Let’s talk about numbers in circuits. Our goal is to develop numeracy:
More precisely, we want to develop a sense for scale i.e. a sense for what kind of numbers are physically reasonable.
We all have an intuitive sense of scale for the physical world. More precisely, we can very easily tell when physical quantities are absurdly large or small. As an example, if we are asked to estimate the weight of a bag of apples, we know immediately that 50 mg or 5000 kg are both absurd numbers. However, because the electrical world is more abstract and not directly accessible to our senses, such intuition is not naturally acquired, but instead must be deliberately cultivated.
In the PEI class, we are concerned with a class of “low frequency” electronic circuits. What frequencies are considered “low”? Specifically this means circuits where the timescale of voltage and current fluctuations is slow enough that transmission line effects can be neglected. This means that we can neglect the fact that electrical disturbances travel at the speed of light which is approximately 1 ft/ns. A sine wave at a frequency of 100 MHz has an oscillation time of 10 ns. At frequencies higher than this (with shorter oscillation times), the finite speed of light cannot be ignored; wires no longer behave like short-circuits nor air gaps as open circuits. This is the regime of RF and microwave circuit design. In our class, we work with frequencies of 10 MHz or less where we can comfortably ignore such “high frequency” effects.
Time. So this is a good place to start: for timescales, in our class we are concerned with timescales significantly longer than 10 ns, and usually shorter than 100 ms or so.
Voltage and current. Voltages are usually within a couple of orders of magnitude of 1 V i.e. 10 mV is a very small voltage and 100 V is fairly large. A Volt is an excellent unit for voltage. Sadly the Amp is a poor unit for current: 1 A is a very large current! 1 mA is a much more comfortable unit and we usually work with currents within a couple of orders of magnitude of this.
Resistance. Once we have the above, it is a matter of simple dimensional analysis (which is a valuable tool for numeracy – learn it!) to figure out reasonable range of values for other circuit quantities. We will illustrate with resistance. A small resistance would produce a small voltage drop even with a fairly large current e.g. 50 mV with 50 mA of current. Thus 1 
What can you infer about other quantities such as charge, energy, power, capacitance and inductance?
hawkee 