Introduction: Current, Resistance, and Voltage

The following information is a single lesson in a larger project. Find more great projects here.

Project Overview:

In this project you will learn the basics of current, resistance, and voltage and how they are related by a rule called "Ohm's Law." Then you will use a variable resistor to create a dimmer switch knob, like you might find on a lamp or on your classroom lights!

Step 1: Introduction

All of the circuits that you have made so far share some basic parts.

A power source (like a battery) Components that DO something (like lights and switches)

Wires that connect components together

Resistors that control current flow In this project we will focus on what is happening inside the conductive parts of your circuit: the wires and resistors. You will learn about electrical current and how it depends on other factors like resistance and voltage.

The rule that ties all of this together is Ohm’s Law.

At the end of the project, you will apply Ohm’s Law to a common household device: a dimmer switch!

Instructions

  1. Continue to the next step.

Step 2: Current Basics

Electrical current is the basis for all circuits! When you apply power to a circuit, tiny negatively charged particles called electrons inside the wires and components start to move, making lights light up or motors spin.

This flow has a rate associated with it, measured in units of Amperes or Amps (A). This is the amount of charge, in Coulombs (C), passing through per second.

1 A = 1 C / sec

A Coulomb actually represents a HUGE number of electrons: over 6x10^18 !!

Current also has a direction associated with it. Electrons tend to flow from the (-) side of the battery to (+) side. However, we usually think of current as being the flow of positive charge. This mathematical difference causes us to describe current as flowing from the (+) side of the battery to (-).

The circuit diagram below matches the circuit in the Workplane. The red arrows show how we traditionally describe the flow of current.

  1. In the circuit diagram the red dotted line represents the flow of current. When the button is pressed, current starts moving in the direction of the arrows. The rate of current flow (in Amps) is the same at all points on this loop. That means that charges are moving at the same rate through the LED, button, and resistor.
  2. Try simulating your circuit by pressing the Start Simulation button.
  3. Instruction image Press the pushbutton to close the circuit and start the flow of current. If you hold down the shift key while pressing the button, it becomes a "latching button" that stays on after you release it. The diagram below will help you visualize the flow of current.
  4. Instruction image In this lesson, and in future lessons, we will imagine circuits as a network of city streets. The wires in your circuit as roads that connect components together. Current is the cars, or traffic, driving around the neighborhood. When you press the button, all of the cars that are parked on the streets start to move at the same time. Just like traffic, sometimes current is slow and sometimes it's fast.
  5. Continue to the next step.

Step 3: Measuring Current

In the simulator, you can measure the flow of current using a device called a multimeter. The multimeter can be used to quantify resistance, voltage, or current in your circuit by connecting wire probes in the right places.

Since current flows through components and wires, we want to pass the same current through the multimeter. We'll do that by breaking one of the wires and inserting the meter.

  1. Start by opening the Components + menu and dragging a multimeter into the Workplane. Using the dropdown menu, change its mode to "Amperage" for measuring current. The small A on the multimeter will be highlighted.
  2. Next, delete or shorten the wire that connects the LED to the button, using the picture as a guide.
  3. Extend the terminals of the multimeter with wires so that it completes the gap in the circuit. Pay attention to the colors of the multimeter terminals, which correspond to the colors of the battery terminals. You want to position the multimeter so current is flowing from red to black, or from positive to negative.
  4. Simulate the circuit and press the pushbutton! The LED will come on again, and you will also have a numerical reading of current.
  5. In our circuits, current will normally have a low value in the milliamp (mA) range. In our example, it's about 14 mA or 14 thousandths of a coulomb per second.
  6. Continue to the next step.

Step 4: Voltage Basics

When a car is driving around a neighborhood, it needs some energy to move! The same is true of electrical charge moving around a circuit. Charges move under the influence of voltage.

Voltage is the difference in potential electrical energy between two points. Current flows from points of high voltage to low voltage, usually through wires and components.

Voltage is measured as the amount of energy (in Joules) available per unit of charge. The units of voltage are volts (V).

1 volt = 1 Joule / coulomb

The battery is a voltage source which sets the amount of energy available per unit charge for the circuit. Each component uses that energy and has a measurable voltage drop across its terminals.

  1. You can imagine voltage levels in a circuit as being like a landscape with height variation. If you turn of your car's engine and start at the top of a hill, you will roll from a tall height to a low height, as long as there is a path for the car. In this circuit the positive side of the battery is like the top of the hill, and current flows through the wires and components to get to the bottom, or the negative terminal.
  2. As the diagrams show, the voltage drops across the components add up to the voltage source. 9V = 2V + 7V
  3. Let's think about the voltage drop across the LED. This is a special component because its voltage drop, which we call a "forward bias," is fairly constant compared to other components. In this project, you will see the voltage drop vary between 1.8 and 2.1 V. We'll describe this further in the next lesson, but in the diagram above we assumed an average voltage drop of 2 V. The resistor uses the remaining 7 V.
  4. What is the voltage drop across the button? It's actually zero! When the button is pressed, you can think of it as a continuation of the wire.
  5. Continue to the next step.
  6. Stuck? HINT: In 123D Circuits all colored LEDs will drop 2.0 volts, but in the physical world each color will have slightly different voltage drop ranging from ~1.5 to ~3 volts. This is because In physical circuits, different colored LEDs are made out of different materials that turn electrical energy into light.

