Question

A fourth grader decides she is going to make an electromagnet for her science fair project. She uses an iron nail, 50.cm of #22 copper wire and one D cell battery. The battery soon becomes so hot it burns her hand. She asks you why? You explain it is due to the internal resistance of the battery.

Objectives:

To design an experiment (do not test the plan) to measure the internal resistance of the battery in the fourth grader's electromagnet.

To plan and communicate an original experimental design.

To demonstrate a working knowledge of measurement techniques, ohm's law and energy dissipation in a DC circuit.

Materials:

None required

Investigate

Read through the entire lesson first. You are not to preform any hands-on experimentation in this assignment.

Design an original experiment that will measure the internal resistance of a D cell battery in the situation described.

Place a detailed description of your experimental design in the class Lab Data Wiki. Make sure you include:

A purpose

A detailed procedure

An equipment list

A safety statement

What data you will collect

What graphs/equations you will employ

What calculations you will make to achieve the stated purpose

Answer 1

Purpose: Experiment to measure the internal resistance of a DC Battery

Equipment list - DC Battery of known voltage, the internal resistance of which needs to be measured, a potentometer, An ammeter, interconnecting wires, voltmeter

Safety Statement - Please ensure to use a fuse in series with the battery to avoid high power dissipation, which could lead to explosions

Detailed Procedure

1. Note the potential of the battery as V_bat.

2. Connect the circuit as shown in the diagram

3. Set the potentiometer to 0 and note the current through the potentiometer on the ammeter. Also measure the voltage across the potentio meter.

Data to be collected

Increase the value of the potentiometer to 50 ohms in steps of 10 ohms and measure the voltage V_pot and current I, in each case.

Graphs/equations

From the circuit, Vbat - Vpot = I x R_i

Internal resistance R_i = (Vbat - Vpot)/I

The R_i calculated in each of the 6 steps (0,10,20,30,40,50 ohms of the potentiometer) can be averaged to account for experimental errors.

Plot V vs I. The negative slope of the graph is the internal resistance of the battery.

Calculations

The R_i calculated in each of the 6 steps can be averaged to account for experimental errors to obtain the final value of R_i