This eighth-grade science lesson is about electric currents and circuits. It is the 10th lesson in a sequence of 30 lessons on electromagnetism. The lesson is 45 minutes in duration. There are 21 students in the class.

00:00:17Please, sit down.
00:00:52Today, first we will review what we've learned about electrical circuits and the laws that apply to the electric circuit.
00:01:00And then, the goal of the lesson will be to learn how to measure electrical current; namely, its symbols, unit or units.
00:01:11That is the goal for today's lesson.
00:01:16We'll review a little at the beginning; and then you'll take a short test for five minutes, in which you display what you remember.
00:01:31How is an electrical current formed? Vladenko?
00:01:34An electrical current is formed by a rectified electrical charge.
00:01:39Yes. And the charge could be different. So, what kind of electrical charge is in metals. Vileme?
00:01:46And what //particles are those?
00:01:47//Free electrons.
00:01:48Yes, those are free electrons. What particles are present in liquids and gases?
00:01:54Free ions.
00:01:55Free ions. Correct. What conditions have to be met for an electrical current to run through a body? Tamaro?
00:02:06A body must maintain an electric field.
00:02:08And how is the electric field secured?
00:02:14The source of electrical voltage must be connected in there.
00:02:16Yes. The source of electrical voltage must be connected in there. Another condition- yes, tell us?
00:02:22It must contain new particles for electrical voltage.
00:02:25Yes. A body must contain specific particles, which are able to form a current. Of what does a simple circuit consist? We'll build the circuit here, later.
00:02:39A battery, a switch, conductors, and some //electrical consumption device.
00:02:42//Aloud. Battery, switch, conductors, and some consumption device. So, what would that be, for example? Lado?
00:02:53Good day.
00:02:54A light bulb.
00:02:55A light bulb, Lado. Correct. What else could it be?
00:02:58Or a bell.
00:02:59A bell. What else did we say? Yes, Vincente?
00:03:04A coil.
00:03:05A coil, for example, a resistor, and so on.
00:03:08We have connected only a bulb or bell in the electrical circuit. And what was the reason for it? Why those two consumption devices? Valerie?
00:03:19Because they are the ones most frequently used.
00:03:22So, they are the ones most frequently used. And then, what was the other reason? Vileme?
00:03:27That we would be able to recognize whether the current flows through the circuits (inaudible).
00:03:32Yes. Whether-
00:03:33Good day. We have our attendance book here.
00:03:35You, please leave. This is unreal. [ laughter ]
00:03:43So, we have recognized that the current runs through it immediately. How does the bulb react? Yes?
00:03:51It lights up.
00:03:52It lights up and the bell will? Tadeasi?
00:03:56The bell will ring. Correct. So, we will still review what the direction of an electrical current in a circuit is. Jakube?
00:04:06From the positive to negative pole.
00:04:08Yes, from positive to negative. Correct. And how do free electrons move in a metal conductor?
00:04:19In the opposite direction.
00:04:20Yes. Correct. That means, from the negative pole of a source, to the positive pole of a source.
00:04:28Each of you will get a sheet of paper and write your name on it. There are two simple tasks on your worksheet. There are some schematic symbols for different parts of the circuit there.
00:04:42And also a simple electrical circuit. Vileme, please, come here.
00:04:54And hand out. Always one- you know how. Here you have test B. It is enough for everyone, so sign the paper; and if you can't read something, please ask me.
00:05:15Or we could do this- who has it already- here the girls have the test A. Read the first problem to us.
00:05:23"Draw an electrical circuit: light bulb, switch, electric cell, conductors and using an arrow mark the flow of electrical current in the circuit."
00:05:32Yes. And I think that the second problem is clear. Read test B.
00:05:37"Draw an electrical circuit: bell, switch, battery, electric cell battery, conductors and arrow. Using an arrow mark the flow of free electrons in the circuit."
00:05:46Correct. So, write everything on that paper. This is only a short task.
00:06:07Valerie, you were not present last time, so write that down. You'll submit it anyway.
00:06:16You may write with a ballpoint pen this time. Fill in what is needed.
