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Overview
In this lab you will learn the basic techniques of constructing circuits and making measurements using the iOLab. Since many of the labs you will perform. involve electrical circuits, you will need to understand how the iOLab can function as a power supply, a device that applies a voltage to a circuit, and a voltmeter and ammeter, devices that can measure the voltage and current at particular places in the circuit.
Let’s begin by defining an electrical circuit. A circuit is literally a loop. An electrical circuit is a loop made of various electrical components like batteries, wires, resistors, light bulbs, capacitors, and diodes. Electrical circuits can comprise multiple loops interconnected in thousands of different ways.
Of course it is always best to start with the simplest case of a single circuit. First you will need to connect some wires that come with the Electricity and Magnetism Accessory Pack. The wires are bundled together and you can simply peel the wires apart as needed. The wires typically have a pin end and an alligator clips end:
The pin ends connect to the connection points on the iOLab or breadboard. The alligator clips can connect to different circuit elements or connect to each other to make wires longer.
Now setup your iOLab device:
● Detach the dongle from the iOLab device and connect to a USB port on your computer.
● Launch the iOLab application installed on your computer. The dongle indicator at the top of the application window should be on.
● Check the Analog 7 in the list of Sensors along the left side of the application window.
● Turn on the power of the iOLab device. The remote indicator at the top of the application window should be on.
● Connect a wire to the 3.3V output of the iOLab device. Connect a second wire to the A7 input of the iOLab device.
Your iOLab setup should look like this:
Part 1 - Breadboard Connections
The breadboard has MANY holes and so an attempt to try every pairwise combination would be tedious. Instead of trying every pair of holes, you are asked to apply intelligence to the issue and try enough holes that the pattern of connections has become absolutely certain.
Here is a video to show you how to test the breadboard connections. The breadboard in the video may differ from the one you have, BUT the concept of testing the connection is the same.
Breadboard Connections (https://youtu.be/ERCLlfAnSZ4)
Remember:
● A measurement of 3.3V means the two points you test on the breadboard are connected. The circuit is closed.
● A measurement of 1.5V means the circuit is open and the two points NOT connected. The circuit is NOT closed.
1. Use the image of the breadboard below and draw lines across all holes you have determined to be connected.
Once you know which holes in the breadboard are connected to each other and which ones are not, we want to use the breadboard to construct circuits. The advantage of using a breadboard is it is easy to connect circuit elements together without soldering.
Part 2 - Voltage and Voltmeters
We build all kinds of circuits to do things. In our home and places of work there are lots of circuits to run the electronic devices we use everyday; to light the lights, power computers, TVs, etc. But all of these things need energy. Where does the energy come from?
The energy that runs electronics comes from the electrical potential difference or voltage applied to the electronics. To see how that works, let’s look at what a voltage is, or better what the unit of voltage is. A voltage is measured in Volts (V):
Right away you should recognize a Joule (J) is the SI unit of energy. A Coulomb (C) is the SI unit of electric charge, so a Volt is the energy per unit charge applied to the circuit. When charges in the circuit have energy, they make the circuit do the things it is supposed to do. The light bulb lights up or the speaker emits sound or music.
Now let’s make a very simple circuit with a light emitting diode (LED). Your E&M accessory pack should have two LEDs; one red and one green. Take out the red LED:
The LED is polarized, which means it only works when the voltage is applied to it a certain way. Remember voltage is an electrical potential difference between two points in a circuit. If one potential is higher (+) than the other potential (-), then the voltage is positive. Notice on the LED one of the terminal wires is longer than the other. Also notice that at the base on the LED “bulb” is flattened. The longer terminal prefers to be connected in the circuit at the higher potential, while the terminal on the flattened side prefers the lower potential.
Construct a circuit using the iOLab, alligator clip wires, breadboard, and red LED.
● Connect the red alligator clip wire between the high side (+) of the LED and 3.3V on the iOLab
● Connect the black alligator clip wire between the low side (-) of the LED and GND on the iOLab.
Your circuit should like something like this:
2. Turn on the iOLab power. Describe what happens to the LED. Take a picture of your circuit and insert your picture here:
If your circuit was correct, the LED should light up.
3. Swap the alligator clips on the LED. With the iOLab power on, describe what happens to the LED. Take a picture and insert your picture here:
What is the voltage across the LED? At first you may say it is 3.3V because the LED is connected to the 3.3V output of the iOLab. But instead of just using the value 3.3V, let’s measure the voltage. How do you measure the voltage? With a voltmeter! A voltmeter is a device that measures the potential difference between two points. Voltmeters need to be connected to two points in the circuit. Whatever is in between those two points is at the voltage between those two points.
