Few questions about basic concepts in electronics

Question

So I'm rather new to electronics and I'm struggling to understand some basic concepts because I picked up little information from many different sources so it's starting to get confusing :/

1. Why does current flow from positive to negative when electrons flow negative to positive? I thought current was the flow of electron

2. I think question 1 will answer this but why do you need to connect both sides positive and negative in order to power something? i.e. led

3. Why does voltage keep on rising and falling in an ac circuit?

4. I heard that ground is basically the negative side of the power source as it has 0 volatage(potential) but I thought when powered by a 1.5 volt battery the entire circuit had 1.5 voltage all around.

thank you :D

Answer 1

First thing you need to do is to understand amps and volts, then it all gets clearer. Electrical power is not like a substance which is supplied, it is a flow. The electrons move around a circuit and as they pass through things they make stuff happen. It is a bit like a hydraulic system where a pump pushes fluid around a system and as it passes through hydraulic motors etc it makes them work. In order for the electrons to go round and round there must be a loop. The battery is like a pump pushing them around. They flow from the high pressure (at the "pump" output) towards the lower pressure ("punp" return). It does not matter how much pressure there is in the system, what counts is how much pressure difference there is between one end and the other, this is what forces the electrons to move. The term for this "pressure" in the electrical circuit is voltage. For this reason, it only makes sense to talk about voltage difference between one place and another, never the absolute voltage at a point. It is a pain to have to keep referring to difference between one point and another when we want to talk about the voltage at some point in the circuit so we just CHOOSE a convenient point in the circuit (often, but not always the negative supply terminal) CALL IT 0V. Then when we say there is 10V at the output, it means "10V difference between the output and the 0V point".

To answer why current flows the opposite way to electrons, in the early days of electricity, cleaver scientists worked out that the strange effects observed in the laboratory (when coils of wire, connected between metal plates in acid, produced magnetic fields) must be produced by something flowing through the wire. They also figured that there must be a force which made the flow happen. They called the force "voltage" and the flow "current". They did not at that stage know what particles were involved so they made an arbitrary decision and assigned + and - to the voltage and a direction to the flow. It turns out that electrons actually flow the other way, but it does not matter, all the maths works and it too late to change it now.

AC is often produced by a rotating machine that uses magnets and coils of wire. As the machine rotates the magnets cause a constantly varying magnetic field to pass through the coils. During 1 rotation, the direction of the magnetic flux will reverse and return to the initial state. This induces varying voltages across the coils. Voltage changes from 0, builds and then falls, reverses polarity, rises then falls back to zero. This cycle repeats. There are some advantages to using AC in transmission of power as it is easy to use a transformer to change the voltage. With a DC system this is more complex, but modern electronics means that it is now easier to change one DC voltage to another, and DC transmission systems are becoming more popular.

Answer 2

1. Why does current flow from positive to negative when electrons flow negative to positive? I thought current was the flow of electron.

Current flow convention was established before the discovery of the electron. There was a 50/50 chance of guessing right. They didn't! We still use the positive to negative current flow convention but keep in the back of our minds that the electron flow is the opposite direction.

2. I think question 1 will answer this but why do you need to connect both sides positive and negative in order to power something? e.g., LED.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Open-circuit.

The current flows in a circuit. If you think of your LED example, the battery can't supply charge without it returning to the other terminal otherwise it would gain or lose electrons with the imbalance getting larger and larger. The atoms wouldn't like that!

The other problem with your idea is that we would never be able to switch anything off!

3. Why does voltage keep on rising and falling in an ac circuit?

By definition it is alternating polarity. The alternation occurs 'naturally' in many rotating machines such as alternators. As the windings move through the magnetic field the voltages and currents rise and fall as the sine of the angle between the winding and the magnetic field. The pattern repeats on each 180° but in the opposite sign (polarity).

4. I heard that ground is basically the negative side of the power source as it has 0 volatage(potential) but I thought when powered by a 1.5 volt battery the entire circuit had 1.5 voltage all around.

When surveying the land a surveyor has to pick a reference height which is his 'zero' or datum. The height of every other point is measured above or below that. She can, if it suits, use sea level as the standard reference.

In an electrical circuit we pick some point as being zero or ground. Usually it's the battery negative (as on a car electrical system) but it can be the positive (famously on the old 6V VW beetle) and in some cases we need a split-rail power supply and we can call the centre one 'zero' and have a '+' and '-' with respect to that.

^{simulate this circuit}

Figure 2. Voltages around a circuit.

Here we have a simple circuit. We wish to power a 6 V lamp from a 9 V battery. If we connect the two up directly the bulb will blow because too high a current will flow. By adding a series resistor we can limit the current to the lamp to a safe value. We've put in a 'ground' symbol on the battery negative to signify that we're referencing our measurements from that point. 'A' will read +9 V relative to circuit ground. 'B' will read + 6 V with respect to ground because of the divider effect of the 100 Ω and 200 Ω resistances. 'G' will read 0 V. In your 1.5 V battery example you will have similar setup and you won't get 1.5 V 'all around'.

Finally, if the circuit common is connected to earth (as in The Earth) we take that as a real zero volts, in much the same way as the surveyor might use sea-level as an absolute reference.

Figure 1. The building on the ground and shot off into space. In the space situation (electrically isolated) we can call any floor the 'ground' floor.