00:07:28Don't forget to mark with an arrow, what you are supposed to mark, while solving the first problem.
00:08:17Please, continue to write; you'll have to finish soon.
00:09:01So, everybody, let's finish. It wasn't as difficult. Lubosi.
00:09:24So, girls, let's finish. Tamara.
00:09:40Give it to me. Give it to me (inaudible). Let's finish, everybody.
00:10:04Is that all?
00:10:08It's okay. Mrs.(inaudible), so tomorrow.
00:10:12Is that all? Done.
00:10:18What did you write there?
00:10:19And now, we will continue with the new material; with additional information in regard to electrical current. So, we'll review, again.
00:10:30How is an electrical current formed, Vitku? Explain, once again.
00:10:33By a rectified motion of free particles with a negative charge.
00:10:37With a negative charge, for example, yes. Ordered motion, rectified motion of free particles with an electrical charge.
00:10:47We'll build a simple electrical circuit. Tadeas wants to come to the front. So, come. Build a simple electrical circuit.
00:11:01Tell us what you are connecting in this circuit.
00:11:05(inaudible) bulb.
00:11:06In addition to that, we'll connect a light bulb so we are able to see what is happening inside of the circuit. Source of voltage, yes. And a switch. Please, try. Okay.
00:11:20The bulb indicates that an electrical current runs through the circuit.
00:11:26We mentioned earlier that we could replace the bulb with the bell. We have demonstrated that already. Just to review it.
00:11:37[ bell ]
00:11:38Yes. So, we are immediately able to find out that an electrical current runs through the circuit.
00:11:45But, if we connect a different component to it, some different consumption device. As we mentioned earlier, for example, a resistor, a coil, or something similar.
00:11:53We don't see that the current runs through it. There is just- the current runs through it, but we are not able to see it or hear it.
00:12:02Therefore we have measuring instruments which allow us to find out if some electrical current runs through a circuit. So, we have to connect some measuring instrument into a circuit.
00:12:13So, I brought one to show you. This is a very simple measuring instrument.
00:12:19The coil and the scale is in there. A little magnet is connected to the pointer.
00:12:27You'll find out later about the principles governing those measuring instruments. So this is one little instrument that we could use and find out if current runs through the circuit.
00:12:41Here we have different measuring instruments. This one will allow us to find out if a current runs through the circuit.
00:12:50And here we have an example of a measuring instrument which has the advantage of having a large scale. Used in demonstrations.
00:12:58Ahh, for this measurement, we'll use this instrument,
00:13:03During your lab, in one of the next lessons, you will use this particular instrument to measure. Therefore you should become familiar with it.
00:13:15What do we call this instrument? It is an ammeter. We recognize it due to a letter on its scale. Which one?
00:13:30A. Which means that it is an ammeter. It'll measure electrical current in particular units. The basic unit is one ampere.
00:13:43As you know, we mentioned that several times. Units of the physical quantities- variables- are named after famous scientists.
00:13:54Not only after physicists, but after scientists from different fields as well.
00:13:59So, the unit of electrical current, one ampere, was named after the French scientist Ampere. Therefore we called it ampere.
00:14:10And the unit abbreviation is capital letter A, which you can find on any ammeter.
00:14:19We already mentioned the schematic symbol of an ammeter. Who can remember?
00:14:26Jakube, come and draw a picture on the blackboard. On that side. Get a piece of chalk.
00:14:36So, ammeter. Even from the symbol we can assume what type of an instrument it is. Yes. The letter A is inside the circle, which enables us to recognize it immediately.
00:14:53Some of the measuring instruments look alike. This one, for example.
00:15:00As you can see, it would be easy to confuse them. They look the same. In spite of that they differ by an A, which is a symbol for ammeter.
00:15:15On this one, we have a different letter, but we'll talk about it in one of the next lessons. Yes.
00:15:20So, a short review. We use an ammeter to measure an electrical current.
00:15:28How can we measure a current within a circuit? You should remember that we can interrupt the circuit at any place and measure the current.
00:15:39Tadeas built this one. And we'll connect the ammeter somewhere in the circuit. So this ammeter- I'll disconnect the circuit, doesn't matter where, anywhere.
