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Circuits I and II

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Topics

DC circuits: Current, resistance, voltage, conductors, insulators, series circuits, parallel circuits, Kirchhoff's loop rule

Points of Emphasis

Illustration of the scientific process of model building.

Prerequisites

Circuits I assumes no prior experience with electrical circuits. Circuits II assumes that the students have a model for electric current that they will enable them to relate bulb brightness to current that is developed in part I.

Lab Activities

Circuits I

Part I - Complete Circuits:

  1. Students are give a single wire, a light bulb and a battery and asked to arrange the three components in various ways and note what arrangements will light the bulb and which will not. Introduces the concept of a complete circuit.
  2. Using the same materials in part a, the students insert various materials into the circuit (coins, paper, pencil lead, etc. to determine which will allow the bulb to light. Introduces the concepts of insulating and conducting materials.

Part II - Bulbs in Series:

  1. Students construct a series circuit with two light bulbs in series with a battery. Based on the relative brightness of the two bulbs, the students are asked to consider whether or not current is used up in the circuit.
  2. The concept of resistance is introduced. Students compare the brightness of the bulbs in the two bulb circuit with the brightness of the bulb in a single bulb circuit. They are asked to consider (1) how the current through the single bulb compares to current through the two bulb circuit and (2) to formulate a rule for predicting how the current through the battery would change as the number of bulbs in the circuits in increased or decreased.

Part III - Bulbs in Parallel:

  1. Students construct a parallel circuit with two light bulbs and a battery. Based on the relative brightness of the two bulbs, the students are asked how the current from the battery is divided up among the two branches in the circuit.
  2. Students compare the brightness in the two bulbs to the brightness of a single bulb and are asked to formulate a rule for predicting how the current through the battery would change as the number of bulbs in parallel branches are increased or decreased.
  3. Finally, the students are asked to construct a circuit that is made up of three bulbs, two of which are connected in parallel and then are put in series with the third bulb. A switch is inserted in the parallel branch of the circuit. The students are asked to predict how the brightness of the three bulbs when the switch is open or closed. Students see that the simple model of circuits developed thus far is inadequate to predict the currents in a circuit composed of both series and parallel circuits.

Circuits II

Part I - Current and Resistance:

Part I is a review of current. Students are given three hypothetical circuits consisting of two or three light bulbs and asked to predict the relative brightness of the bulbs in each circuit. Students are asked what their observations imply about the relative currents through the batteries.

Part II - Potential Difference:

  1. In part a, students construct a series circuit consisting of one battery and one light bulb. Using a voltmeter, the student mearsure the voltage across the battery and then across the bulb. The two values are compared.
  2. In part b, students construct a series circuit consisting of one battery and two light bulb. Using a voltmeter, the student mearsure the voltage across the battery and then across each of the bulbs. The students compare the voltage across the battery in this circuit to the voltage measured in part a. They also compare the brightness of each bulb in circuit with the brightness of the bulb in part a. A connection is made between the brightness of the bulb and the potential difference across each bulb.
  3. In part c, the students compare the total potential difference across both bulbs with the voltage across the terminials of the battery.
  4. In part d, the students construct a parallel circuit consisting of a battery and two light bulbs. Voltage measurement similar to those done in parts a, b and c are made. Based on their observations, the students are asked to answer two questions: (1) Does the current through the battery depend on the circuit to which it is connected? and (2) Does the potential difference across the battery depend on the circuit to which it is connected?

Part III - Extending the Model:

The students are guided to formulate Kirchhoff's loop rule.

Course Level

Introductory level courses: Physics 4, Physics 14 and Physics 16.

Student Handouts

Faculty may see either David Abbott or Jan Largent for a copy of the write-up and the lab homework.

Equipment

Number of set-ups available: 20

Per lab station:

1 multimeter
2 D-cells
2 battery holders
2 banana leads
2 alligator clips
5 clip leads
3 miniature light bulbs (> 3V rating)
3 miniature light bulb sockets

Circuit's apparatus

Notes

  1. This lab exercise comes from Tutorials in Introductory Physics by Lillian C. McDermott, Peter S. Shaffer and the Physics Education Group, Department of Physics, University of Washington.
  2. The miniature bulbs used in this lab should have a voltage rating of greater than 3 V.

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Site contact: largent@Dartmouth.EDU

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