So my plan at this point is to set up a series of structured well-document experiments for my son to make it easier for him to understand what we're doing.
Otherwise he will just get frustrated and do it for the sake of doing it. I really want him to grasp this information and turn it into an exciting experience.
With so many distractions these days it is getting really hard for a parent to get a child excited about learning something, especially at this crucial age just before entering middle school.

So....starting with the most basic circuit (and taking into account what I've learned about internal DMM resistances), refer to the image below for Experiment #1.
Does anything in the procedure look out of place?

View attachment 84494

Part of a well-documented experiment involves a clear statement of what the objective of the experiment is and how the data will be used to support those objectives. Without that, it is impossible to evaluate a list of procedures because they have no context. I might give you a perfectly clear set of procedures that you follow flawlessly but they are worthless if they walk you through installing a bathroom light fixture when the objective was to unclog the sink drain. But if the objective isn't clearly stated, how can anyone tell you that this nice set of procedures is worthless?

 

Part of a well-documented experiment involves a clear statement of what the objective of the experiment is and how the data will be used to support those objectives. Without that, it is impossible to evaluate a list of procedures because they have no context. I might give you a perfectly clear set of procedures that you follow flawlessly but they are worthless if they walk you through installing a bathroom light fixture when the objective was to unclog the sink drain. But if the objective isn't clearly stated, how can anyone tell you that this nice set of procedures is worthless?

What I posted was basically a quick outline I whipped up in 10 minutes. Obviously I will add more guts to it.

 

What I posted was basically a quick outline I whipped up in 10 minutes. Obviously I will add more guts to it.

You missed the point. How can I evaluate the procedure you've given when you don't state what the objective of the procedure is? I am NOT going to read through a bunch of steps to determine if they are leading to the correct place in the correct way when I have no idea what the correct place that they are supposed to lead even is! And I'm not interested in guessing what the objective is based on the procedures.

 

You missed the point. How can I evaluate the procedure you've given when you don't state what the objective of the procedure is? I am NOT going to read through a bunch of steps to determine if they are leading to the correct place in the correct way when I have no idea what the correct place that they are supposed to lead even is! And I'm not interested in guessing what the objective is based on the procedures.

The objective it to teach a kid how to calculate and then measure current in a basic electrical circuit.

 

The objective it to teach a kid how to calculate and then measure current in a basic electrical circuit.

So that's the objective of THAT set of procedures?

The objective for an experiment needs to be specific to that experiment. Why is THAT set of procedures being used?

If the objective of the experiment is just to teach a kid how to calculate and then measure current in a basic electrical circuit, then design the experiment so that the effect of power supply resistance is negligible as are the perturbations introduced by the measurement methods. Boom. Easy done. Pick resistance values that are at least 100x that of the supply and meter resistances and then ignore them, because the stated objective can now be achieved to the desired level.

 

So that's the objective of THAT set of procedures?

The objective for an experiment needs to be specific to that experiment. Why is THAT set of procedures being used?

If the objective of the experiment is just to teach a kid how to calculate and then measure current in a basic electrical circuit, then design the experiment so that the effect of power supply resistance is negligible as are the perturbations introduced by the measurement methods. Boom. Easy done. Pick resistance values that are at least 100x that of the supply and meter resistances and then ignore them, because the stated objective can now be achieved to the desired level.

Let me elaborate. The purpose of the first experiment is to introduce the basic methodology of how we will be measuring voltage drops and currents in our final experimental stage...that which involves the current limiting 10W resistors and the 1/8 watt load resistor. This is basically just an introduction.... think of it like a set of training wheels. I wanted to use resistor values that will be used in the final experiment. For the 10W resistors we are using 100Ω, 47Ω, and 5.1Ω while for the load resistor we are using a 3.9Ω resistor. So we start with simple stuff....a 100 Ω resistor circuit. We then add a 2nd identical resistor in series for the 2nd experiment. In the 3rd experiment we use a 100Ω and a 47Ω in series. Baby steps. Eventually we introduce the low power load resistor. At this point the training wheels get removed. The fact that we have to work with 100Ω resistors and less is just a consequence of trying to prove the new objective. I would have like to use high value resistors but do they come in 10W ratings? Would I have been about to burn out a 1kΩ 1/8 watt resistor easily?

 

Note that the ammeter resistance may change as you change the scale setting.
Thus you may see that the current reading change some when you change the scale.

 

Have you thought of using TWO meters? One for voltage and the other for current. By having two meters, the circuit would not be changed while various measurements are taken. The current meter would remain in the circuit all the time and you could even measure the voltage across the current meter to determine its internal resistance using Ohm's law. One other point is the resistance of the test leads for the current meter must be accounted for. The test leads on my Fluke 77 measure 0.2 ohms.

 

We simplified the project a bit to make the procedure a little more straightforward. By doing what he is doing for his project it already exceeds what others in his class are doing in terms of research required. So rather than measuring actual current in the circuit with an ammeter in series (and having to account for the DMM's internal resistance), we are just setting up an Excel table that contains preset voltage values (3, 6, 9, 12) and three different values for the current limiting resistor R1 (100Ω, 47Ω and 5.1Ω). After calculating the theoretical current (i.e. which assumes the power supply preset is exact and the two resistors are exact), the circuit will be connected and the actual voltage of the power supply will be measured. That will be compared to the sum of the voltage drops across the two resistors using DMM1 and DMM2. They should be fairly close if not exact. The exact resistances of R1 and R2 will also be measured as well prior to connecting the circuits. Knowing the measured voltage drops and the actual current in each circuit (based on the measured voltage of the supply and the combined measured restistances R1 and R2) the power dissipated in each resistor will be calculated using both formulas for power: P = V2/R and P = I*I*R. Two two results should be identical. The final (17th actually) column in the table will be used for recording observations of senses (sight, touch, smell) to describe how the quanitity of power dissipated in the load resistor for each circuit affects it (it is cool, warm, hot, does it smell, did it go up in smoke, etc).

 

Sounds reasonable and sidesteps some math that he probably isn't ready for.