Step 5: Measuring Voltage

Let's consider the hill and traffic analogy again. On a steep hill, a car will roll down very quickly. On a hill with a lower pitch, cars will roll down slowly.

The same is true for voltage and current! A lower voltage(i.e. potential energy drop) results in a lower current. In this step you will visualize the current through the LED becoming dimmer when a 3V battery is used. Voltage is measured across a component. We'll measure the voltage drops across the LED and resistor with multimeters and see how they add up.

  1. Start by deleting the multimeter that measures current - you won't need it for this step! Remember to close the gap in the circuit with a wire! It will look like the circuit that was originally in the Workplane.
  2. Drag a multimeter into the Workplane and place it above the LED. Change its mode to "Voltage" using the dropdown menu.
  3. Extend wires from the multimeter terminals to either side of the LED, as the picture shows. We used blue wires for the multimeters measuring voltage across a component.
  4. Drag one more multimeter into the Workplane and place it next to the resistor. Change its mode to "Voltage" as well.
  5. Extend wires from the multimeter terminals to the top and bottom leads of the resistor, as the picture shows.
  6. Now simulate the circuit and press the button! The voltage drops across the LED and resistor should add up to 9 volts. (See hint for more information.)
  7. Stop the simulation and replace the 9V battery with a 3V coin cell, using the picture as a guide. Be sure to match the positive (+) terminal to the red voltage rail and the negative (-) terminal to black.
  8. Start the simulation and press the button! Do the voltage drops add up to 3V? What happens to the brightness of the LED? You should see that the brightness decreases! This happens
  9. Continue to the next step.
  10. Stuck? HINT: In reality, the voltages will add up to slightly less than 9 volts. The battery has what's called an internal resistance -- it effectively acts as a resistor inside the battery, using some of the voltage, and leaving less than 9 volts to distribute to the other components.

Step 6: Resistance Basics

Now you know a little about current and how it originates from a voltage source. A resistor is what you use to control the rate of that current flow.

Current flows through a wire because it is made out of a conductive material. Conductive materials simply allow electrons to move through them. A typical wire is made out of a strip of copper metal, which allows electrons to move quite easily.

A resistor is made out of a conductive material - but not a very good one! In a circuit, resistors restrict the flow of current by lowering the amount of charge that can pass by every second. A resistor will lower the brightness of a light or the volume of a buzzer.

Resistors are measured in units of ohms. Higher values limit current more strongly.

  1. A resistor can be thought of as a road that slows down traffic because it is hard to drive on. If a wire is a two lane highway with a high speed limit, a resistor would be a winding, one lane dirt road. The circuit diagram symbol for a resistor, shown above, actually looks like a rough windy road!
  2. Simulate the circuit and press the button to turn on the LED. Keep the LED on by holding down the shift key while pressing the button.
  3. Highlight the resistor by clicking on it. You will see that the original value is 500 ohms. Change its value in ohms using the dropdown menu. Try several values between 400 and 10,000 ohms. What happens to the brightness of the LED? tip: you can change the resistance value while the simulator is running and the button is pressed!
  4. Change the resistance to 1 ohm and then use the dropdown menu to change the units. This will change the resistance by factors of 1000 1 m-ohm = 0.001 ohms 1 k-ohm = 1,000 ohms 1 M-ohm = 1,000,000 ohms
  5. When the resistance is really low (less than 100 ohms), you should see a starburst symbol over the LED, as if it is bursting. In the next lesson, you will learn how to use a resistor to prevent this from happening!
  6. Continue to the next step.
  7. Stuck? HINT:

Step 7: Measuring Resistance

Resistance is a property of a conductive object, so you will actually measure it while the circuit is not running, (in this case, while the button is not pressed).

To measure resistance with a multimeter, you will measureacross the component by connecting one wire to each side of the resistor.

  1. To simplify the Workplane, delete the multimeter measuring voltage across the LED - you won't need it for this step.
  2. The resistor already has a multimeter across it! Use the dropdown menu to change its mode to "Resistance." The R symbol will be highlighted.
  3. Simulate the circuit! The meter will immediately start showing a resistance value.
  4. Highlight the resistor and use the dropdown menu to change its value or its units. Does the number on the multimeter change to match the menu?
  5. Bonus: If you have a multimeter in your classroom, set it to resistance mode and try measuring different objects to see if they are good or poor conductors. Some suggestions include paper clips, coins, different parts of a pencil, or your own skin!
  6. Continue to the next step.

Step 8: Review

In this lesson you learned a little about current flow through a simple LED circuit, and how resistance and voltage play a role, and how to measure current, resistance, and voltage with a multimeter.

In your time remaining, try changing the values of the resistor and switching between the 9V and 3V batteries to see the following trends:

Higher resistance results in a dimmer LED and lower current

Higher voltage results in a brighter LED and higher current

Keep these trends in mind when you move onto the next lesson! You will learn how voltage, resistance and current are related to each other mathematically by Ohm's Law.

  1. Delete the last multimeter to return to the original circuit layout. Simulate the circuit again and explore different resistor and battery values.
  2. Try exchanging the LED for another output component like a motor. What does changing the resistor or battery do to this component?
  3. Continue to the next lesson to learn about Ohm's Law!

Next Lesson:Ohm's Law

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