All matters are made of atoms

All matters including the humans are composed of atoms. Yes, all are the sum of bunch of atoms and atoms have nucleus, electronis and protons. We will talk about electronis for automobile applications.

Electrons move from negative to positive

Yes, electrons move from negative to positive charge. But then when electrons move along the different atoms, energy is produced. This energy is electricity. The funny thing is that although electronics flow from negative to positive, energy actually flows in the opposite direction. Yes, energy (electricity) flows from positive to negative.

What is voltage (V)?

Voltage expressed in V is nothing but the difference between energy or electricity. Energy flows from higher voltage to lower voltage. It is like a waterfall. Energy will not flow if there is no voltage or energy difference.

What is current (A)?

Current shown in ampere (A) is the amount of energy or electricity. There are two kinds of currents which are direct current (DC) and alternating current (AC). In automobile, mostly 12VDC is used. However, some vehicles use 48DCV as well.

What is resistance (Ω)?

Resistance shown as Ω is the obstable to flow of current. In automobile, fuel guage sending unit and throttle body use resistance to calculate the values.

Whey should I disconnect battery negative post first?

The big concernt is the spark that could damage the senstive electronic sensors and circuit board used in the automobile. If negative connector is disconnected first, then there is no place for energy to flow to. This will eliminate the possibility of spark. That is why disconnecting the negative battery terminal is the must.

Ok, then why connect battery positive first when installing the battery?

If battery negative post is connected first, then the entire vehicle becomes a huge battery negative or ground. There is so much room for energy or electricity to flow into. However, if battery postitive post is first connected then there is no place from energy to flow into, which eliminates the spark possibility.