Basic concepts in electricity

Question

I'm fairly new to electricity and just learning the basic concepts, and I wanted to ask a few questions:

1. One of the things that I couldn't understand from all I've read is the relation between Amperage and Voltage. Does certain voltage predict certain amperage across a wire? How is it calculated?

2. Voltage - From what I understand (which could be wrong), voltage can be understood as the strength in which electrons flow in a circuit. It is explained in few places as "The force that pushes electrons within a circuit". Volt is a measure of Joule/Coulomb - meaning - how much work can a group of electrons do. In this video, he uses the water analogy to explain voltage in the following manner:

Voltage is like pressure in a water pipe. The more pressure you have - the more water can flow. The more voltage you have - the more can flow.

But as far as I understand, the more correct way to put it is:

The more pressure you have - the harder water flow. The more voltage you have - the harder electrons flow.

Isn't it?

3. Again the water analogy. The guy in the above video describes voltage like the pressure built when you put water in a tank. The weight of the water is what creating the pressure - so once you let water out, the pressure is constantly decreasing. The question is - is it the same with electricity? When I use a battery, does the voltage decrease as the energy is wasting?

Answer 1

1. Yes, depending on wire material, thickness and length, it will have resistance. What ties voltage, current and resistance together is simply Ohm's law.

2. Yes the more voltage you have there will be more current flowing into a fixed load resistance (see Ohm's law again).

3. Yes batteries will deplete and give lower voltage out when used compared to fresh batteries.

Answer 2

1. Does certain voltage predict certain amperage across a wire? How is it calculated?

Current flows through a wire. Voltage is measured across the wire. The current that flows through the wire depends on the voltage applied to the circuit and the total circuit resistance - not just that of the wire. Ohm's law then allows you to calculate the resultant current from \$ I = \frac V R \$.

2. Voltage ... can be understood as the strength in which electrons flow in a circuit.

Don't get hung up on electrons. While they are the most common mobile charge carriers there are times when positive charges flow too such as in liquids. Think of voltage causing current to flow from positive to negative.

The more pressure you have - the harder water flow. The more voltage you have - the harder electrons flow.

The more pressure / voltage you have the more current will flow as shown by \$ I = \frac V R \$.

3. The question is - is it the same with electricity? When I use a battery, does the voltage decrease as the energy is wasting?

Not quite. A battery is made of chemical cells. The output voltage is a property of the chemistry and an ideal battery would maintain its rated voltage until all the chemical energy was consumed and then go suddenly flat. Practical cells exhibit a voltage droop as shown in Figure 1.

Figure 1. Battery discharge curves for various chemistries. Image source: MPowerUK.

You might find the linked article of interest.

Answer 3

Does certain voltage predict certain amperage across a wire? How is it calculated?

Yes, voltage does "predict" the current. Current = Voltage / Resistance.

Higher the voltage - Higher the current.

2.The more pressure you have - the harder water flow. The more voltage you have - the harder electrons flow.

Well, not really, as in the water analogy, the more pressure you have, the more water can flow through the pipe. Again, as I described with the formula, higher the voltage (the pressure in the pipes), higher the current (the water flowing in the pipes).

3.Is it the same with electricity? When I use a battery, does the voltage decrease as the energy is wasting?

Yes, when you use a battery it's voltage goes down and by that the current that it can supply to the circuit.