00:15:52And we'll build a simple circuit, including the ammeter, again. Now, I'll disconnect it so that you can see the indicator's motion. It's visible.
00:16:10The light bulb lights up. If the light bulb were not there, then the measuring instrument would indicate that current runs through. Okay. In this way we will identify the current.
00:16:20What is the case for a current in a simple circuit? Yes, Emo?
00:16:25It is the same everywhere.
00:16:27Its value is the same everywhere. We can try it. Look in here. The students in the back won't be able to see it. But Tadeas and Vilem should see the motion of the pointer-
00:16:39One point five.
00:16:40Yes, it shows. So, I'll place the ammeter somewhere else. It doesn't matter where. Between the switch and the battery from the other side. The circuit is closed. //The same.
00:17:03//The same.
00:17:04Therefore anywhere in that circuit. And what did we say about a parallel circuit? What is the case for a current then? What's the rule? Tamaro?
00:17:15An electric current is smaller in its branches.
00:17:16Yes. An electric current is smaller in its branches.
00:17:21So, that's a sufficient review of an electric current. How it is formed, how and with what it is measured and you've been introduced to another variable.
00:17:30As we mentioned already, each physical variable has its symbol and its unit.
00:17:36The unit, as we said, is one ampere and the symbol is indicated by the capital letter I. Okay.
00:17:45So, I repeat, once again. The symbol for an electric current is I, the unit is one ampere.
00:17:52In addition to the ampere, other derived units are used. As you know, very often instead of the fundamental one, other units are used.
00:18:01For this electrical current smaller units are used. What is a smaller unit than an ampere?
00:18:11What is a unit smaller than a meter? The really small one?
00:18:17A millimeter.
00:18:18A millimeter. And a smaller unit than ampere is?
00:18:22A milliampere.
00:18:23A milliampere. Correct. What increment of an ampere is it? One milliampere? What increment of an ampere is it? Pavel? So, Vincente?
00:18:36One thousandth.
00:18:37Yes. It is one thousandth. And besides this one, another smaller unit is used. What is it? We've already talked about a similar unit in regard to a charge.
00:18:51You. Do you know?
00:18:52A microampere.
00:18:53Yes, a microampere. And this microampere is designated by the Greek letter mu and capitol A. Where did we hear about the Greek letter mu?
00:19:06Alongside what unit? Who remembers? Yes?
00:19:09Alongside the coulomb.
00:19:10Yes. Coulomb. And what kind of unit is it?
00:19:16Well, that is the symbol for an electrical charge. Okay.
00:19:18A microcoulomb.
00:19:19Yes. A microcoulomb. And what size is that in relationship to the fundamental unit? How big is a microampere to ampere? Yes.
00:19:27One millionth.
00:19:28A millionth, yes.
00:19:29Take your notebooks, please. Let's write: Measuring an electrical current and its units. Measuring an electrical current and its units.
00:20:16Write down that the symbol for an electrical current is I. Unit of an electrical current is A, which is an ampere.
00:20:49As mentioned, smaller units are: milliamperes, designated by a small m and an A.
00:21:15What unit is designated by capital M? Instead of this one? What would it be? It is not used in practice anymore, but what is it?
00:21:26A megaampere.
00:21:27A megaampere. So, you should write a small m. Milliampere and microampere. This is the Greek mu and an A.
00:21:45And the relationship. One milliampere is one thousandth of an ampere. One microampere is millionth. Write it down.
00:22:09We won't write down that the unit is named after the French scientist. We'll remember it.
00:22:42Also we'll write how to identify an electrical current. Not how to measure it, but how to specify it.
00:22:51For that, we would have to know how big a charge runs through a conductor and for how long.
00:22:58Afterward, we would determine the current, by dividing the total charge which runs through by the total time.
00:23:08We'll write down the formula- we won't solve any specific problems- only to let you know, how to calculate a current.
00:23:17So, write it down. If the particles with total electrical charge Q pass through the cross-section of a conductor
00:23:50We already talked about it today- Ema mentioned that's the symbol-
00:23:56During time t, current I passes through a conductor, which is equal to Q divided by t. Okay. Write it down.
