Basic Electricity

Simple Circuits

41 questions By Tony R. Kuphaldt

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Question 16 of 41

In this circuit, where would you expect to measure full battery voltage (between what pairs of test points)?

Reveal answer

You should expect to measure full battery voltage with one test lead of your voltmeter touching any of the points along the top wire of the circuit (points A through E), and with the other test lead touching any of the points along the bottom wire of the circuit (points F through J).

Notes:
This circuit provides an excellent opportunity to discuss the concept of “electrically common” points. Any points in a circuit directly connected together with wire are considered “electrically common” to each other: a voltage measurement referenced at any one of those points should be identical if referenced any of the other points as well.
Question 17 of 41

In this circuit, where would you not expect to measure significant voltage (between what pairs of test points)?

Reveal answer

You should not measure any significant voltage between any of the test points along the upper wire (A to B, A to C, A to D, etc.), nor between any of the test points along the lower wire (F to G, F to H, F to I, etc.). As a general rule, points in a circuit that are electrically common to each other should never have voltage between them.

Notes:
The answer uses a concept which I’ve found to be very helpful in understanding electrical circuits: the idea of points in a circuit being electrically common to each other. Simply put, this means the points are connected together by conductors of negligible resistance. Having nearly 0 ohms of resistance between points assures insignificant voltage drop, even for large currents.

Conversely, if significant voltage is measured between points in a circuit, you can be assured that those points are not electrically common to each other. Engage your students in a discussion of electrical commonality and expected voltage drops:

• If voltage is measured between two points in a circuit, are those two points electrically common to each other? Why or why not?

• If no voltage is measured between two points in a circuit, are those two points electrically common to each other? Why or why not?
Question 18 of 41

Suppose this battery and light bulb circuit failed to work. Using nothing but a voltmeter, how would you check the circuit to determine where the problem is located? Note: the letters indicate “test points” along the wiring where you may probe with the circuit with your voltmeter.

Reveal answer

There are several strategies which may be employed to find the location of the problem in this circuit. One popular technique is to “divide the circuit in half” by testing for voltage between points C and H first. The presence of absence of voltage between these two points will indicate whether the problem lies between those points and the battery, or between those points and the light bulb (assuming there is but a single problem in the circuit - a large assumption!).

Notes:
A circuit like this is very easy to construct, and makes for an excellent classroom demonstration piece. I’ve used such a circuit, constructed on a piece of pegboard 2 feet by 4 feet, with metal screws acting as test points, for students to develop their troubleshooting skills in front of the class where everyone may observe and learn together.

It has been my experience that students who experience difficulty troubleshooting circuits in general usually experience difficulty troubleshooting this simple circuit in particular. Although the circuit itself couldn’t be simpler, the fundamental concept of voltage as a quantity measurable only between 2 points is confusing for many. Spending lots of time learning to troubleshoot a circuit such as this will be greatly beneficial in the future!