So, you hear all around you about electricity in everyday life from the toasters to hand-held gadgets. You hear terms like voltage, current, watts and wonder what these things mean and how they are related? The difficulty is most likely that you can’t readily see electricity with you eyes. You only see the effects of electricity at work.
So, at the risk of over-simplification, we’ll relate this electricity to something more tangible and to something most people are more familiar with: garden hose.
First of all, here are some basic electrical terminologies:
- Voltage–units in volts(V)
- Current– units in amperes(A)
- Resistance — units in ohms(Ω)
- Power– units in watts(W)
People often confuse voltage and current and say that “the voltage is flowing in the wires.” Well, that’s completely incorrect. Voltage doesn’t flow. The current flows.
Now, imagine a common garden hose with water running through it.
- Think of the garden hose as the electrical cord.
- Think of the flowing water as the current.
- Think of the kink in the hose as the resistance.
- Think of the water pressure that you feel (when you pinch off the flowing water with your finger), as voltage.
Now, you see that voltage doesn’t flow–it’s the water(current) that flows.
So, now imagine the hose connected to an impeller or a paddle wheel. When the water flows, the wheel moves. More water, more movement. By restricting the water flow in the hose (resistance), less water gets to the wheel and the wheel moves less.
Consider the wheel movement as power produced to do some work, like running a mill. The power measured at the output shaft of the wheel is the output power. Consider the volume of water used as the input power. Less pressure, the less water flow, the less power going into the wheel. Thus the wheel moves less and produces less output power.
Now, let’s look at the water example as compared to the real electronic circuit. Take an electric DC motor and connect it to an electrical source, a battery. When everything is connected, the motor will spin at some maximum speed with the given voltage from the source.
If a resistance is placed in the wire, current flow is reduced and the motor spins slower. Power is the voltage at the point of entry (to the motor in this example) multiplied by the current going into the motor. The output power is at the spinning shaft of the motor. You can’t measure output power electrically. You have to refer to the input side. The only catch here is that when the motor converts electrical power to mechanical power, it looses some power in the process. The efficiency of the motor is output power divided by the input power and it’s always less than 1.0 or 100%.
There is also a misconception about power as well. Many people would measure the input power to the motor and say that the motor is producing the power that was measured. That is not exactly correct. It’s the efficiency thing again. If the motor is very inefficient, the output power is way lower than the input power.
For example: if the input power measured 100Watts of power and the efficiency of the motor is 0.75 or 75%, then the motor is not producing 100W–the output power is only 75Watts.
So, there it is in a nutshell.