement of Current and Voltage, this is the first lesson of our suite of physics lessons covering the topic of Miscellaneous, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Measurement of Current and Voltage

In tutorial 15.4 "Electric Circuits. Series and Parallel Circuits. Short Circuits" we have shortly explained what ammeter and voltmeter are used for. Thus, we have explained that ammeter is used to measure the value of electric current in the circuit. It is connected in series with the operating appliance in the circuit. On the other hand, a voltmeter is used to measure the value of potential difference in the circuit. It is connected in parallel to the operating appliance in the circuit.

But why this is so? Why ammeter is connected in series to appliances and voltmeter is parallel to them? Let's explain the logic of this setup.

a. Ammeter

Since ammeter is used to measure the current in the circuit, it can be placed at any position of the conductor since the current is the same at every point of a circuit branch. An ammeter has a very low internal resistance because and therefore, the potential difference across its terminals is negligible, otherwise this (a high resistance) would affect the value of potential difference in the operating device. This would result in wrong values not only for the potential difference but also for electric power, efficiency and so on.

The figure below shows the connection of an ammeter in a circuit in both real and simplified view (in symbols).

Physics Tutorials: This image provides visual information for the physics tutorial Miscellaneous

b. Voltmeter

Since voltmeter is used to measure the potential difference across the terminals of a circuit element, it must be connected in parallel to it that is one wire of voltmeter is connected before the appliance and the other wire after the appliance. This is because the voltmeter has a very high resistance in order to affect as less as possible the value of current flowing in the circuit. In other words, a very high resistance allows only a negligible amount of current to flow through the voltmeter. Look at the figure.

Example 1

What are the readings of ammeter and voltmeter in the circuit below if the resistance of battery is 1 Ω and that of the conducting wire is 0.5 Ω?

Solution 1

The ammeter measures the current in the main branch (I_tot) while the voltmeter measures the potential difference ΔV in the parallel branch.

First, we must calculate the equivalent resistance in the parallel branch. We have

1/R_eq = 1/R₁ + 1/R₂
= 1/2 + 1/6
= 2 + 1/6
= 3/6
= 1/2

Thus, R_eq = 2 Ω.

The total resistance in the circuit is

R_tot = R_eq + R_wire + r_battery
= 2Ω + 0.5Ω + 1Ω
= 3.5Ω

The reading of ammeter that shows the current in the main branch therefore is

I_tot = ε/R_tot
= 21 V/3.5 Ω
= 6 A

As for the reading of voltmeter, it shows the potential difference ΔV in the parallel branch, which is calculated by

ΔV = ε - I ∙ R_w - I ∙ r
= 21V - 6A ∙ 0.5Ω - 6A ∙ 1Ω
= 21V - 3V - 6V
= 12V

Therefore, the ammeter reads 6A and the voltmeter reads 12V.

You have reached the end of Physics lesson 15.8.1 Measurement of Current and Voltage. There are 5 lessons in this physics tutorial covering Miscellaneous, you can access all the lessons from this tutorial below.

More Miscellaneous Lessons and Learning Resources

Electrodynamics Learning Material
Tutorial ID	Physics Tutorial Title	Revision Notes	Revision Questions
15.8	Miscellaneous
Lesson ID	Physics Lesson Title	Lesson	Video Lesson
15.8.1	Measurement of Current and Voltage
15.8.2	Superconductivity
15.8.3	Electric Current in Liquids
15.8.4	Faraday's Law for Electrolysis
15.8.5	Electric Current in Gases

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