The simple apparatus on your lab bench will be used over the
next three weeks to study standing waves. The wire is fixed at
one end and has a movable fulcrum at the other end that allows
you to change the length of the wire. Tension is provided by a
mass hanging over a pulley at the adjustable end. There is a magnet
at the fixed end and a function generator that passes alternating
current through the wire. The AC current in the presence of the
magnet produces an alternating force on the wire, which is your
means of driving the wire back and forth.. When the function generator
is operating at a frequency that matches one of the resonant frequencies
of the wire, a standing wave will appear.
Your first measurement should proceed as follows:
The whole class should compare the measured resonant frequency.
Do you all agree with one another? If not, figure out why and
correct the issue.
The physical system being studied here is a wire held fixed
at its two ends. Transverse waves traveling along the wire are
reflected from each end and as the waves bounce back and forth
from end to end they usually interfere with each other destructively.
At certain frequencies each reflected wave interferes constructively
with the incoming wave and these build up into a standing wave.
This is an example of a resonance phenomenon. For a fixed length
of wire, the resonances are evenly spaced in frequency (denoted
F1, F2, F3...)
so the relationship between them is
Fn = n * F1
wher F1 is the lowest resonant frequency.
Find at least three resonant frequencies for the wire length
of 70 cm and a mass of 400 grams. Are the frequencies in agreement
with the equation above?
Try a few other wire lengths to build up a set of data on resonant
frequency versus wavelength, noting that
wavelength = (2/n) * (length of wire)
You need only a few different harmonics at each length to build a
complete data set. Most importantly, try to span as wide a range of resonant
frequency as possible.
Develop a linerized plot to show the relationship between frequency and
wavelength (plotted as frequency versus something). Once you have a linearized plot, comment on the value and
meaning of the slope.
assessment of how to do the measurment and how to determine uncertainty in the measurements: 3 mark
and comparing in part II 2 mark
Wide-ranging data set and graph 4 mark
rescaling to demonstrate frequency-wavelength relationship: 2
conclusion about frequency-wavelength relationship and comment on meaning of the slope: 2 marks