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Voltage Current and Resistance — Transcript

by The Organic Chemistry Tutor · 2,674 words · 513 segments · language en · Watch on YouTube

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0:01in this video we're going to talk about
0:03voltage current and resistance
0:06so let's begin our discussion with
0:08voltage
0:10what is voltage
0:12voltage
0:14is the electric potential energy
0:16difference per unit charge
0:18the unit for voltage is the volt
0:21one volt is equal to one joule
0:24of electric potential energy
0:26per one coulomb of charge
0:29so let's say if you have a 5 volt
0:31battery
0:33that means that
0:34each column of charge
0:36carries 5 joules of energy
0:38or this 5 joules per 1 column of charge
0:42and so the electrons they have more
0:44energy when
0:45they're operating at a higher voltage
0:49a 10 volt battery
0:52can pump 10 joules of electric potential
0:55energy
0:56to every one coulomb of charge
0:58and so voltage
1:00is related to electric potential energy
1:04now what about current
1:08how can we describe
1:09current
1:13current represents the flow of electrons
1:16it tells you the rate at which
1:18electrons are flowing
1:21current is represented by the symbol i
1:22is equal to the charge divided by the
1:25time the unit for current is the amp
1:29one amp is equal to one column of charge
1:33that flows per second
1:36if you have five amps of current
1:39then you have five coulombs of charge
1:42flowing per second
1:44now when i think of current i think of
1:46water
1:47flowing
1:49and so water can flow very slowly
1:52or
1:53it can flow very quickly
1:55and so you could think of that as
1:57current if you have a lot of water
1:59flowing at any given point the current
2:01is high if you have a small amount of
2:03water or trickle flowing through then
2:06the current is low
2:07and so electric current and the flow of
2:09water
2:10they have some similarities in that
2:13instance
2:15now the next topic we have is resistance
2:19and when you think of resistance what do
2:20you think of
2:23resistance
2:26they can be provided in a circuit by
2:28something called or devices called
2:30resistors and they resist the flow of
2:32current
2:35resistance is measured in the units ohms
2:37represented by the greek symbol omega
2:42and
2:43that's basically what you need to know
2:44about resistors they resist the flow of
2:46current
2:48that's a question for you which type of
2:51wire will have more resistance
2:53let's say
2:54a long wire
2:56or
2:57a short wire
3:02well you know that a long wire will have
3:04more resistance
3:05than a short wire
3:07because the electrons they have a
3:09greater distance to travel through
3:11and so longer wires have more resistance
3:14than short wires you have a greater
3:15distance to travel to get from one part
3:17of the circuit to the other
3:19now
3:21here's another question for you
3:23which one will have more resistance
3:27a thin wire
3:29or
3:31a thick wire
3:36it turns out that the thin wire has more
3:39resistance
3:40than a thick wire and to illustrate this
3:43resistance and current they're inversely
3:45related
3:46as you increase the resistance of a
3:48circuit the current decreases
3:51so the rate at which electrons can
3:53travel through the circuit decreases if
3:56you increase the resistance
3:58and
3:59think of a highway with cars flowing
4:02think of the cars as being the electrons
4:05in which case will the rate at which
4:07cars flow through any given point be
4:10greater
4:11let's say if you have
4:13a one lane highway
4:16or
4:16let's say a seven lane highway
4:20by the way
4:21the green part here represents
4:23resistance not current so just keep that
4:25in mind
4:29in a one-lane highway
4:32there's not many cars that can get
4:33through
4:34and so
4:35the current which i'm going to highlight
4:36in red
4:37will be low in a one-lane highway
4:39however in a seven-lane highway you can
4:42get more cars um
4:44passing through any given point in a
4:46several lane highway so the current will
4:48be high but the resistance
4:51will be low
4:53in a several highway because a lot of
4:54cars can get through
4:56and so think of resistance as being that
4:59one lane highway the cars are restricted
5:02on traveling on that highway
5:04but
5:05in the seven lane highway there's not
5:06much resistance and so you can get a lot
5:08of cars a lot of current flowing through
5:10so keep this in mind resistance and
5:12current they are inversely related
5:16now there is an equation
5:18that relates a voltage
5:20current and resistance
5:22together
5:23and this equation is known as ohm's law
5:27now
5:28let's say if the resistance is kept
5:31constant
5:32what's going to happen to the current in
5:33the circuit if we increase the voltage
5:37increasing the voltage
5:39will cause an increase in current
5:42and to illustrate this
5:43imagine if you have let's say a pipe
5:46that's filled with water
5:48so you have water in this pipe
5:52now what's going to happen if
5:55you increase the pressure on this side
5:57so let's say on the left side
5:59the pressure is high
6:01and on the right side the pressure is
6:03low
6:03but imagine this pipe is completely
6:05filled with water so we really don't
6:07have any uh space here but it's just
6:10filled with water
6:12if the pressure on the left side is high
6:15then
6:16the water
6:18is going to be forced to move in one
6:20direction
6:22it's going to flow from a region of high
6:25pressure
6:26to
6:27a region of low pressure
6:30and the same is true with um
6:32with voltage
6:36current will flow from a region of high
6:38voltage to a region of low voltage
6:41so let's say
6:42if the potential on the left side is 10
6:45volts
6:46and on the right side the potential is 2
6:48volts
6:49in which direction
