Consider the below circuit with it solution. We need to find the current io3.
My question is why we have a -ve sign in the equation of i03 ?
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\$\begingroup\$ because the current flows in the direction that is opposite to the arrow \$\endgroup\$jsotola– jsotola2020-03-26 21:28:09 +00:00Commented Mar 26, 2020 at 21:28
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\$\begingroup\$ @jsotola how did you know that ? \$\endgroup\$AAAA– AAAA2020-03-26 21:35:54 +00:00Commented Mar 26, 2020 at 21:35
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\$\begingroup\$ 7/9A is entering the node and 2/9A and 5/9A are leaving the node. +5/9A leaving the node is same as saying -5/9A entering the node. \$\endgroup\$across– across2020-03-26 21:40:12 +00:00Commented Mar 26, 2020 at 21:40
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\$\begingroup\$ Thanks all for the help. \$\endgroup\$AAAA– AAAA2020-03-26 21:54:55 +00:00Commented Mar 26, 2020 at 21:54
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1 Answer
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Your schematic has only one power source, the 2 A current source.
Since the current source is driving current to the left across the top of the circuit then i2 will have a positive sign and io3 will have a negative sign since its arrow is pointing in the direction opposite to the real current flow.
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\$\begingroup\$ How did you know that io3 should move to the left ? why it shouldn't flow to the right as its shown ? \$\endgroup\$AAAA– AAAA2020-03-26 21:38:28 +00:00Commented Mar 26, 2020 at 21:38
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\$\begingroup\$ Follow all the possible paths for the 2 A current to split and return to its lower terminal: some will flow through the 5 Ω resistor, the rest will flow right to left through the two top resistors and then split, some through the 10 Ω and the remainder through the 4 Ω. All the current ends up returning to the source via the bottom rail flowing from right to left. \$\endgroup\$Transistor– Transistor2020-03-26 21:41:54 +00:00Commented Mar 26, 2020 at 21:41
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\$\begingroup\$ @AAAA Sometimes you can not be sure ahead of time which way the current flows, so you make an assumption about which direction it flows. If you are careful analyzing the circuit, and the value of the current turns out to be negative, then you know that the current actually flows in the opposite direction. \$\endgroup\$Elliot Alderson– Elliot Alderson2020-03-26 21:48:57 +00:00Commented Mar 26, 2020 at 21:48
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\$\begingroup\$ Thanks all for the help. \$\endgroup\$AAAA– AAAA2020-03-26 21:54:41 +00:00Commented Mar 26, 2020 at 21:54