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in this video we're gonna talk about
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Ohm's law so what is Ohm's law Ohm's law
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describes the relationship between
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voltage current and resistance perhaps
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you've seen this equation V is equal to
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I times R V stands for voltage I stands
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for the current r is resistance voltage
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is measured in the units of volts I
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which represents the current is measured
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in amps and are the resistance is
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measured in ohms now you need to know
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that as the voltage in a circuit
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increases the current will increase
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provided that the resistance stays the
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same if the resistance goes up the
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current will go down if the voltage is
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held constant so voltage and current
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they are proportional to each other and
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resistance and current they're inversely
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related to each other
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now let's work on a practice problem
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let's say if we have a 12 volt battery
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connected across a four ohm resistor
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what is the current flowing in this
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circuit conventional current flows from
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the positive terminal the battery to the
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negative terminal of the battery this is
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the opposite direction to electron flow
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so to find the current in a circuit we
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can use Ohm's law V is equal to IR
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so the voltage is 12 we're looking for
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the current the resistance is 4 so we
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need to solve for the variable I let's
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divide both sides by 4 12 divided by 4
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is 3 so the current is going to be 3
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amps
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now let's say if we have three resistors
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connected in series like this let's say
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this is r1 r2 and r3 and it's connected
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across a 60 volt battery now let's say
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that r1 has a value of 3 ohms and r2 is
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4 ohms and r3 is 5 ohms so what is the
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current flowing in a circuit in order to
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find the current flowing in a circuit
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where the resistors are connected in
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series you need to find the total
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resistance and the total resistance is
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going to be r1 plus r2 plus r3 you just
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need to add the values of the three
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resistors so 3 plus 4 plus 5 that's
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going to give us 12 so the total
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resistance in a circuit is 12 ohms next
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you need to calculate the SIRT on the
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current so we could use the formula V is
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equal to IR so V is 60 that's the
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voltage across the three resistors we're
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looking for the current and then the
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total resistance is 12 so you can treat
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this as if it's one big resistor and you
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have a 60 volt battery source across a
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12 ohm resistor what is the current in
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that resistor so what we need to do is
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divide both sides by 12 to get the
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current by itself 60 divided by 12 is 5
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so we have a current of 5 amps flowing
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in this circuit now once we have the
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current
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we can calculate the voltage drop across
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each resistor what is the voltage drop
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across the first resistor now in this
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series circuit the current that flows in
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a circuit is the same as the current
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flowing through r3 r2 and r1 because
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there's only one path for the current to
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flow it's going to be the same five amps
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so to find the voltage across the first
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resistor we can use the current that
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flows through the first resistor times
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the resistance to the value of that
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resistor so we're using Ohm's law but in
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a different way I 1 is going to be the
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same as I because that 5 amp current is
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flowing through each resistor but r1 is
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different r1 is going to be 3 so it's 5
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times 3 so we have 15 volts across r1
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now what about across r2 what is the
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voltage across r2 well we could follow
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the same pattern so we can say v2 is
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equal to i2 times r2 so the current is
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still going to be 5 amps but this time
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the resistance is 4 ohms so 5 times 4
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that's going to give us 20 so we have 20
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volts across r2 now across R 3 it's
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going to be v3 is equal to I 3 times r3
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so I 3 is going to be the same as i2 and
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i1 so that's 5 amps r3 is 5 so 5 times 5
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is 25 so notice that if you add up 15
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plus 20 plus 25 it gives you 60 and so
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the voltage of the battery is equal to
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the sum of all of the voltage drops
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across those resistors and there's
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something called Kirchhoff's voltage law
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which
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basically states that as you go around a
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circuit in a loop the total voltage will
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be zero and make sense because the
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battery it increases the energy of the
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circuit because it supplies energy to
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the circuit so increases it by 16 the
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resistors consume energy from the
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circuit so they decrease it does they
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have a negative value so if you add a
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positive 60 with a negative 15 negative
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20 and negative 25 you get zero because
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the energy that flows into a circuit
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must equal the energy that comes out of
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your circuit thus kirchoff's voltage law
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it always applies whenever you have a
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closed loop so the sum of all the
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voltages in a closed loop will always
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add up to zero now what's going to
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happen if we connect three resistors in
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a parallel circuit let's calculate the
