Objectives for This Lab

Industrial Electricity

Name______

Due Friday 2/09/18

Lab 4: Investigating Series DC Circuits

Objectives for this lab:

Construct a series electrical circuit in a safe and logical manner

Practice using a DMM to measuring voltage, current and resistance in a series electrical circuit.

Verify the relationships between voltage, current and resistance in a series electrical circuit.

Use measured values of voltage and current to determine the ohmic value of an unknown resistance.

Communicate information (calculation steps / English word) effectively

Grading for this lab:

Complete pre-calculations (neatly) and populate tables prior beginning lab build 3 points _____

Follow all instructions and complete the lab build safely and logically

(you may work in pairs or alone) 10 points _____

Correctly, neatly answer the follow-up questions at the end of the lab assignment 10 points _____

Submit neatly completed lab document as well as supporting documents 2 points _____

TOTAL /25

Introduction

A series circuit is defined as one in which all of the circuit components are connected end-to-end as shown:

The total resistance in a series circuit is found as

RT = R1 + R2 + R3 + … RN.

By virtue of the fact that all circuit components are end to end, there is only one path in which current can flow. The implication of this leads us to an important concept regarding series circuits:

The current in a series circuit is the same at all points in that circuit

Kirchhoff’s voltage law, or loop rule, states that the sum of all the voltage drops in a closed loop must equal zero. Or, equivalently, the sum of all the voltage drops in a series circuit must equal the supply voltage. The application of Ohm’s law I =E/R to the individual circuit components provides proof that this is so. Thus, the total voltage in a series circuit can be written as:

ET = E1 + E2 + E3 + … EN.

where E1, E2, ..., EN are the voltages across the individual resistors.

Objectives

In this lab you are about to verify the properties of series circuits by:

· Connecting components together in series configuration using a breadboard

· Measuring resistance using a DMM (circuit off, components disconnected from power supply

· Measuring current and voltage using a DMM (live circuit)

· Determine the value of an unknown resistor using Ohm’s Law

Equipment

(1) Power Supply (1) DMM; Digital Multimeter (1) Protoboard

Components

(4) Resistors: (2) 1.0kW, (1) 2.0kW, (1) 3.0kW and (1) “mystery” resistor

(~10) Jumper wires

Procedure

1. Connect four resistors in series as shown in Figure 1 below. Calculate the total resistance and record this value in Table 1.

Figure 1.

2. Use the DMM to measure the individual resistance values, the combination of R1 + R2 and the total resistance of the circuit. Record these values Table 1.

3. Calculate the theoretical current flowing in the circuit. Record this value in Table 3.

Figure 2.

4. Calculate the theoretical voltage drops for the circuit shown in Figure 2. Record these calculated values in Table 2.

5. Turn on the power supply and connect the DMM to the output. Adjust the output voltage to 10V. If you are unsure of this procedure ask the lab instructor for assistance.

6. Connect the power supply to the circuit. Measure and record the voltages across the individual resistors and across the resistor combination specified in Table 2.

7. Power down the circuit and remove the jumper wire between R2 and R3.

8. Configure the DMM to measure current. Depending on the DMM you are using, this will involve turning the dial to amps or milliamps and may involve moving the leads to the appropriate holes. If you are unsure of this procedure, check with the lab instructor. Improper connection could result in equipment damage or, at the least, a blown fuse.

9. Connect the DMM between R2 and R3 in order to measure the circuit current. Turn on the power supply and record the current reading in Table 3.

10. Obtain a “mystery” resistor from the lab instructor. Replace R2 with the mystery resistor.

11. Measure & record the current as in step 9. Record this reading in Table 3.

12. Power down and replace the jumper between R2 and R3.

13. Using the DMM, measure the voltage across the mystery resistor. Record this value in Table 3.

14. Power down the circuit and disconnect the equipment & components. Return them to their proper place.

Table 1: Ohmmeter Measurements

R1 / R2 / R3 / R4 / R1 + R2
Nominal Values
Measured Values
Calculated Total Resistance of Figure 1 (from step 1)
Measured Total Resistance of Figure 1

Table 2: Series Circuit Voltage Measurements

Across
R1 / Across
R2 / Across
R3 / Across
R4 / Across
R1 + R2
Calculated Voltages (from step 4)
Measured Voltages

Table 3: Current / Voltage Measurements

Calculated Current (from step 3)
Measured Current
! CAUTION! Remember the rules for measuring current
Measured Current through “mystery” resistor
Measured Voltage across “mystery” resistor

Follow-up Questions

1. Use Ohm’s Law and the measurements made during the lab to calculate the value of the mystery resistor. SHOW YOUR CALCULATION(S) CLEARLY BELOW.

2. Refer to Tables 1 & 2. The nominal sum of R1 + R2 = 2 kΩ. Is the sum of the voltages across these resistors approximately equal to the voltage across the 2 kΩ resistor? Should it be? Explain and/or support your answer with the measurements gathered in this lab.

3. Refer to Tables 1 & 2. R4 is three times larger than R1. Is the voltage across the R4 three times larger than the voltage across the 1.0 kW resistor? Should it be? Explain and/or support your answer with the measurements gathered in this lab.

4. If additional resistance is placed into a series circuit but the voltage is held constant, does circuit current increase or decrease? Give a simple example to prove you understand the question.

5. If the voltage supplied to a circuit is increased, but the resistance is held constant, does circuit current increase or decrease? Give a simple example to prove you understand the question.