ROBERT COLLEGE SCIENCE DEPARTMENT ALISON OGUZ

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ROBERT COLLEGESCIENCE DEPARTMENTALISON OGUZ

• USE OF OSCILLOSCOPE IN
• HIGH SCHOOL PHYSICS

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Aim To demonstrate how to operate an
oscilloscope and incorporate its use in the
teaching of various physics topics.The
presentation will cover the following

• Introduction to use of oscilloscope
• Use with mechanical waves (with electric guitar)
• Use with magnetism (with bar magnets)
• Use for measuring ac/dc signals
• Use to show rectification and smoothing of ac
  voltages (dependence of capacitor charge-time on
  C)
• Use to show that effective resistance of a
  capacitor (Xc) is frequency dependent (with R-C
  filter circuit)
• Integration of a constant voltage, a sine
  function and a square function (with op-amp) - if
  time available!

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Oscilloscope schematichttp//solidstate.physics.
sunysb.edu/teach/phy132/lab_instructions/scope/sco
pe.htm
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Setting the time base
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Applying a voltage
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Demonstration of waves produced on stretched
stringsElectric Guitar
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To demonstrate harmonics

• To show the fundamental
• (1st harmonic - a pure sine wave)
• For one string use finger to change
  vibrating length so that pick-up coils are in the
  center, then pluck at center

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Playing the fundamental frequency
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To show mixture of harmonics

• Release string and pluck in various positions, or
  play chords
• Resulting wave is harmonic (repetitive) but
  complex.
• Can use this to lead into
• discussion of synthesized
• music.

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To demonstrate force on a charge moving in a
magnetic field

• Magnitude of force (F) on a charge (q) moving
  with speed (v) in a magnetic field (B) is given
  by
• F q v B Sin ?
• (where ? is the angle between v and B)
• In this case the charges are negatively
  charged electrons so
• F q v x B - e v x B

• Direction of force
• http//ffden-2.phys.uaf.edu
• or F q v x B - e v x B

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Bar magnet approaching oscilloscopeNote
Time-base turned OFF

• N-pole approaching from left electron beam
  deflection?
• downwards
• N-pole approaching from above electron beam
  deflection?
• to left

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To measure a d. c. (direct) voltage

• Connect coaxial cable from oscilloscope Ch1 input
  to battery (croc clip to battery - since this
  is earthed)
• Set Ch1 slide switch to GND to observe position
  of 0V adjust as necessary with y-shift
• Set Ch1 slide switch to DC and adjust y-gain
  knob to cal (calibrated) position, selecting a
  suitable volts/div setting
• Deduce voltage from measurement of number of
  divisions.

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To measure an a. c. (alternating) voltage

• Connect coaxial cable from oscilloscope Ch1 input
  to function generator output (croc clip to black
  low terminal)

• Set trigger Ch input to Ch1 so that time-base
  function adjusts according to applied input

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Measuring a. c. voltage

• Set Ch1 slide switch to AC and adjust y-gain
  knob to cal (calibrated) position, selecting a
  suitable volts/div setting

• Deduce peak voltage from measurement of number of
  divisions.
• Then Vrms Vpeak/v2

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Measuring a. c. frequency

• - Check that time-base knob is set on cal, and
  select a suitable time/div setting
• Deduce frequency of signal from measurement of
  number of divisions.

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Rectification and smoothing of a. c. voltages

• 1) With no capacitor, observe half-wave
  rectification

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• With capacitors of various values (0.01µF -
  1000µF) added in parallel to the load resistor,
  observe smoothing
• Note how the larger the C value the longer it
  takes the capacitor to charge discharge and
  therefore the smoother the output voltage

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Smoothing of rectified signal with i) C 0.1µF,
ii) C 1µF, iii)
C 10µF, iv) C
100µF
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To demonstrate that the effective resistance of a
capacitor is frequency dependentXC 1/(2pf
C)A) HIGH-PASS FILTER

• As f is increased, XC decreases
• so that a larger fraction of the same voltage
  input (from the function generator) is dropped
  across R
• So that the voltage output (as measured by the
  oscilloscope) increases i.e. high frequencies are
  passed

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HIGH-PASS FILTERNote amplitude of input kept
constanti) f 300Hz,
ii) f 3kHz, iii) f 7kHz,
iv) f 30kHz
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B) LOW-PASS FILTERXC 1/(2pf C)

• As f is increased, XC decreases
• so that a smaller fraction of the same voltage
  input (from the function generator) is dropped
  across C
• So that the voltage output (as measured by the
  oscilloscope) decreases i.e. low frequencies are
  passed

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LOW-PASS FILTERNote amplitude of input kept
constanti) f 150Hz,
ii) f 300Hz,iii) f 1kHz,
iv) f 3kHz
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Integrator Circuit using Op-Amp

• Theory
• - An op-amp amplifiers the difference between
  its inputs
• Because of huge open-loop gain, can take point P
  as a virtual earth
• Because of large impedances at inputs can assume
  no current drawn by op-amp
• Thus Vi IR and Vo - Q/C
• (minus sign since output inverted)
• And I dQ/dt so dQ/dt Vi/R
• So Q ? Vi/R dt
• Therefore Vo - 1/CR ? Vi dt
• i.e. integrated and inverted (take CR
  1µF.100kO 0.1s)

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Integrating a constantInput 2.5 x 0.2 0.5V
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Integrating a constant

• Expected shape of output?
• a negative ramp
• output - 1/CR x Vi t
• - 1/0.1 x 0.5t - 5t i.e.
  negative ramp saturating close to battery voltage
  
• From video final Vo 3 x 2V 6V,
• so time expected 6/5 1.2s.
• Time measured 6 x 0.2 1.2s!

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Integrating a sine functionOutput ?
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Integrating a square waveOutput ?