# DC Voltage Polarity and Current Flow?

So I'm getting pretty familiar with Electrical Theory but one issue keeps bringing me problems. In a DC circuit, there is a negative (-) and a positive (+). I am aware that in electron flow theory, current flows from the negative (-) to positive (+). See Figure 1. But in a car battery, the positive terminal is considered the hot lead and the negative terminal is ground. If you take a wrench and connect the negative (-) terminal to the metal chassis of the car, it won't short, but when you short the positive (+) terminal to the metal chassis, it does. See Figure 2. My question is.. How is it possible for the positive terminal of the battery to be hot when, there is positive charge on the terminal? ? If electrons are supposed to flow from negative(-) to positive(+), wouldn't the negative terminal be considered hot and not at ground potential? Also, when the positive(+) terminal is shorted to ground, is that positively charged terminal just allowing negatively charged electrons to flow from the ground? Please Help!

simulate this circuit – Schematic created using CircuitLab

• In fig 2, is your battery -ve connected to ground, or isolated as you've shown? It makes a big difference. – Neil_UK Jul 20 '17 at 5:32
• isolated, therfore current should not flow. Right? – bittersweet Jul 21 '17 at 4:41

Electrons are flowing from negative to positive when there is a connection for them to flow. You can also think of it as "holes" (or the absence of electrons in an atom) flowing from positive to negative.

Because of historic reasons, we generally think of conventional current, or the "holes", rather than the actual electron current. You are correct that the positive terminal just "pulls negatively charged electrons."

There is also no magical node called "ground." We simply assign a point that is easy to keep track of to refer to as ground. In a circuit, you could refer to the positive terminal of the battery as ground, and simply be working with negative voltages (though it would likely cause some confusion for others interpreting your work).

Likely in the case of shorting the battery to the chassis of the car, the negative terminal of the battery is already connected to the chassis. Possibly this is done for some sort of EM shielding (vehicles are inherently very noisy environments). When you make another connection from the negative terminal of the battery to the chassis, no current flows. When you connect the positive terminal, current flows through the chassis of the vehicle to the negative terminal.

• Some older vehicles has a 6v battery with its positive side connected to the chassis. These were called "positive ground" systems. – Chris Stratton Jul 20 '17 at 4:02

In your figure 2, there will be no current flow, as there is no complete circuit - the negative terminal of the battery is not connected to anything.

"Ground" has no magic properties - it is (for most circuits) just the point in the circuit that we choose to call "Zero Volts".

Please try to forget about electron flow - most people speak in terms of conventional (positive) current, although we are aware that, in most materials, current is actually a movement of negative charges.

• Ok, I will think in terms of conventional flow theory . But back to referring to figure 2, if the negative terminal was connected to "ground", let's say the metal chassis of the vehicle, current will flow and that is the reason why it would short. Right? – bittersweet Jul 20 '17 at 4:21
• Yes - if both terminals of the battery are connected to ground (or connected to each other by any other means) you will have a short circuit, and a very large current will flow. – Peter Bennett Jul 20 '17 at 4:55

Ok Thanks!, I get it now. The reason why the positive terminal would be considered "hot" is because the negative terminal which is connected to the metal chassis at 0 volt potential will short to the positive terminal if something metal were to allow electrical continuity. AND yes, I now understand that positive voltage is just the potential difference between a positively charged area and a negatively charged source with the voltage "pulling" the electrons from the negatively charged source to the positively charged area.

simulate this circuit – Schematic created using CircuitLab