Voltage, Current, and Resistance | Course of Microprocessor

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Fig 1. Voltage in an electrical circuit is analogous to pressure in a fluid.

In figure 1, a battery provides a voltage source for electricity, much like a pump provides a pressure source for a fluid. Voltage, or pressure, is required to produce current flow in the circuit. The voltage source provides the pressure "motivation," if you will, for current flow.
Resistance provides a limiting constraint on the amount of current that will actually flow. The resistor will allow a current to flow through it that is proportional to the voltage across it, and inversely proportional to the resistance value. Higher resistance is like a smaller aperture for the fluid to flow through. The resistance results in a voltage, or pressure drop, across the resistance as long as current is flowing in the resistor, Figure 2 bellow.


Fig 2. Resistance in an electrical circuit is analogous to a restriction in the flow of a fluid.

The wiring connecting the components in a circuit is like the piping connecting plumbing components that let a fluid flow. The flow of current in the circuit is controlled by the magnitude of the voltage (pressure) and the resistance (pressure drop) in the circuit. In Figure 3, the battery provides a voltage to force current through the resistor. The magnitude of the voltage (V) generated by the battery is developed across the resistor, and the magnitude of the resistance (R), determine the current (I). Note the "return" current path is often shown as "ground," which is the reference voltage used as the "zero volts" point. In this case, current flows from the positive battery terminal, through the wire, then the resistor, then through the "ground" connection to the minus terminal of the battery. This is usually not the same as earth ground, which provides a connection to a stake or pipe literally stuck in the ground. The magnitude of the current in this case is I = V / R by re-arranging the equation V : I * R, as shown in figure bellow . This is known as Ohm's law. Another way to look at it is that whenever current flows through a resistor, there is a drop in voltage across the resistor due to the restriction in current.

Fig 3. Voltage across R is equal to current multiplied by resistance.

Real components are not the perfect voltage sources, resistances, etc. we have discussed so far. They have parasitic values that limit their performance in the real world and are subject to other limitations, such as operating temperature, power limits, etc. Current flows only through a complete circuit, and in most cases (for a positive power supply) current flows from the power source through the circuitry and returns to the power supply through the common "ground" connection. Current flowing through any resistance results in the dissipation of power as heat. The power dissipated is P = I²R = V*I = V²/R. Note that voltage is sometimes denoted by the variable V and sometimes by E, for electromotive force.

All practical components have some resistance. Real batteries have an internal resistance, for example, which provides an upper limit to the current the battery can supply to an external circuit. Real wires have resistance as well, so the actual performance of a circuit will deviate somewhat from the ideal. These effects are obvious in some cases, but not in others. In an automobile starting circuit, it's not surprising that the battery, supplying 12 volts to a starter with internal resistance on the order of 0.01 to 0.1 ohms, will result in currents of hundreds of amperes in order to start the engine. On the other hand, while consulting with a prominent notebook computer manufacturer, I uncovered a design error resulting in an internal current of hundreds of amperes flowing in the circuit for a few nanoseconds. Obviously, this wreaked havoc on the operation of the computer, and generated a great deal of electromagnetic noise!