Wire Gauges, Ground Wire, Circuits, and Ohm's Law

The current density of a copper wire increases as the diameter of the wire decreases. As a result, the higher the current density, the hotter the wire, and the more collisions that occur. Sometimes, current density can overpower the copper-lattice binding energy resulting in wire meltdown, so you also need to select and appropriate wire size. The current capacity increases as the wire size increases. Here's the answer:

A load device is known to vary in output power from 0.1mW to 5W and is to be connected to a 12V source that was 10 ft. away from the load. What is the minimum wire gauge that can safely support the current?

I = P/V which is power / voltage which is 5W/12V = 0.42A. Then a 22 gauge wire with a 0.914 ampere rating will work because because we want at least 2x the current capacity for safety measures. 

We shouldn't connect a wire across a voltage source because you can melt the wire and possibly cause a nasty shock. This might also trip a circuit breaker. This might heat the battery or destroy the battery due to the levels of resistance in the battery. 

The next topic is on grounds. Understanding voltage is a relativity game. You often compare voltages to a 0V reference point, and this point is often called the ground. The ground is the reference point in which to base all the other voltage measurements. There are various ways in which to define voltages by selecting a ground. The 0V ground reference acts like a common return, and is often necessary where signals are sinusoidal and alternate between positive and negative voltage relative to a 0V reference. As a result, a symbol's dual meaning has survived. 

The ground symbol is supposed to represent a physical connection to earth through a conductive material buried by the earth. The earth ground wire is sent to various ac outlets in one's home through a green-coated or copper wire, and can be accessed through a ground socket. This is important because the earth is an electrically neutral body.

Due to earth's infinity charge neutrality, attempts of changing this will not really help, and the earth is zero ground. When a device is plugged in, the ground wire from the power plug (in green) is typically linking to the equipment chassis (frame) then bought out as a lead ground terminal, or a return portion emanating from interior circuitry, bought out as the ground lead terminal. Various power sources also share the same ground reference. To ground the DC supply, use a jumper cable, otherwise you are floating. 

Grounding reduces the chance that the electrical shock will fail and the chassis will become hot. The path of current flow will go to the ground but not through your body. DC ground is a ground system to prevent shock. It also helps eliminate electrostatic discharge when a statically charged body comes in contact with sensitive equipment. Use a grounded wrist strap to avoid destroying chips. The ground symbol. You should connect a load between the positive and negative terminals directly, creating a floating load, or using a jumper wire between ground and negative thus creating a grounded load. This is a common grounding error for beginners. The connection between negative and positive terminals results in a floating return which doesn't really affect circuit performance. The earth grounded symbol is used too generally and generically and can mean several things. 

The next thing I want to discuss is different types of ground symbols. The point is, the ground has to be, in a way, connected to the circuit.

Before you put the ground, always close the circuit. Here are the ground symbols: 

Here are the types of ground symbols: 

The loose ends on grounding is using a shock hazard caused by leakage, using the earth ground to eliminate the shock hazard, generating the ground loops, having a single point ground, and having a bus bar return. 

The most common cause of noise is the lack of good grounding practices, which is a major issue for practicing design and system engineers. Differences in potential points are caused by impedance in the ground line, that can cause troublesome ground loops. We can fix that by using a single point ground. This can be substituted with a ground bus. A bus bar carries the sum of all the load currents to the power supply. We want to make sure that the lead and ground connections are both secure. Bad connections lead to noise. 

Devices also should have analog or digital grounds separate and eventually connected together at a single point. Digital circuits generate spikes of current when signals change state, and analog circuits generate spikes when the load current changes. Ground impedance have resistance, inductance, and capacitance, and a high frequency AC voltage. The voltage are injected in local circuits and are considered noise. We want to keep the digital and analog ground separated and attach them at a single point.

Let's then define circuits in basic terms. Connecting load elements one after the other forms a series circuits and the current through all the loads in the series will be the same. The voltage drops in accordance to the resistance. A parallel circuit has load elements attached so that the voltage across each elements are the same. The effective resistances are 3x and 1/3 of a resistive element, respectfully. The electrons flow in the opposite direction as the current.  

And here's the combinations of series and parallel. 

The first is an analog circuit ground that is terminated at the supply to an actual earth ground connection. The second is a chassis ground to prevent shock hazards. The third is the analog ground linked to both the chassis and earth ground. The next is a floating earth ground connected to the return ground. The fifth is separate analog and digital grounds.