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current in such a circuit
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in the series circuit the current
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flowing through the resistors that are
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connected in series is the same because
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the current only has one path and what
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you could flow in the parallel circuit
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the current has multiple paths and so it
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could vary however notice that whenever
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resistors are connected in parallel the
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voltage across those resistors is the
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same so let's say if we have in this
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case a 12 volt battery each resistor is
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connected across that 12 volt battery
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and so all of them have 12 volts across
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their terminals let's call this r1 r2
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and r3 and so let's say that r1 has a
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value of 3 ohms and r2 is going to be 4
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ohms and r3 is going to be 6 ohms what
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is the current flowing through each
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resistor so we can use this formula V 1
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is equal to i1 times R 1 so remember in
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a parallel circuit the voltage across
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the resistors connected in parallel is
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the same but in the series circuit the
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current flowing and resistors that are
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connected in series will be the same in
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this case V 1 is 12 because we have 12
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volts connected across R 1 and to find
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the current flowing through this
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resistor we need to use that formula R 1
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s 3 so the current is going to be 12
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divided by 3 so we have a current of 4
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amps flowing through R 1 now let's do
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the same for R 2 so let's use the
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formula V 2 is equal to i2 times R 2
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it's basically owns a labo of different
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subscripts so V 2 is still 12 and R 2 is
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not 4 so it's going to be 12 divided by
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4 which will give us a current of 3 amps
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now to calculate I 3 it's going to be 12
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the
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by 6:00 following the same pattern and
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so that's a current of two amps now
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notice that as the resistance increases
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the current decreases as we mentioned in
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the beginning of this video here notice
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that our one has the lowest value and it
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has the highest current our three has
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the highest value but it has a lowest
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current so as you increase the
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resistance the current decreases if we
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increase it to six the current decreases
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the two and if we decrease the
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resistance the current will increase if
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we decrease it to three the current goes
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up to four provided that the voltage is
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held constant and so you'll see this
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relationship in a parallel circuit as
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you can see it here or in a series
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circuit - you can see that relationship
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there as well now what is the current
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that leaves the battery how can we
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determine the current that's leaving the
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battery the total current in the circuit
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which we'll call I T that leaves the
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battery it's going to be the sum of the
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individual currents so it's going to be
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four plus three plus two four plus three
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plus two is nine so the total current
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here is nine amps now let's focus on
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this point what is the current that is
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flowing through that branch right here
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along let's say this wire what is the
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current in that region so now we need to
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use something called Kirchhoff's to
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current law we saw in a last example
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that kirchoff's voltage law which
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basically states that the sum of all the
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voltages around the loop adds up to zero
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while kirchoff's current law is very
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similar the current that enters the
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junction is equal to the current that
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leaves the junction now let's draw a
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picture
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so we have 9 amps of current flowing to
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this point and we have 4 amps that's
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leaving it now the current that is
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flowing to a junction must equal the
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total current that is leaving the
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junction so we have to have current
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leavin in this direction and it has to
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be 5 amps because 5 plus 4 is 9 so we
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have a total of nine amps of current
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that answers disjunction and 9 amps a
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current that leaves it and so that's the
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basic idea behind Kirchhoff's current
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law so we have five amps flow into the
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right in this direction now out of those
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five amps three amps is going this way
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so that means the other two amps flows
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in this direction and you can see how
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it's like a river splitting off into
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three directions so in this section we
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still have two amps of the current
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that's traveling here and then when it
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joins up with a three amp current three
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amps you will add up to five and so we
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have five amps flowing in this region
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and then the five and four will get
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together and so we're gonna have a total
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of nine amps of current flowing in this
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region and so as you can see the current
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that's going this way it's gonna be the
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same as this current here not amps and
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so hopefully this all makes sense it's I
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want to give you a basic idea of how to
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use Ohm's law in a simple circuit in a
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series circuit and also in a parallel
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circuit so that's it for this video
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hopefully you found it to be helpful and
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they gave you a good understanding of
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kirchoff's voltage law and its current
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law as well thanks for watching
