Download Series and Parallel Circuits 2 - Instructor Outline

Instructor Outline:
Series and Parallel Circuits II
UM Physics Demo Lab 07/2013
Lab length: 70 minutes
Lab objective: Additional instruction about combined series and parallel circuits,
voltage, current, Kirchhoff’s Loop Rule, Kirchhoff’s Junction Rule, Ohm’s Law and the
correlation of power and brightness for light bulbs.
Materials
1 green multimeter (with leads)
1 battery board
1 alligator lead card
Suggested Demonstrations: None
Introduction: 10 minutes – Lecture
The concepts of series circuit elements, parallel circuit elements, voltage, current, and
power are reviewed and discussed in further detail. Kirchhoff’s Loop Rule and Junction
Rule are reviewed along with a qualitative discussion of Ohm’s Law. The connection is
made between the Loop Rule and the conservative nature of the electric force, a
concept previously introduced for gravity. The junction rule is compared to the
condition of continuity for a flowing fluid.
Exploration stage: 55 minutes – Group Lab-Work
The students measure current and voltage for a circuit that combines series
and parallel light bulbs. They observe where the current flows based on what happens
when they unscrew various bulbs.
The also make observations regarding bulb
brightness and relate them to the power dissipated in each bulb.
Summary: 5 minutes – Lecture
The relationship between power, brightness and current for light bulbs is reviewed in
the context of the Junction Rule and current (charge) conservation.
Concepts developed:
1. The total voltage available to a circuit is determined by the number of cells in
series for the source battery.
2. The electric force in a DC circuit is conservative.
3. The sum of the component’s voltages in a series circuit is equal to the voltage
of the source (Kirchhoff’s Loop Rule).
The Loop Rule results from the
conservative nature of the electric force and is equivalent to the statement that
the electric force does no net work over a closed loop path, exactly the same as
for gravity.
4. The current is the same throughout each element of a series circuit.
5. The current is inversely proportional to the number of resistors (bulbs) in the
circuit (Ohm’ Law).
6. The current divides in a parallel circuit, but the total current must be conserved
(continuity, charge conservation and the Junction Rule).
7. The voltage across each component in a parallel circuit is the same and equal
to the source voltage. This implies that the voltage across each element in a
series circuit is less than that for a parallel circuit for the same source potential
(battery voltage).
8. Power/brightness is the product of the voltage across a bulb and the current
flowing through it.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109