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Electrical Power and Answers to the Meter Scaling.

Two resistors in series

Before talking about power I probably need to define several terms. Energy is the ability to do work and energy used is equivalent to the work already done minus the energy wasted.  Power is energy divided by time or said another way, it is how fast the work can be done.  In Electricity power is measure in a unit called Watts.  (It is the same everywhere…. no metric to English divide!!!)  Electricity usage, energy consumed, is measured in a unit called watt-hours.

Watts is usually given the letter P and defined as Volts X Amps .  So in the circuit above the Source is supplying  10 Vots X 1 Amp or 10 Watts of power.  Each resistor has 5 Volts drop across it so each resistor is consuming 5 Volts X 1 Amp or 5 Watts of Power.  The power consumed = the power supplied.   If this happened for 2 hours we would have 20 Watt hours of energy produced and 20 Watt hours consumed.

If we had elements in the circuit that stored energy this would not be the case.  It is very similar to over-eating. If energy intake, in the form of food, then your waistline will expand. If you exercise more than you eat, you may have to cut some more holes in your belt.  We have yet to talk about those energy storage elements, capacitors and inductors, but it is coming eventually.  The point I want to drive home right now is that in our simple circuits so far we are at “steady state” conditions and power and energy in = power and energy consumed.

The 5 watts of power consumed by each resistor is being converted into heat.  One of the specified values of a resistor (and other electrical components) is its power dissipation value.   In the case of resistors, this is how much power it can radiate without overheating in a 70 deg C environment (158 deg F).

If by Ohms Law V=I*R and P=V*I  then P = I*I*R.   Similarly, P= V*V/R.  Both of those equations are other commonly used versions of the power equation.   P=I*I/R is very useful once we get into AC electricity.

In the next Electrical Post I will be making a shopping list for you should you decide to experiment.  In that I will be making some recommendations for resistors and part of the specification will be the power rating for the resistors.

Voltmeter Scaling Calculations.

I promised last week I would show the calculations for the resistors for the Voltmeter and Ammeter scales.   The Voltmeter calculations are shown to the right.   The additional R and Normal units value at the values of an additional resistor added in series to those already used for the next lower range value.


Ammeter calculations for a constant 0.25 V drop across the Ammeter.

The Ammeter resistors are shown for an additional resistor added in parallel to the already existing network obtained for the previous lower range.  You will note these values are very odd numbers and very small on the higher ranges.

Looking at the actual voltage drop figures Simpson provided, I understand what they did.  They would add an extra resistor in series with the network obtained for the previous range.  This keeps the resistor values in a more workable value.   I have calculated the values but have not drawn the schematic yet.

As stated above I will come up with a shopping list for the next Electrical post and then we go off into some simple but real electronics.


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