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For more information, please see full course syllabus of AP Physics 1 & 2
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Lecture Comments (8)

1 answer

Last reply by: Professor Dan Fullerton
Wed Apr 13, 2016 6:18 PM

Post by Shikha Bansal on April 13 at 05:33:57 PM

So for any example like example 1, is it a general principle to put the anameter as close to the negative side of the battery as possible so all of the current goes through it?

1 answer

Last reply by: Professor Dan Fullerton
Tue Mar 22, 2016 6:55 AM

Post by john lee on March 21 at 11:04:40 PM

Why can't we see a combination of cells as one big cell? I thought they can work in the same way.

1 answer

Last reply by: Professor Dan Fullerton
Wed Apr 30, 2014 8:38 PM

Post by Tyler Zhang on April 30, 2014

In example 1, if the ammeter is placed at position 3, wouldn't all the current flow through it instead of any of the other two resistors? That case placing the ammeter at position 3 will also be accurate in measuring the total current.

1 answer

Last reply by: Professor Dan Fullerton
Wed May 8, 2013 6:11 AM

Post by Nawaphan Jedjomnongkit on May 8, 2013

From ex1 if put voltmeter at position 3 it will measure potential different of the whole circuit or just on that resistor that we connect voltmeter on? Or in this case it will equal because resistor connected in parallel?

Circuits & Electrical Meters

  • Electrical circuits are closed-loop paths through which current can flow.
  • Conventional current flows from high potential to low potential.
  • Circuit schematics are two-dimensional representations of three-dimensional circuits.
  • A source of potential difference is required for current to flow.
  • Voltmeters measure the potential difference between two points in a circuit. Voltmeters are connected in parallel, and have very high resistance.
  • Ammeters measure the current flowing through a circuit element. Ammeters are connected in series, and have very low resistance.

Circuits & Electrical Meters

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Objectives 0:08
  • Electrical Circuits 0:21
    • A Closed-Loop Path Through Which Current Can Flow
    • Can Be Made Up of Most Any Materials, But Typically Comprised of Electrical Devices
  • Circuit Schematics 1:09
    • Symbols Represent Circuit Elements
    • Lines Represent Wires
    • Sources for Potential Difference: Voltaic Cells, Batteries, Power Supplies
  • Complete Conducting Paths 2:43
  • Voltmeters 3:20
    • Measure the Potential Difference Between Two Points in a Circuit
    • Connected in Parallel with the Element to be Measured
    • Have Very High Resistance
  • Ammeters 4:19
    • Measure the Current Flowing Through an Element of a Circuit
    • Connected in Series with the Circuit
    • Have Very Low Resistance
  • Example 1: Ammeter and Voltmeter Placement 4:56
  • Example 2: Analyzing R 6:27
  • Example 3: Voltmeter Placement 7:12
  • Example 4: Behavior or Electrical Meters 7:31

Transcription: Circuits & Electrical Meters

Hi Everyone. Welcome back to Educator.com.0000

In this lesson we are going to talk about circuits and electrical meters. 0003

So it should be a fairly short lesson. 0007

Our objectives are going to be to identify the path and direction of current flow in a circuit, drawing and interpreting schematic diagrams of circuits and effectively using and analyzing voltmeters and ammeters. 0009

Electrical circuit is a closed-loop path through which current can flow. 0022

An electrical circuit can be made of most any materials, but practically speaking, circuits are typically comprised of electrical devices. 0026

This includes things like wires, batteries, resistors, and switches. 0033

However, if you are not careful, even humans can be part of an electrical circuit, so let us try to avoid that. 0036

Now, conventional current flows from high potential to low potential, but if you ever want to talk about electron flow, electrons are going to flow in the opposite direction. 0043

So electrons flow from low potential to high potential, but conventional current flows from high potential to low potential. 0054

So, circuit schematics are two-dimensional representations of three-dimensional physical circuits. 0070

Looking at a physical circuit, things can get complicated in a hurry. 0077

A circuit schematic is a way of representing that on paper or on a computer. 0080

It allows us to much more neatly organize what we are going to do with that circuit. 0085

Symbols represent these circuit elements and lines represent wires, conductive wires. 0090

Now sources for potential difference are required for current to flow and those sources for potential difference can be things like voltaic cells, batteries, or power supplies. 0096

And typically we are going to use a symbol for a battery for all of them, where the longer side is positive and the shorter side is negative. 0105

Here is how I remember that -- when you draw the battery schematic symbol, you have to use more ink over on the longer side -- you also need to use more ink to make a plus, than you do a minus. 0115

So the long side of a battery is the positive side or voltage sources and current sources can also be used. 0126

Here is a single cell. A battery is actually a combination of cells, which is why you get this symbol. 0134

A switch is often times shown opening and closing that way. 0140

A voltmeter with a v in it measures potential difference or voltage. 0144

An ammeter measures current flow in amps. A resistor kind of looks like the Charlie Brown shirt. 0148

Variable resistor just means you can change the resistance and a lamp is a resistor that gives off light. 0155

So, current only flows in complete paths. 0163

Over here on this side, we have the positive side of our cell, the negative side of our cell -- we have a resistor -- a light bulb here, but we do not have anything happening. 0166

