LAB Using Ohm S Law to Calculate Resistance

LAB – Using Ohm’s Law to Calculate Resistance

in Series and Parallel Circuits

BACKGROUND:

SERIES CIRCUITS:

During this investigation, you will use three resistors to determine how these resistors combine when they are connected in series. A series circuit consists of resistors connected in a circuit with the source in such a way that the current must flow through each resister in turn because a single pathway has been created. The total resistance of a series circuit is, therefore, the sum of the resistances of the individual resistance in the circuit.

RT = R1 + R2 + R3

PARALLEL CIRCUITS:

During this investigation, you will use three resistors to determine how these resistors combine when they are connected in parallel. A parallel circuit consists of resistors connected in a circuit with the source in such a way that the current can flow through all resistors simultaneously. The total resistance of a parallel circuit is, therefore, calculated as follows:

OHM’S LAW:

Ohm’s Law states that if the temperature of the resistance remains constant, the electric current (I) flowing in a circuit is directly proportional to the voltage (V) and inversely proportional to the resistance (R) of the circuit.

I = V/R or V=IR

During this investigation, you will use a voltmeter to measure the voltage across a resistance and an ammeter to measure the current flowing through a resistance. You will then apply Ohm’s Law to calculate resistance.

PURPOSE:

To determine the values of resistors by applying Ohm’s Law.

To observe how resistors add in series circuits and diminish in parallel circuits.

To observe how voltage and current are affected by resistors arranged in series and parallel.

MATERIALS:

Battery (D-cell) and holder

Jumper wires (8)

Multimeter (measures current and voltage)

Resistors (10 W, 15 W, 22 W)

PROCEDURES:

SERIES CIRCUIT

1.  Study Table 13-2: Schematic Diagram Symbols on p. 448 of your text to help you understand the schematic diagrams in this lab.

2.  Study the scales on the meters until you can read them properly.

3.  Arrange three resistors in series with each other as shown in Figure 1 and measure the total current through the circuit and total voltage across the circuit. Since you only have one multimeter, you must measure voltage and current in two separate tests. Notice that the multimeter must be connected in series after the resistors to measure current, and it must be connected in parallel across the resistors to measure voltage, or potential difference. Record both total current and total voltage.

NOTE: When your circuit is connected, quickly read the meter and then open the circuit by disconnecting one of the wires. The meters you are using are expensive and can easily be damaged.

4.  Calculate the value of the total resistance by using Ohm’s Law. Record this value.

5.  Use Figure 2 to measure the current through and voltage across Resistor #1. Use Ohm’s Law to calculate R1.

6.  Use Figure 3 to measure the current through and voltage across Resistor #2. Use Ohm’s Law to calculate R2.

7.  Use Figure 4 to measure the current through and voltage across Resistor #3. Use Ohm’s Law to calculate R3.

PARALLEL CIRCUIT

8.  Repeat steps 1-7 with the resistors connected in parallel. Use figures 5-8 for reference.

SCHEMATIC DIAGRAMS:

SERIES CIRCUIT

SCHEMATIC DIAGRAMS:

PARALLEL CIRCUIT

Name :______Period:______Date:______

LAB – Using Ohm’s Law to Calculate Resistance in Series and Parallel Circuits

OBSERVATIONS AND DATA:

Actual value of R1 = 10 W

Actual value of R2 = 15 W

Actual value of R3 = 22 W

Data Table 1: Resistors in SERIES

Data Table 2: Resistors in PARALLEL

* Measured across/through the battery

** Calculate using your measured values

QUESTIONS AND CONCLUSIONS:

1.  While measuring the resistance of a resistor (by use of a voltmeter and an ammeter), which is placed in series with the circuit and which is placed in parallel?

2.  Voltage (or electric potential) is said to be measured “across” a resistor while current is measured “through” a resistor. Why do you think that is?

3.  What is the same for all resistors in a series circuit?

4.  What is the same for all resistors in a parallel circuit?

5.  For a parallel circuit, how does the total current measured across the battery compare to the total current calculated by adding up the current for each resistor (IT = I1 + I2 + I3)?

6.  For a series circuit, how does the total voltage measured from the battery compare to the total voltage calculated by adding up the voltage for each resistor (VT = V1 + V2 + V3)?

7.  Calculate the actual RT for both the series and parallel circuits using the manufacturer’s values for R1, R2 and R3. (R1 = 10 W, R2 = 15 W, R3 = 22 W)

8.  Calculate the percent error for the both circuits.