Direct Current (DC) Circuits Lab

Name:

Group Partners:

Date:

Objective:

In this lab, we will investigate the behavior of direct current (DC) electrical circuits. We will study the flow of electrical current in a circuit from the cell, through the wires, and through various combinations of light bulbs and resistors.

By the end of the lab, you should be able to:

·  identify the components of a circuit;

·  understand the relationships that occur between the components;

·  be able to use all variables associated with Ohm’s Law (V, I, R).

Background:

Before we start the lab, we will talk about electrical charge, voltage, current and resistance, to make sure that you are familiar with these concepts and terms, and the units we use to measure them.

Electrical circuits usually have a power (energy) source such as a cell or battery, and one or more resistors such as a light bulb. A circuit is a path along which electrons can flow. If the circuit is closed, there is a continuous flow of current until the power source is exhausted (finished). Current does not flow in an open circuit. A circuit in which there is a single pathway is known as a series circuit whereas a circuit that has multiple (more than one) possible paths is known as a parallel circuit.

Resistors provide resistance to the flow of current in a circuit. Many resistors obey Ohm’s Law (V = IR), which states that the current I through a resistance R is proportional to the voltage V across the resistor.

Apparatus:

You should have the following equipment in your setup:

·  One 1.5V cell

·  One cell holder

·  Set of ten “alligator-alligator” leads

·  Two small light bulbs (2.5V, 0.3A)

·  Two light bulb holders

·  One digital multimeter to take current and voltage readings

·  Assorted resistors (5)

Procedure:

This lab is designed to have several sections, each of which builds upon the work of the previous sections.

  1. Look at the various components you have in your experimental apparatus and make sure that you can identify each of them in the list above.
  1. Make a simple circuit using the cell (and its holder), a light bulb (and its holder), and some of the connecting leads.

a). What happens to the light bulb?

______

______

b). Draw a circuit diagram representing your circuit:

Conductors and Insulators (Non-Conductors):

  1. Add another lead to your circuit but do not connect two of the alligator clips together (leave the circuit open). Now, look around you for three objects that you can connect the alligator clips to, and connect one to each side. Try to find at least one that is a metal.

a). Does the lamp light for each object or not?

Object 1: ______, does the bulb light or not?: ______

Object 2: ______, does the bulb light or not?: ______

Object 3: ______, does the bulb light or not?: ______

b). What do you think this indicates about whether your test objects conduct electricity or not?

______

______

______

Simple Circuits and Resistance:

  1. Now, make a simple circuit using the cell (and its holder), one of the 20W (ohm) resistors, and some of the connecting leads. (The 20W resistors will have a red ring followed by two black rings (and a gold and red ring) painted on them.) Your circuit should look like your circuit from Section 2, but with the resistor replacing the light bulb.

a). Draw a circuit diagram representing your new circuit:

b). We will now use the multimeter to measure the voltage across the resistor, and the current passing through it. In order to measure the voltage across a component, we need to place the two probe tips on either side of the component – we connect the multimeter in parallel. See if you can measure the voltage across the resistor. In order to measure the current through a component, we need to place the multimeter so that it is part of the current path – we connect the multimeter in series. See if you can measure the current through the resistor.

c). Fill in the table below, measuring the voltage across the 20W resistor, and the current through it. Use Ohm’s Law (V = IR) to calculate the resistance from these two values, and then measure it directly using the resistance option on your multimeter.

Resistor / Measured voltage (V) / Measured current (A) / Calculated resistance (W) / Measured resistance (W)
20W

Resistors in Series and Parallel:

  1. Next we return to studying light bulbs. Reassemble your simple circuit from Section 2 with a cell and a single light bulb. Try to remember how brightly the bulb is shining.

a). Now add a second identical bulb in series. What do you observe about the light intensity (brightness) in each bulb compared to a single bulb?

______

b). What happens if you remove one of the light bulbs from its holder?

______c). Instead of adding the second bulb in series, add it in parallel to the first. What do you observe about the light intensity in each bulb compared to a single bulb?

______

d). What happens if you remove one of the light bulbs from its holder?

______

Effective (Equivalent) Resistance:

Resistors in Series

  1. We look now at two resistors in series. Take two 20W resistors and make a circuit with them connected in series.

a). Draw a circuit diagram representing your new circuit:

b). We can describe the effective resistance of two resistors R1 and R2 in series using the equation: R = R1 + R2. Measure the effective resistance R across both resistors using the resistance option on your multimeter and note the value below:

______

c). Calculate the expected value for R from the equation above and note the value below:

______

d). Is the effective resistance (R) of two resistors in series greater or smaller than the individual resistances?

______

Resistors in Parallel

  1. We will now look at two resistors in parallel. Take two 20W resistors and make a circuit with them connected in parallel.

a). Draw a circuit diagram representing your new circuit:

b). We can describe the effective resistance of two resistors R1 and R2 in parallel using the equation: 1/R = 1/R1 + 1/R2. Measure the effective resistance R across both resistors using the resistance option on your multimeter and note the value below:

______

c). Calculate the expected value for R from the equation above and note the value below:

______

d). Is the effective resistance (R) of two resistors in parallel greater or smaller than the individual resistances?

______

  1. Finally, take a “mystery” resistor (it should have rings that are coloured either brown; black; brown; gold or orange; black; brown; gold; red). Choose any method of measurement except the resistance option on your multimeter to determine its value. Once you have your answer, ask a member of the Physics Emasondosondo team if you are correct!