Principles of Electricity

Units

(V) : Voltage

Measure in volts.

Think of this as the “pressure” in the electrical circuit.

USB devices run at 5 volts. Typical American household circuits are 110 volts. Larger appliance “two phase” circuits that run a dryer, for example, are 240 volts.

Water Analogy for Volts

In a water-based analogy think of controlling the voltage as turning the spigot on a garden hose.

Sidebar about Current: In this example assume our current , the diameter of the hose, stay steady. To help visualize this think of amps as one of several settings on a garden spray adapter; a small hole (low amperage) , a medium hole (moderate amperage), and a large hole (high amperage). We are going to ignore the effects of the increased pressure in a water-based system and focus on the size of this hole as the diameter of the hose overall. Think of this spray handle as being set to the medium hole (mid-amperage) as we adjust voltage.

Turn it just a little and a trickle comes out the hose due to the lower pressure = 5 volts. The water comes out at a stead but slow pace, barely making out of the end of the house and making a puddle at your feet.

Turn it half way open and you get a lot more pressure causing the water to flow faster, think of this as 120 volts. Here the water shoots halfway across the yard making a larger puddle.

Turn it up “full blast” and you get a lot of pressure and a deluge of water, this would be 240 volts. On this setting the water shoots all the way across the yard making a big puddle.

(I) or (A) : Current

Measured in amps.

The instantaneous “volume” or carrying capacity. How many electrons are flowing past a single point at one time.

A fast charger for your iPhone or tablet is around 2 amps. Typical American residential circuits are 15 amps for most outlets that will run a television, table lamps, and other “plug in” devices. Larger appliances like a dryer or oven tend to be placed on 30 amp circuits.

Water Analogy for Current

In our above example for volts, we talked about the amps being the garden spray adapter on the end of the hose. For the examples here, assume the voltage — how far we “opened the spigot” to be steady and set to the halfway mark.

At low amperage, the “2 amp setting”, we have this set to the smaller hole. The voltage is midway at 120 volts. Our circuit only allows water droplets to flow through it at a smaller rate due to the size of the opening. We get a moderate sized puddle on the other end due to the low amperage. It is a little less than halfway across the yard due to the lower amps of the circuit.

We can crank up the amount of power we have by increasing the amperage (the “10 amp setting”). With the voltage steady , turn the spray adapter to the middle setting with a mid-sized hole. This is like increasing the amps in the circuit. More water flows out, it shoots the same distance as the pressure (voltage) is the same but our puddle gets bigger faster. More water in less time thanks to the higher amperage. It is halfway across the yard as our voltage example (midway open) and amperage example (middle diameter spray opening) match the voltage assumptions above in that middle example.

Turn this to the adapter to the largest diameter setting (“30 amps”) and we get a big puddle a little more than halfway across the yard fairly quickly.

(R) : Resistance

Measured in ohms.

The “back pressure” of a circuit. Typically larger conduits (wires) have lower resistance and allow the electricity to run more freely. The size of the wire, known as the guage, the length of the wire, and the material it is made of have an influence on the resistance.

Larger diameter wire lowers resistance.

Shorter lengths of wire lowers resistance.

Metals with lots of free electrons in the outer shell of the atom lower resistance.

Water Analogy for Resistance

In our voltage and current example above we can easily visualize the rate of flow of water based on how far we open the spigot (voltage in volts) and how much we open the diameter of the garden sprayer (current in amps). It is a bit harder to visualize the resistance.

The resistance of the circuit is the back pressure. What does the water have to overcome to make it out the end of the house.

For resistance, think of it as crimping the hose. The initial state is very low resistance. There is some thanks to the garden spray head as well as the original diameter of the hose, but let’s consider that nominal.

To add resistance, which will slow the flow of water and ultimately reduce the overall output (Power) of the system, you can partly crimp the hose — just fold it a bit with your hands. Here you increase resistance and reduce the overall output.

You could also make the hose a lot longer or just us a smaller diameter hose to increase resistance. To overcome that you’d need to add more “push” the system by increasing voltage to get the water out the other end at the same rate.

(P) or (W) : Power

Measured in watts.

Amount consumed in an instance.

When referring to total amount consumed, “the volume”, the common term is Watt-Hour (Wh). The larger unit Kilowatt-hour (kWh) is often cited since we tend to use so much of it in modern society.

A typical small home in America will use around 1 kWh every month.

Water Analogy for Power

Here this is the “size of the puddle” we are making and how quickly it happens , representing the work that has been done. Think of it as the size of the puddle in a very short amount of time that we cannot measure easily — a 1-inch diameter puddle that “happens in an instant”.

If we turn up the spigot twice as much, doubling the voltage, and assume the garden sprayer opening remains the same, we would get a puddle twice the size.

Alternatively we could keep the spigot set to the initial setting and open up our garden sprayer to a hole twice as big, we would also get a puddle twice the size.

The measure of how much power we used, the total volume of water, is a measure of the size of the puddle over time. The total volume of water is the “Watt-hours” consumed.

Relationships Between Units – The Formulas

Depending on the source of information and the time period used the unit of measure can be represented in different ways. There is probably a formal explanation and “proper use” for each , but for simplification this article will be using W (instead of P) for watts and A (instead of I) for amps. That is far easier to remember.

V * A = W

Multiplying volts (V) and Amps (A) gives you Watts (W)

A tablet charger using USB-standard 5 volts at 2 Amps (5 V * 2A ) = 10 W of power.

Charge your tablet for an hour and it will use 10 Watt-hours (Wh) of power.

W / A = V

Dividing Watts (W) by Amps (A) gives you Volts (V)

W / V = A

Dividing Watts (W) by Volts (V) gives you Amps (A)

A 100 watt solar panel that outputs 12 volts generates (100 W / 12 V) = 8.33 A.

In an hour it will push approximately 8 Amp-hours into a 21Ah battery. It will take about 2 1/2 hours to fill the battery at that rate.