Let’s measure the voltage across the LED. Connect two more alligator clips to the LED.
● Connect the red alligator clip from the high side of the LED to 3.3V on the iOLab
● Connect the yellow alligator clip from the high side of the LED to A7 on the iOLab.
● Connect the black alligator clip from the low side of the LED to GND on the iOLab.
● Connect the white alligator clip from the low side of the LED to GND on the iOLab.
● Launch the iOLab application and choose A7 from the list of sensors.
A7 is a voltmeter built into the iOLab. It measures the voltage between the point A7 is connected and GND.
● Click the Record button and record the A7 voltage for about 5 seconds then stop .
4. Measure the voltage across the LED.
a. Use the Analysis Mode to measure the voltage for the flat segment of data.
b. Take a print screen (PrtScr) of the the iOLab app and paste it here:
5. Record the average value () +/- uncertainty () of the voltage across the LED in box below:
LED Voltage (V) |
+/- |
6. When you connect a voltmeter to a circuit, is it connected in series or parallel? Explain why in terms of potential difference.
Part 3 - Current and Ammeters
Now we know what a voltage is and how it is measured, let’s consider electric current. Just like we did with voltage, let’s look at the units of electric current. Current is measured in Ampere or Amp (A):
Again, you should recognize that Coulomb (C) is the SI unit of electric charge, so an Amp is the amount of charge moving through the circuit per second. The higher the Amp the more charges are moving. Be careful not to assume Amps measure how fast the charges are moving. It just measures how much charge is moving; not how fast they are moving.
Not surprising, the current moving through the circuit is measured by an ammeter. Ammeter only measures the current through one point in the circuit. If there are multiple circuits connected, the current through each circuit can be different.
Let’s measure the current through the LED. First we will need to change the circuit and add a resistor to the circuit. We will use a 1 ohm resistor from the E&M accessory pack. The 1 ohm resistor is in the pack with the label ending with code J0109. Look closely at the resistor and you will see 4 colored bands. the bands correspond to digits that represent the value of the resistance. A 1 ohm resistor has the colors brown(1), black(0), gold(0.1), and gold(5%). There is a formula for calculating the resistance:
(1×10 + 0)×0.1 ± 5% = 1 ohm ± 5%
We will cover the value of resistance in more detail in Lab 2. For now we just want to use a 1 ohm resistor.
Now construct a circuit with the red LED and the 1 ohm resistor. Connect one terminal of the resistor to the low (-) side of the LED; i.e., the terminal of the resistor and the low (-) side of the LED should be in the same row of holes on the breadboard.
● Connect the red alligator clip from the high side of the LED to 3.3V on the iOLab
● Connect the yellow alligator clip from the high side of the resistor to A7 on the iOLab.
● Connect the black alligator clip from the low side of the resistor to GND on the iOLab.
● Connect the white alligator clip from the low side of the resistor to GND on the iOLab.
Your circuit should like something like this:
● Click the Record button and record the A7 voltage for about 5 seconds then stop .
7. Measure the voltage across the 1 ohm resistor.
a. Use the Analysis Mode to measure the current for each flat segment of data.
b. Take a print screen (PrtScr) of the the iOLab app and paste it here:
8. Record the average value () +/- uncertainty () of the voltage across the resistor in box below:
1 Ohm Resistor Voltage (V) |
+/- |
Wait a minute???? Why are we measuring the voltage across the 1 ohm resistor? How do we know how much current is flowing through the circuit? It turns out that an ammeter is a device with a very low resistance (<1 ohm). When an ammeter is connected to a circuit, it is as if a 1 ohm resistor is connected, then by measuring the voltage across the 1 ohm resistor you can calculate the current through the resistor. The formula that connects the current through and the voltage across a resistor is known as Ohm’s Law. We will study Ohm’s Law in more detail in the next lab. For now all you need to know is the current I is equal to the voltage V divided by 1 ohm:
I = V/(1 ohm)
9. Calculate the average value () +/- the uncertainty () of current through the 1 ohm resistor and record in the box below:
1 Ohm Resistor Current (A) |
+/- |
10. When you connect an ammeter (the 1 ohm resistor) to the circuit, is it connected in series or parallel? Explain how this determines the current through the LED.