6:51will the current flow
6:53well the voltage is well the electric
6:55potential rather
6:57is high on the left side
6:59and low on the right side so the current
7:01will flow from
7:03a region of high electric potential to a
7:05region of low electric potential
7:08by the way current and electron flow
7:11they're different
7:13let's say if the electrons are flowing
7:14in this direction
7:16conventional current is defined as the
7:18flow of positive charge
7:20so it's flowing in the other direction
7:22so even though current flows from
7:25high potential to low potential
7:27electrons
7:28they flow from low potential to high
7:31potential electrons are more attracted
7:33to
7:34a positive charge as opposed to a
7:36negative charge
7:37so they're going to flow towards the
7:38more positive side
7:40but when dealing with current
7:43i'm going to focus on conventional
7:45current the flow of positive charge even
7:47though negatively charged electrons are
7:49flowing in a circuit
7:51now let me give you another picture
7:55let's say
7:57this is
7:59negative 12 volts on the left
8:02and negative 18 volts
8:06let's make it uh actually let's make
8:08this negative four volts on the right
8:13so in which direction is the current
8:14flowing
8:15that is the flow of positive charge is
8:17it flowing to the right or is it flowing
8:19to the left
8:22feel free to pause the video and think
8:23about it
8:25now
8:26which side has the high potential and
8:28which side has a low potential
8:30on a number line which number is greater
8:33negative 12 or negative four
8:37if we draw a number line let's say this
8:40is 0 this is 5
8:42this would be negative 4 and negative 12
8:45will be somewhere over here
8:47so the value increases as you go towards
8:49the right on a number line so therefore
8:52negative 4 is higher
8:55on a number line than negative 12.
8:58and so current is going to flow from a
9:00region of high potential to a region of
9:03low potential in this case it's going to
9:05flow towards left
9:07towards the lower potential which is
9:09negative 12 volts
9:11so current flows to the more
9:14negative side or the less positive side
9:17as in the case of this example
9:21here this side is more negative so
9:23current is going to flow in that
9:24direction
9:25so current flows from high potential to
9:27low potential
9:29now here's a question for you let's
9:31focus on
9:32this picture
9:34what is the voltage across the resistor
9:37you need to understand the difference
9:39between electric potential
9:42and voltage
9:45now let's call this point
9:47point
9:48a and this point
9:51point b
9:54now the electric potential at point a
9:58is positive 10 volts
10:00the electric potential at point b
10:03is 2 volts
10:04voltage
10:06is the difference
10:08between the electric potentials of two
10:10points so it's the electric potential
10:12difference of two points so the voltage
10:14across the resistor
10:17is the difference between a and b
10:20so in that case
10:22the voltage across that resistor we can
10:24call it vr
10:26is eight volts
10:30it's the difference between those two
10:32points
10:35now what about the voltage across this
10:37resistor
10:38it's also eight volts
10:42now sometimes this could be negative
10:43eight depending on how you connect it so
10:46let's say if you connect the positive
10:48terminal
10:50of let's say a meter
10:51to
10:52point a and the negative terminal to
10:54point b it's going to read positive 8
10:56volts however if you connected the
10:59negative terminal of a meter
11:01to point a and a positive terminal of
11:03the meter to point b
11:05the current will
11:06basically be reversed in that meter and
11:09so the voltage that it's going to read
11:10will be negative eight
11:12so depending on the way you connect it
11:14you can get a reading of positive 8 or
11:16negative 8. but if you want to get a
11:18positive reading
11:19connect the positive terminal to the
11:21high potential part of the circuit and
11:24the negative terminal to the low
11:25potential part of the circuit and then
11:27you'll get a positive reading
11:30now let's go back to this so we said
11:32that if we increase the voltage in a
11:34circuit
11:35the current will increase
11:38and also if you increase the resistance
11:40of a circuit
11:41the current will decrease
11:43so make sure you understand
11:45these statements from ohm's law so
11:47voltage is directly related to current
11:50and resistance is inversely related to
11:53current
11:54so if you double the voltage in a
11:55circuit with everything else being the
11:57same the current will double if you
12:00double the resistance the current will
12:02reduce by a factor of two it will be
12:04half of what it used to be if you triple
12:06the voltage
12:07the current will triple if you triple
12:10the resistance the current will be one
12:12third of its original value
12:15now let's work on some practice problems
12:18a 12 volt battery is connected across a
12:214 ohm resistor
12:23how much current will flow in the
12:24circuit
12:26so
12:27this is the electrical symbol of a
12:29battery
12:30and let's draw
12:32the electrical symbol of a resistor
12:33which looks like that
12:36and so this is a 12 volt battery
12:40and we have a 4 ohm
12:42resistor
12:44now this is the negative terminal of the
12:46battery and this is the positive
12:48terminal
12:49so current
12:50will flow from the positive terminal to
12:53the negative terminal but keep in mind
12:56the electrons are flowing in the
12:57opposite direction
13:01now using ohm's law v is equal to ir we
13:04can get the answer i like using it in
13:07this form because i can easily solve for
13:08any variable that i need just using some
13:12simple algebra
13:14so the voltage is 12
13:17the current
13:18in the circuit is what we're looking for
13:20and the resistance is 4.
