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Discussion in 'General Electronics Discussion' started by Paul Nelthorpe, Aug 7, 2013.

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  1. Paul Nelthorpe

    Paul Nelthorpe

    Aug 7, 2013
    Hey all, I'm very new to electronics and I'm still yet to get a grasp on some of the concepts. I purchased the book "Hacking Electronics," And so far it has done a pretty good job at explaining things.

    However I can't seem to wrap my head around the current flow of a circuit diagram. When it's a relatively simple diagram such as this one I totally get it
    so in something like this, I can easily see that the current flows from the positive end of the battery, around to the switch, through the lightbulb and back into the negative end of the battery.

    Now where I get confused is with something like this
    or this [​IMG]

    I simply don't grasp the concept of the current flowing in multiple directions at once. I know a lot of the symbols on the diagram (Resistors, transistors, capacitors) and I've done a good amount of research into what they actually do, but I feel like that knowledge is totally lost on me when I can't grasp the concept of a circuit diagram that seems to have a current flow which branches out in more than one direction. How do I know which of these wires are positive and which are negative?

    I apologize if some of my terminology has you guys rolling on the floor (either laughing or crying) but if anyone can understand where I am having trouble and explain it to me, that would be great, I feel like a simple knowledge of current flow would help me out a great deal.
  2. john monks

    john monks

    Mar 9, 2012
    Perhaps you are confused because you don't fully understand what voltage is and current is. So let us begin.
    Voltage or electromotive force. This should be called pressure, not force. It is similar to water pressure in a hose or in a water line like in a house. So let's say that a house has 30 pounds of pressure in the water line and you have the kitchen faucet running very slowly. You would agree that this would not affect the pressure. And if you had the garden hose running slowly at the same time. You can see that one faucet will not affect.
    Current or amperage. This is the rate of flow through some device or another independent of of the pressure. For example your sink may have one flow and your garden hose another. But the pressure is the same in both.
    Resistance or opposition to the flow of current. This is similar to the resistance of your kitchen faucet and your garden faucet. The more the resistance to slower the flow and this will not affect the pressure very much as long as the water is running slowly.
    Your last circuit shows many things that is in affect leaking current through them .each branch has current flowing through is inversely proportional the the resistance in the device. And like faucets the pressure or voltage across the devices is the same.

    So please tell me where you are confused and I will try again.
  3. Paul Nelthorpe

    Paul Nelthorpe

    Aug 7, 2013
    Cheers for the reply john.

    I'm pretty sure that I grasp the concept of voltage, resistance and current, and I have heard many analogies to describe it such as water flowing through a stream, or plumbing and faucets around the house (your analogy).

    Where these analogies don't work for me is in the polarities. When you have a water faucet running water, the circuit ends where the water comes out.

    The way I understand circuitry is that the power supply provides both a positive and negative charge. The positively charged electrons flows through the circuit in order to reach the negative charge. This makes perfect sense to me when the circuit is a simple, one direction flow (such as the first picture.)

    Now look at the second picture. My logic at the moment would say that the current flows through the resistor and then back to the power supply, but I can't understand why the current diverts through the multimeter (V) and then back to the resistor again. In my mind, it makes sense that the positive electrons would go straight past the multimeter and back to the negative end of the power supply, as this seems to be the fastest way for the positive charge to become balanced.

    I know my explaining here is pretty abysmal, but I hope I've explained where my confusion lies.

    In short. Why does some of the positive charge divert directions instead of just going straight to the negative end of the power supply? As this would seem the fastest way for the positively charged electrons to become balanced.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Think of +ve as the source of the water, and -ve (or ground) as the drain through which it escapes, or perhaps think of the power source as a pump, with the +ve being the outlet and the -ve the inlet.

    No water gets created or destroyed (or lost) it just keeps circulating. There's always just enough to keep the pipes and things full.

    It's kind of easier to think of pipes full of water rather than taps here.

    Imagine you have 2 pipes at one end both connect to the outlet of a pump, and at the other both connect to the inlet of the pump. One of those pipes is very narrow, the other very large.

    Does all the water flow through the large fat pipe? Or does some water (perhaps an insignificant amount) flow through the smaller pipe?

    The latter is the case.

    There is a certain pressure across all sections of pipe and water will always flow through them as long as that pressure is not zero.

    Also note that the water only flows one way at a time, from the higher pressure to the lower.

    In your case, the voltmeter is a very (theoretically infinitely tiny) skinny pipe. Hardly any current flows. Almost all flows through the resistor. Because the voltmeter is a very skinny pipe, the amount of flow through it is small compared to what it is placed across. This means that the amount of current flowing through it does not affect (significantly) the current flow through the circuit.

    In contrast, the ammeter is a very fat (theoretically infinitely fat) pipe. It has the minimum of pressure (voltage) difference across it so as not to disrupt the flow of current around the circuit.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Just reading that, you might think "But what about wires? There is no voltage across them in a circuit, yet current flows through them?"

    The answer is that yes, there is a voltage drop across wires. It is very small, but it is finite. It is caused because wires have resistance (again, very small). And when a circuit is operating the voltage drop across all elements is completely in balance with the resistance of the circuit and the current flowing through it.

    This is called Ohms law (as opposed to Plumbers law, which, if it existed, would probably say something about pressure, flow rate and pipe sizes)

    Ohms law says that Voltage = Current x Resistance

    Other important laws are Kirchoff's laws which basically state that -n + n = 0

    In plumbing terms they state that the pressure drop in a pipe is equal to the sum of all the pressure drops in each section of that pipe, AND the amount of water going into a section of pipe is the same as the amount of water coming out of it.

    All of these things have their subtleties, but once you "get" them, they're about as subtle as a sledge-hammer.
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