Lesson 0: Section 2: Voltage, Current, Resistance, Power

Section 2: Voltage, Current, Resistance, and Power

This section will describe some of the ways that main concepts in electronics, using mechanical analogies.

Figure 1.3.1: Ubiquitous Water Tower Analogy

Voltage

While the water analogy isn't perfect, the first analogy I'll provide does help many with understanding these concepts- consider a water tank and a water tower. The water represents electrons. Water in a tank on the ground can provide a certain amount of water pressure at ground level. Move the same amount of water up to to top of the tower, and we have increased the pressure that can be provided at ground level. The increase in water pressure is caused by the increase in potential energy. We did work on that water, so we can get more work out of the water. Voltage is the level of pressure (relative to ground) per the amount of water... that is, the amount of energy per unit of charge.

Current

If we let water flow through a pipe, the amount of water going through the pipe at any given cross-section is the current. At your faucet, if you open your tap a little, you get a weak current. If you open your tap a lot, you get a strong current. It's one of those happy times the words and the concepts match up.

Figure 1.3.2: Current from a small tank versus a big water tower

Resistance

If you think about how much pressure is in your pipes, a lot more water could flow than your tap is letting. Your tap is putting up a resistance to the current flow. In electronics, resistors resist the flow of current- bigger resistors allow less current to flow. The faucet resists water flowing more than a garden hose, which keeps the current lower. The faucet is actually a variable resistor, allowing more or less current to flow.

Figure 1.3.3: The water pipe and the water drain resist the flow of current

Capacitance

The basin in Figure 1.3.3 acts as a capacitor, it stores the water if the current is too much for the drain to handle. As the basin gets full, the water in the basin applies more pressure on the drain. If the current stops flowing from the faucet, the basin will keep current from through the drain for some time.

Inductance

Inductance is a little more difficult to describe with a water analogy. Imagine if a turbine is placed inline between your faucet and the shut-off valve (Figure 1.3.4). When the tap is open at the faucet, it will take some time to spin up the turbine. The water will not flow quickly at first. The turbine reacts against the start of the water flow by taking some of the energy from the moving water and turning into rotational spin. If the shut-off valve is turned off, the stored energy in the turbine will pull water out of the reservoir and keep moving water out of the pipe for some time. Again, the turbine reacts against the stop of the water flow by taking energy from the rotational spin and turning it into moving water.

Figure 1.3.4: The turbine behaves like an inductor (click to make bigger)

Power

Let's say you have a pipe coming out of your water tower, and it is going to power a turbine. You would like to know how much power you could reasonably get. Intuitively, it should be a function of the pressure (our ability to do work), and it should have something to do with how how fast the water can flow... a trickle won't provide a lot of power. It turns out that:

Mechanical: Power = (Force from pressure) * Velocity

Electrical: Power = Voltage * Current

Circuit Model

In the water basin analogy above (Figure 1.3.4), we drew a series of pictures and lines to represent a system. In electronics, circuits are drawn using a set of standard drawings which represent the components. A representation of a circuit is called a schematic, and the schematic version of Figure 1.3.4 is shown in Figure 1.3.5.

Figure 1.3.5: The circuit model equivalency