1.7. Voltage versus Current
Let’s say there are two terminals separated by a space. Assume one terminal has more charge relative to the other, so the charge between the terminals is different. Different charges attract each other with a force that is related to the magnitude of the charge difference. This force creates an electric field between the terminals.
The force between these two terminals is defined as voltage. If we connected the two terminals together with a conductor, charge would flow from one terminal to the other. That charge flow is defined as current. In general, current cannot flow without a force “pushing” it along. If the two terminals had exactly the same charge (no force or voltage between them) and we connected them together with a conductor, nothing would happen.
An ordinary garden hose is a good analogy. Say the hose is connected to a faucet that is turned off. This is like a wire connected to the positive terminal of a battery through a switch that is turned off. The other end of the hose is lying open on the ground. Now turn the faucet on.
There is water pressure at the faucet (analogous to voltage) that is much higher than the pressure at the other end of the hose. So water flows through the hose in a manner analogous to current flowing through a wire. If we disconnected the hose while there was still water in it and left it lying on level ground, there would be no force pushing the water out of the hose. But if we raised one end of the hose higher than other end, gravity would provide the force to drain any remaining water from the hose.
There is a very well-defined relationship between the voltage (force) between two points and the current that can flow between those two points, depending on the impedance to current of the path that connects those points. This relationship is called Ohm’s Law, which is discussed in Chapter 3.
The nomenclature we use to define voltage and current can sometimes be confusing. For example, we say that a battery is a 9-volt DC (direct current) battery. This means the force at the battery terminal (absent a very heavy load) is 9 volts. But without a circuit between the two terminals, there is no current flow, direct or otherwise. The DC means that if and when we do connect a circuit between the terminals, current will only flow in one direction.
We often draw pictures of voltage and current waveforms without carefully distinguishing between them. We can get away with this because, at least with resistive circuits, the voltage and current waveforms look very much alike. But sometimes they don’t. When creating or looking at pictures of waveforms, it is important to be clear about whether they are voltage waveforms or current waveforms.