13:22so to get i by itself
13:24to solve for the value of i i need to
13:26divide both sides by 4. on the right
13:28side 4 divided by 4 is 1 giving me just
13:31i
13:32on the left side 12 divided by 4 is 3
13:35thus the current in the circuit is 3
13:37amps
13:39and that's the answer
13:41number 2
13:42a battery is connected across a light
13:44bulb
13:45with an internal resistance of 75 ohms
13:49using an ammeter the current flowing in
13:51the circuit was measured to be 120
13:55milliamps if you see ma that's milliamps
13:57what is the voltage of the battery
14:00so
14:00feel free to pause the video
14:02and draw a circuit with the appropriate
14:04elements
14:05and then
14:07solve it
14:10now let's start with the battery
14:13so here's the battery
14:15and
14:17here is the ammeter
14:20you can just put an a in a circle
14:23and then
14:24let's draw the lipo which this is one
14:26way you can draw a light bulb
14:32or you can draw a light bulb like this
14:33if you want
14:40and let's
14:42turn the light bulb on
14:44so what is the voltage
14:46of the battery
14:49now the ammeter detects a current of 120
14:53milliamps
14:55how can we convert that to amps
15:01now it's important to understand that
15:03one amp
15:05is equal
15:06to a current of a thousand milliamps
15:09so to convert milliamps
15:12to amps
15:14you need to
15:15divide by a thousand
15:19so if we take 120
15:21and divided by a thousand
15:24all we need to do is take the decimal
15:25point and move it
15:27three spaces to the left and that will
15:29give us a current
15:31of 0.12 amps
15:33and so that's a quick way in which you
15:35can convert milliamps to amps
15:38now the resistance of the light bulb
15:40we said it was 75 ohms
15:44so now we have everything that we need
15:45in order to calculate the voltage of the
15:47battery
15:48so v equals ir
15:51so the current is 0.12
15:53the resistance is 75
15:56and so we just need to
15:57multiply those two
16:00so 0.12 times 75
16:02what we have is a 9 volt battery
16:06so whenever you're using ohm's law
16:08just remember
16:10if v is in volts
16:13then the current has to be in amps and
16:15the resistance has to be in ohms
16:19let's say if the
16:20resistance was in ohms but if you plug
16:23in milliamps instead then this will give
16:26you millivolts
16:28so you have to be careful when you're
16:29using different units but just to be on
16:31the safe side use volts for v
16:34amps for the current and ohms for the
16:36resistance
16:38number three
16:40a hairdryer pulls a current of 0.8 amps
16:44from a 120 volt power source
16:47what is the internal resistance of the
16:50hair dry
16:52so let's go ahead and go directly to the
16:54formula
16:55v equals ir
16:56so we have the voltage it's 120 volts we
17:00have the current 0.8 amps and our goal
17:02is to solve for r the internal
17:04resistance of the hair dry so v is 120
17:09the current is 0.8 and let's calculate r
17:12so to get r by itself we just need to
17:14divide both sides by 0.8
17:18and so 120
17:20divided by 0.8 is 150
17:24and so the answer is 150 ohms
17:27and that's the resistance of the
17:29hairdryer
17:30and so it's very simple to use ohm's law
17:32particularly in this format you can
17:34easily calculate the voltage the current
17:37or the resistance
17:40now perhaps you've seen
17:42this
17:44symbol before
17:45maybe like a triangle or something
17:47with
17:49v on top
17:51and i and r on the bottom
17:53so let's say if you wish to calculate
17:56r
17:57looking at what um the stuff that's left
17:59over let's say if you put your thumb on
18:01r you can see that the resistance is
18:03voltage divided by current
18:06now let's say if you block off the
18:08current if you put your thumb on the
18:10current you can see that the current is
18:13voltage divided by resistance
18:16now if you put your thumb on let's say
18:17the voltage
18:19then that tells you that
18:21voltage is current times resistance
18:24which we could see in this form
18:27but let's say if you
18:29divide by i the voltage divided by
18:31current is equal to resistance if you
18:33need to calculate r or if you need to
18:35calculate current is voltage over
18:37resistance
18:38so those are the other forms if you
18:40prefer
18:41to use that way but
18:42if you just use this formula you could
18:44find all three just by doing a little
18:46algebra but for those of you who don't
18:48like algebra
18:49you may want to use
18:51this chart if it helps
18:53i think it does help but it just
18:55it all depends on you which method is
18:57easy for you but thanks for watching
19:20you

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