We do not have a completely closed path, therefore current cannot flow. 0175

On the right hand side, though, once we have closed that switch, we now have a complete loop -- current is going to flow this way from positive potential to negative -- from high to low potential. 0179

Now keep in mind though, that means that the actual electrons are flowing in the opposite direction. 0191

Voltmeters measure the potential difference between two points in a circuit. 0200

They are connected in parallel with the element that you want to measure. 0205

And if a voltmeter is connected correctly, you can remove it from the circuit without breaking the circuit. 0210

For example, as we look at this circuit over here, the voltmeter would measure the potential difference from one side of our light bulb to the other side of our light bulb. 0215

And if we want to make sure we have it installed correctly, imagine what would happen if you cut the circuit there and there. 0223

When you do that, the circuit would still operate -- the voltmeter would not measure anything, but you would still have an operating circuit. 0230

If that is the case, you have done it correctly. 0236

Voltmeters have very high resistance. 0239

They have that high resistance so that very little current flows through them and they do not impact your circuit performance much at all -- just a tiny amount, a negligible amount. 0242

You want it as little as possible so that you get an accurate reading of how the circuit would function without the voltmeter in there. 0251

Ammeters, on the other hand, measure the current that is flowing through an element of a circuit. 0260

They are connected in series with the circuit, so that whatever current is flowing has to flow through the ammeters so that you can measure it. 0264

You want all of the current that you want to measure to flow through the ammeter. 0271

The ammeter must be broken to correctly insert an ammeter. 0275

If you can pull the ammeter out of the circuit and the circuit still functions, that portion of the circuit, then you have done something wrong. 0279

Ammeters have extremely low resistance, therefore they do not have much potential drop and they minimize their effect on the circuit as a whole as well. 0285

So let us talk about ammeter and voltmeter placement in the circuit. 0296

In the electric circuit diagram here, possible locations of an ammeter and voltmeter are indicated by the circles 1, 2, 3, and 4. 0299

Where should we place an ammeter in order to measure the total current and where should a voltmeter be located to measure the total voltage? 0307

Well, the ammeter -- if we want to measure the total current -- if we put it here at 2, we would only measure the current through this resistor and if we put it here at 3, we would only measure the current going through here -- that would not be a good idea. 0315

If we put it here at 4, we would measure current through here, but really we want our current to be flowing this way and that way and then coming back. 0328

So at the one spot is where we could place our ammeter where we would get the total current flow. 0339

So right away I can get rid of B and C. 0344

If we want to measure the total voltage in our circuit, where should we put the voltmeter? 0348

Well, let us take a look here. If I put my voltmeter here at 4, I would get the total voltage of my circuit, so that looks good. 0353

If I put it at one, that is in series -- that is bad. That cannot be it. 0363

At 2 -- that is in series with the air resistor. That is bad. 0367

I could also put it at 3 and that would work. 0372

So I need the voltmeter at either 4 or 3 and it looks like I only have one answer left -- Answer A because it does not say I can put an ammeter at 1 and a voltmeter at 3. 0373

So this is our best answer. 0383

Let us take a look here. 0388

Which circuit diagram below, correctly shows the connection of ammeter (A) and voltmeter (V) to measure the current through and the potential difference across resistor(R)?0390

Well, if we want the current through (R) that means that the ammeter must be in series with our resistor and the voltmeter must be in parallel with our resistor. 0398

Where does that happen? 0409

Well, if I look down here at 4 -- current goes from positive to negative -- that is what we want for our basic circuit. 0412

The current is going through the ammeter -- that is good -- and the voltmeter over here is connected in parallel with (R) and if I was to disconnect it, the circuit would still work. 0418

So 4 must be our best answer here. 0427

Voltmeter placement -- a student uses a voltmeter to measure the potential difference across a resistor. 0434

To obtain a correct reading, the student must connect the voltmeter. 0439

Well, right away you should know the voltmeter must be connected in parallel with the element you want to measure -- in parallel with the resistor. 0442

One more. Which statement about ammeters and voltmeters is correct? 0450

The internal resistance of both meters should be low. 0455

No, that is not the case. If a voltmeter had very low resistance, it would have a lot of current going through it and we would probably burn it up. 0458

That is a bad answer. 0465

Both meters should have a negligible effect on the circuit being measured. 0467

Well, yes, we kind of want that negligible effect. 0470

We do not want the voltmeter and the ammeter to impact the performance of the circuit. 0473

We are going to use these as tools to help us understand what is going on in the circuit as if we did not have them there. 0477

So B looks like a good answer. 0483

Let us check C -- The potential drop across both meters should be made as large as possible. 0485

If you do that across an ammeter -- because it does so little resistance, you are going to have a ton of current going through it. 0490

You are going to make it nice and toasty and probably melt it. That cannot be right. 0496

And the scale range on both meters must be the same. 0501

Well, that is just silly. We have not even talked about scale ranges. 0503

So our best answer here is B. We want the meters to have a negligible effect on the circuit being measured. 0506

Hopefully that gets you just a very brief introduction to basic circuits and how we are going to use meters -- electrical meters -voltmeters and ammeters to measure their performance. 0513

Thanks so much for your time. Make it a great day.0521