00:25:07Somebody will come to the blackboard and draw a simple circuit for us. For now leaving out the measuring instrument that we connected on the table.
00:25:16So, who'll come to draw, Valerie? Please, come. Then go. You can't get here. Vileme, come.
00:25:32Do it here, on the right. Draw a bigger picture of a circuit, please. We'll add a measuring instrument, later. Don't draw it yet. Wait a minute.
00:25:46Tell us what you are drawing.
00:25:48A light bulb.
00:25:49A light bulb, yes. This is?
00:25:53A switch.
00:25:54A switch. Or a circuit breaker could be there. We've noted the difference.
00:26:01An electric cell.
00:26:03Yes. An electric cell or any source of an electrical voltage. Correct. Sit down.
00:26:13So, we can mark it there, for us to remember what a current's direction in a circuit is. What is a current's direction in a circuit, Emo?
00:26:25From positive to negative.
00:26:27Yes. From a positive pole source to a negative one. Come here- take some colored chalk and mark it there- one arrow is enough. Yes, from a positive pole to a negative one.
00:26:44What is the motion of free electrons? Is it the same? Jakube?
00:26:51No, it is opposite.
00:26:52In the opposite direction. Correct.
00:26:54And now, how do we draw an ammeter in there? I mentioned that a circuit can be interrupted anytime and anywhere.
00:27:02Because the value of a current is the same, we'll just connect an ammeter. So, how will we draw it? Tadeasi, please, come.
00:27:19So, it is easy to do it here. Let's erase a bit. Correct.
00:27:25The ammeter could be, for example, between a cell and a switch, between a light bulb and a switch. Just anywhere. Yes.
00:27:34Therefore, you draw a circuit even with an ammeter. I'll put this aside, you can't see, probably.
00:27:50So, draw the picture and let's write.
00:27:56An electrical current is measured by an ammeter. We won't write a schematic symbol. You already have it from a previous lesson. Write down at least this.
00:29:12Are you done? I'll show you a small, simple instrument. There-
00:29:23Hopefully, it will work.
00:29:36Well, can you see?
00:29:40If we turn it on, the bulb is not lighting up, due to coil's huge resistance. But the measuring instrument indicates a movement, therefore current is passing through the circuit.
00:29:56We don't see how big it is, yes. Either amperes or milliamperes, because there is only a scale which is not numbered.
00:30:04We can see only that the current is passing through. What would happen if I were to exchange or turn the battery around, would I change the pole? Lado?
00:30:16The indicator will move in the opposite direction.
00:30:18The indicator will move in the opposite direction. Let's try that. So, we turn that around.
00:30:30So, now the current runs in the opposite direction.
00:30:36However, if we use a cell, a battery or other similar types as our source- we'll talk about them later.
00:30:47And if current passes through- it still passes in the same direction. We called this unidirectional or direct current.
00:30:58Is the direct current in the power supply? Boys should know it for sure.
00:31:06It is not.
00:31:07Possibly not. What type of current do we use? Did anyone hear about it?
00:31:12An alternating current.
00:31:13Yes, it is alternating current and we'll talk about it later.
00:31:17Therefore, this source contains direct current. Write that down and then we'll talk more about an ammeter. So, write it down.
00:31:38An electrical current that moves in one direction is called direct current.
00:32:16Just write it this way.
00:32:24A direct current.
00:32:34These measuring devices, which you can see, are old types. We use them to conduct students' experiments.
00:32:45Otherwise, for real life measurements more sophisticated equipment is used.
00:32:52These are instruments which can be used not only to measure current but also to measure other physical values, such as voltage or resistance of a conductor.
00:33:03On the other hand, as teaching tools for conducting an experiment, these types are very tough and are more durable.
00:33:12They can survive the abuse of being improperly connected. So, this measuring instrument is durable.
00:33:22Different types of instruments exist. For example, digital ones, which are very easy to operate. The value of the passing current is visible on its display.
00:33:33These instruments are much more difficult to operate. They have a scale and we have to know at the start, how big a current we are looking for.
