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¤The statement above this circuit in my book reads 'the (conventional) current should flow into the ammeter through the positive red terminal and leave through the negative black terminal', as the switch is open here, in a real setup ammeter would not show the electric current measurement as the current won't be able to pass to it, so why do we still use Conventional current,isn't it actually wrong?

ieycha
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  • leycha, I'm not sure I understand your question. To be clear, there is zero current (conventional current or electron current) through the ammeter when the switch is open. I don't think that the book is saying otherwise. – Alfred Centauri Nov 06 '20 at 16:21
  • Why not? Because if current flows from positive terminal then current is passing through the ammter in that circuit....? – ieycha Nov 06 '20 at 17:06
  • ieycha, (1) in the circuit pictured, the switch is in series with the ammeter which means that the current through each is identical, (2) an open switch has zero current through (by definition), (3) thus the current through the ammeter is zero when the switch is open. – Alfred Centauri Nov 06 '20 at 18:40
  • @ieycha I think the text is assuming "when the switch is closed". By "conventional current" it means as if it was positive charges; electrons themselves are negatively charged and flow the opposite way. Does that help? – Guy Inchbald Nov 06 '20 at 20:52
  • Thank you! That helped a lot. – ieycha Nov 07 '20 at 08:49

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“Conventional” current (from + to - ) is used because it is more convenient mathematically to think in terms of positive charge flow. In most cases, it makes no difference which charge is moving, but for calculations some right-hand-rules change to left-hand-rules. A measurement of voltage polarity in the Hall effect does tell us that it is the negative charges which are moving in a conductor.

R.W. Bird
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in a real setup ammeter would not show the electric current measurement as the current won't be able to pass to it, so why do we still use Conventional current, isn't it actually wrong?

Not exactly. One thing is clear that the current leave from positive terminal of the battery to negative terminal of bettery. Now for the ammeter, It has two terminal, One postive and other negative. Now If current get into from positive and leave from negative means if the terminal sign match, the current shown in ammeter will be positive and if they not, the current shown in ammeter will be negative. That's doesn't matter (that's doesn't change the magntitude of the current).

Young Kindaichi
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  • But isn't electron flow from negative terminal to positive terminal the 'real' current and no current/charge is actually coming from the positive terminal,or there are...? I am sorry i m really confused....additionally, if current does flow from positive terminal will an ammeter be able to show reading even when switch is open(as the current is passing thought it)? – ieycha Nov 06 '20 at 17:04
  • If there is no current flowing, of course ammeter will show no current. First let's be clear is you attach the battery to a circuit with switch on. The current will flow from postive to negative terminal and electron flow in the opposite direction. The direction of current and electron flow is opposite in circuit. – Young Kindaichi Nov 06 '20 at 17:11
  • @ieycha, electron flow is necessarily an electric current but an electric current is not necessarily a flow of electrons. Within a cell, for example, the electric current is due the flow of charged ions which may be positively or negatively charged. Indeed, the electric current can be due to the flow of two oppositely charged ion flows in opposite directions. This is why there is a convention that the direction of electric current is in the direction of positive charge flow. It's just not always the case that electric current is due to electron flow. – Alfred Centauri Nov 06 '20 at 23:38
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In reality, it is negatively charged electrons that flow. There is no flow with the switch open. When the switch is closed electrons flow from the - battery terminal though the ammeter (- to + direction) back to the + battery terminal. The actual flow of - charges is equivalent to the flow of + charges in the opposite direction, and circuit theory models current as flow of + charges (when in reality - charges flow in the opposite direction). So circuit theory would say current flows from the + battery terminal though the ammeter (+ to - direction) back to the - battery terminal, when the switch is closed. With the switch open, the side of the switch connected to the - battery terminal has an electric potential equal to the electric potential of the battery - terminal, and the other side of the switch and both sides of the ammeter all have an electric potential equal to the electric potential of the battery + terminal.

John Darby
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