ohm lab worksheet

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Northwestern University *

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Electrical Engineering

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May 23, 2024

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Student name: Lab 4: Ohm’s “Law” This is the first of three circuit labs. At the end of this lab, you should be able to construct a simple circuit and make measurements of current and voltage in that circuit. You should also be able to decide whether data are consistent with theory or expectations and support that decision with evidence. Introduction: Circuit elements and functions Electronic circuits are part of our everyday lives, but you likely haven’t had much introduction to them in lecture yet, so let’s go over some of the basics. A circuit is a path (or paths) of wire and other elements that electrons travel through and around. In order for electrons to travel and electricity to flow, the circuit must be a closed loop . If you have a closed loop and then another wire attached to that loop on one end, but dangling on the other end, electricity won’t flow through that dangling wire. Electrons flow because an electric potential is set up within the wires of the circuit. This potential is often created by a battery or a voltage supply – it’s no coincidence that electric potential has units of volts (V) and the batteries you can buy at the store are described as 12 V batteries. A voltmeter measures the potential difference between two different points in the circuit. If you hook a voltmeter up across a 5 V power supply, it should read 5 V because the power supply is applying a potential difference of 5 V. If you want to compare the flow of electrons with the flow of water, the battery is like the water pump raising water to a higher level so that it has more potential energy. Current describes how quickly and in which direction the electrons are flowing. Confusingly, current is described as the motion of positive charge, so it flows in one direction while the electrons flow in the exact opposite direction. Current flows from high electric potential to low electric potential. We measure current with a device called an ammeter and the units of current are amperes (A). The battery provides potential energy to the flowing charges and resistors dissipate that energy. Resistors that dissipate more energy have a higher resistance . A circuit diagram is a drawing that shows how different circuit elements are connected – it’s an instruction manual for how to construct a circuit. Every circuit element has a symbol. Those symbols are connected with lines that represent the connecting wires between the physics circuit elements. Here’s a circuit diagram Page | 1

that has a resistor and a battery connected together. The resistor symbol is the zig-zag and the battery is represented by the double line. This next diagram has the same circuit, but now with a tool measuring the current through the circuit and another tool measuring the potential difference across the resistor. Two of the circuit elements are the same as in the previous circuit diagram – make sure you can identify those. You should also be able to identify the two measurement tools in the circuit – both of them are represented as a circle with a letter in it. When these tools are used correctly, they don’t disturb the circuit’s behavior, so you’re effectively measuring the properties of the circuit as well. Introduction: Ohm’s Law German physicist Georg Simon Ohm was the first to publish his observation that the electric current I that flows through an object is proportional to the difference in electric potential V between the object’s ends. Mathematically, Ohm’s Law is: V = IR . (1) The constant of proportionality R is the resistance of the object, and it depends on that object’s physical properties. Resistance is measured in units of ohms. The abbreviation for the unit is the Greek capital letter omega ( Ω ). In fact, Ohm’s “law” has exceptions and therefore is not a universal physical law. It doesn’t apply to all materials or all circuit elements. Semiconductor diodes and transistors, for example, do not obey Ohm’s Law, and this ‘nonlinear’ behavior is key to their use in modern electronics. Most conductive plastics do not obey Ohm’s Law either. However, most metallic conductors and simple circuit elements do follow Ohm’s Law, so it is very useful in practice for controlling electric current precisely even if it is not universally applicable. When a circuit element is called a resistor, that indicates that it obeys Ohm’s Law. Additionally, sufficiently small signals approximately obey Ohm’s law in non-linear devices. Familiarization and Setup First, download the “Ohm’s Law.cap” Capstone file and execute it. This will allow you to control the potential difference across the test resistance and to observe the potential difference that exists and the electric current that flows. Page | 2

Figure 1 below shows the same circuit as a circuit diagram (a) and a pictorial (b). Compare the two figures and be sure you can identify the following circuit elements in both the circuit diagram and the pictorial: resistor, battery/power supply voltage, ammeter, and voltmeter (in the circuit diagram, this is just indicated by where the voltmeter would measure the potential difference, not by an element exactly). If you can’t do this, confer with your lab partner or your TA until you are confident. Select one of the three resistors and construct the circuit in Figure 1. Before continuing, ask your TA to check your circuit. a) The electric current, I , is measured by the ammeter and the potential difference across the device, V , is measured by the voltmeter. b) Pasco’s “High Current Sensor” is the ammeter (A), that Pasco’s “Voltage Sensor” is the voltmeter ( V ), and that Pasco’s “Output 1” supplies the battery voltage ( V B ). All of these are controlled by Pasco’s 850 Universal (computer) Interface. c) The ‘Record’ button at the bottom left allows you to apply the potential difference (“voltage”) and to begin recording data. d) Note the units: Current is in mA=0.001A and potential difference is in V. The Ampere is a very large current and would burn up our devices. e) The “Signal Generator” at the left contains the battery voltage controls ( V B ). You will need to vary “Output 1’s” values. Figure 1: A schematic diagram of a simple circuit (a) and an instructional aid (b) for constructing the circuit. We will use this circuit to study Ohm’s law and to study the electrical properties of five devices. Page | 3

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