00:33:47This is not an ammeter.
00:33:49An estimate of the size of the current to be measured will enable us to choose the appropriate measuring range for the current.
00:34:00If you were to connect an extremely sensitive instrument to a circuit in which the current is larger than it should be, the instrument would be damaged.
00:34:12Therefore we have to understand what instrument and what scale range to choose to connect them correctly.
00:34:19So, on this instrument, for example, there are three clamps. Therefore we can use two scales, which are on this instrument, okay. Above the symbol A.
00:34:36On one scale we can measure up to sixth tenths of an ampere. There we would read, if we were to connect the circuit with the help of these two clamps.
00:34:50If the passing current was bigger or if our estimate was bigger; we would connect these two clamps and be able to measure up to a three ampere value, okay.
00:35:05These scales are not arranged so that one increment is one milliampere or one ampere. We have to choose the range for one increment.
00:35:18This may create some problems for us. Well, let's see. We'll try to build a simple circuit in the next lesson.
00:35:29And you'll subtract values on the measuring instrument and some of you will face a lot of difficulties,
00:35:35Because some of the increments could be two tenths of an ampere.
00:35:45Someone will realize that immediately- someone will face difficulties even to read the value of an electric current on the scale.
00:35:56That would be all of the new material for today.
00:36:00Now we'll review it quickly and try to do an exercise to practice the units of an electrical current.
00:36:10You can see it on the blackboard in front of you, so you shouldn't have any problem. So, one more time, please, because you still don't understand it.
00:36:18What is- how is an electrical current formed? What forms an electrical current? What is it, the electrical current? Yes, Tamara.
00:36:28An electrical current is formed by a direct current movement of a molecule's //particles with an electrical charge.
00:36:32//Particles with an electrical charge. Correct.
00:36:37How can we measure the value of an electrical current? Yes, Emo?
00:36:43With an ammeter.
00:36:44We measure it with an ammeter. How is an ammeter connected? We didn't describe it, you were able to see it here. We can't write everything down. You have it in your book.
00:36:58It is connected //in a circuit.
00:37:01//In a circuit. So, we can interrupt a circuit anywhere and connect an ammeter to the circuit. What is the symbol for an electrical current?
00:37:13Capital I.
00:37:14Capital I. What is the fundamental unit? Vincente?
00:37:17An ampere.
00:37:18One ampere. What other units could be used? Valerie?
00:37:23A milliampere, a microampere.
00:37:24Yes. A milliampere, a microampere. So, these are the fundamentals.
00:37:31Take your book, part B. I'll write the page number on the board in a minute. It is page 37.
00:37:58So, book B, page 37 and exercise number three.
00:38:14Draw a table and fill in the values of the current.
00:38:27When you are finished, please raise your hand.
00:38:36The relations are written right here. By now, you should know the conversion of units, any units. Either it is a length, a volume, a mass.
00:39:06What kind of a notebook is that?
00:39:09You'll buy a new one, okay.
00:39:17Valerie, just draw a simple table. It should have rows and columns exclusively. It doesn't have to be precise.
00:40:37This one, okay.
00:40:45In order to do the conversion correctly while you're filling it in,
00:40:47You have to identify whether you are converting a small value to a larger one, or the opposite.
00:41:05Is Vilem finished?
00:41:44You have it right-
00:41:58Who else is done? The best way is to convert to amperes first- then you'll convert the rest easily.
00:42:28Is Jakub done?
00:42:41You should convert to this, at first. That's the easiest way.
00:42:45At first, convert to amperes in the first column and then it will be easy to follow the instructions on the blackboard.
00:43:05So, you are still able to get an A grade if you answered correctly.
00:43:31Yes. So, Ema is the second. And who else? Tadeas and Lubos are done?
00:43:47Give it to me, Lubosi.
00:44:05You have mistakes here, Tadeasi.
00:44:09Tadeas made a mistake here. Here. And Lubos.
00:44:24Lubos has everything correct. He is the third one. Therefore, Ema, Lubos and Vilem get an A.
00:44:30Study for the next lesson. We'll review.
00:44:46Wait. This way.