Demonstration Manual

Electricity and Magnetism

DEM20.1 - Temperature Coefficient of Resistance (Method II)

Objective:

- To model the movement of electrons through a conductor and the effect of temperature on the resistance of a conductor.

Electrons are modeled using steel balls, the conductor is modeled using a grid of nails, voltage is modeled by sloping the grid and temperature is modeled by causing the grid of nails to vibrate along an axis perpendicular to the slope.

Apparatus:

• audio generator (Wavetek model 182A)
• audio amplifier (Kenwood model KA 3500)
• large speaker with mounting post attached to cone (as used in the
• vibrating wire demonstration)
• a pair of long spade/spade leads (connects amp to speaker)
• RCA/BNC lead (connects audio generator to the amp)
• lab stand and rod
• 90° clamp
• wooden support for speaker
• custom nail grid
• five 3/4" steel balls, five 5/8" steel balls and five 1/2" steel balls
• stopwatch

Method:

Amplifier settings:

• speaker knob: set to "A"
• Tone knobs: 0
• Balance knob: full LEFT (counterclockwise)
• Loudness: off
• Input selector: Tuner button depressed
• Volume Control: initially at 0 never > 3.
• tape monitor: source

Wave generator settings:

• Dial: 1.5
• Frequency multiplier (Hz): 10
• Mode buttons: none depressed
• Function: depress sine wave
• DC Offset: OFF
• Amplitude: about 1/3 of range

Set up the apparatus and controls as shown above. The nail grid should be leveled along the axis of vibration, and have a slope of 5-10 with respect to the horizontal, along its length.

Place all five 3/4" steel balls into the top of the grid at the same time, and measure the time required for 3 balls to reach the bottom (median time for 5 balls to reach bottom).

Turn the power ON for the amplifier first and then the wave generator. Turn the volume control of the amplifier to 3. The grid should vibrate at 15Hz. Place all five 3/4" balls into the top of the grid at the same time, and again measure the time required for 3 balls to reach the bottom.

It should take longer for the balls to pass through the grid when it is vibrating than when it is stationary.

Try varying the slope, amplitude, ball size and/or frequency and observe the results.