Voltage and Current

The easiest way to think about voltage and current is the analogy of electricity to water, and the analogy of a conductor to a pipe. In simple terms, voltage is the electrical "pressure" that pushes charge carriers through a conductor, while current refers to the volume of charge carriers moving through the conductor. The standard measurement for voltage is the volt, abbreviated V. The standard measurement of current is the ampere, abbreviated to amp or A.

To complicate matters, there are multiple terms for voltage, although they all mean the same thing. Alternate references to voltage include electro-motive force or EMF, potential, and less commonly these days, tension.

A DC voltage source has a positive pole and a negative pole. You can't really say the the current flows from one to the other, since you could either say that electrons flow from the negative pole to the positive pole, or that charge carriers called holes flow from the positive pole to the negative pole. Knowing this isn't really important, but it's surprising how many people think electricity flows in some arbitrary direction. Despite their common meaning, there's nothing inherently positive or negative about the positive or negative charges. It's just a convenient label, similar to the concept of using up and down to refer to different types of quarks. Positive and negative voltages may be referenced to each other, or to ground, which is the electrical potential of the actual ground, and is always considered to be 0V. Many rectifiers that convert AC to DC only provide either a positive or negative pole, and allow the ground to act as the other pole.

An AC voltage is one that is constantly varying. Common household electricity in the US alternates between positive and negative 60 times per second (60 cycles/sec or 60 Hz), with the voltage referenced to ground. An audio signal from an amplifier is another example of AC. Measuring AC voltage is somewhat more complicated than measuring DC, since just giving the "average" voltage would result in 0 for an AC voltage with a constant frequency, and would change with frequency with a variable voltage like an audio signal. Fixed frequency AC voltages like mains power are usually given in a measurement called root mean square (RMS). A standard US wall outlet operates at 120V RMS. Another method of measuring AC voltage is to measure the peak to peak voltage, that is, the difference between the maximum positive potential and the maximum negative potential. It is commonly abreviated pk-pk. This is the measurement most commonly used in measuring signals, for example a standard composite video signal has a voltage of about 1V pk-pk. It's important not to substitute one for the other, since the differences can be dramatic. For example, the 120V RMS wall outlet mentioned earlier would be over 300V pk-pk.

Resistance

Whether AC or DC, the current traveling through a line is affected by its resistance, which is measured in ohms, abbreviated as Ω. Higher resistance offers more impedance to current without affecting voltage. If you know the voltage and resistance, you can calculate the amount of current that can flow. Let's try it now, using 12 volts and 1000Ω of resistance, using the formula known as Ohm's law: current = voltage / resistance. Calculating 12V/1000Ω gives us 0.012 amps or 12mA, meaning that 12mA will flow through a 1000Ω resistor connected to a 12V source.

If you know the voltage and current used by the system, you can calculate its power consumption, which is usually measured in watts and abbreviated W. The formula to do this is voltage × amperage. For example, a lamp drawing 1 amp of current at 1 volt is using 1 watt of electricity. Another term for this is volt-ampere, which is abbreviated as VA. Since all of these equations are reciprocal, you can rearrange them to find out an unknown third value when two or more variables are known. For example, if we know that a lamp is using 60W at 120V, we can calculate the current flowing through it as 60W/120V = 0.5A. Now that we know the lamp's current, we can even calculate its resistance by dividing current and voltage 120V/0.5A= 240Ω. Same deal if we knew the current and resistance, and wanted to find the voltage, 0.5A × 240Ω = 120V. Once you're familiar with these formulas, you'll be surprised how often they come in handy.

Balanced and Single Ended Signals

There are two ways of determining the amplitude of a signal; balanced, and single ended. In the type of electronics engineering you'll be doing, the two major things that can be balanced or single ended are transmission lines and amplifiers.

In a single ended signal, there is a single waveform, and its voltage is referenced to ground. Single ended transmission lines usually have an outer shield that serves as the ground, as in coaxial cable. Single ended amplifiers have a one conductor input with all internal and external voltages referenced to a common ground.

A balanced or differential signal has two identical waveforms, which are 180° out of phase with each other,  which means that the peak of one wave is happening at the same time as the trough of the other one. A balanced circuit may have an external ground, or the voltage can be derived from the difference in the two waveforms. The advantage to a balanced circuit is a reduction in noise, especially for transmission lines. When the differential signal is received by something like an amplifier, one waveform is inverted, bringing them into phase with each other, and the two waveforms can be summed. Since the noise riding on one of the waveforms was inverted and the other one wasn't, the noise is 180° out of phase itself, and adding the two waveforms together cancels out the noise without damaging the signal. Common examples of balanced circuits are phone lines, ethernet networks, XLR cables, 300Ω twin lead antenna cable,  and most amplifiers-on-a-chip.