Circuits 1
Current Direction
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by Goncalo Martins
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Directions
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(click me)
If you see a click me...
This presentation is
not about
how electrons flow...
Conventional Flow
Electron Flow
This presentation is about
understanding
current
directions
in a circuit while
analyzing
it.
Which
direction
should we pick?
Before answering that question, let's
review
a couple of
rules
...
Rule 1
Rule 2
(click me)
(click me)
The sum of
currents entering
a node...
is
equal
to the sum of
currents leaving
the node.
Kirchhoff's Current Law
(
KCL
)
(click on the node)
Passive Sign Example...
(click on the voltage drop)
KCL Example...
(click on the nodes)
Passive Sign Convention
Current
entering
the
positive
terminal...
Current
entering
the
negative
terminal...
... has a
positive
voltage drop.
... has a
negative
voltage drop.
(click on the voltage drop)
Let's cover an
example
where all
these
concepts
are connected
together
...
(circuit
link
)
Notice there are
no signs
or
currents
assigned
in this circuit...
Start by choosing
any
direction of
current
that you
want,
and don't
overthink
...
Assign
+ and 
around
the resistors.
Don't
overthink
here neither...
+
+
+



Hint
: it makes it easier if you assign passive convention, but you don't need to.
From now on,
apply
KCL and passive sign to
get
the
equations
out of the circuit.
+
+
+



To make things
less cluttered
...
The
analysis
will be
split
into
2
schematics.
+
+
+



A
KCL at A:
+
+
+



Passive Sign:
I_3=0.05 A
(click here)
+
+
+



Passive Sign:
Voltage Differential:
KCL at A:
We can write \(V_1\) and \(V_2\) in terms of \(V_A\)
V_A
For this particular case, we don't need to worry about \(V_3\) since we already know the value of \(I_3\)
KCL at A:
Passive Sign:
\frac{V_1}{680}+0.05=\frac{V_2}{4.7k}
Step 1:
Substitute
EQs A
into
KCL
Step 2:
Substitute
EQs B
into
Step 1
Voltage Differential:
\frac{5V_A}{680}+0.05=\frac{V_A0}{4.7k}
(EQs A)
(EQs B)
I_3=0.05 A
Step 3:
Solve for \(V_A\)
V_A=34.071 \> V
Now that we have \(V_A\)...
You can adjust and see the real direction of the currents.
Conventional Flow
Electron Flow
No matter which combination of currents you pick, the value of \(V_A\) will always be the same...
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(Bonus View)
Summary
Adjust the direction of the currents accordingly.
Apply any direction of currents in your circuit.
(hover here)
Find all the voltages in your circuit.
(Don't Overthink)
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