basic electrical sciences module 4
TRANSCRIPT
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ZZ1003-Basic Electrical SciencesModule IV
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Outline
Background
Magnetic effect of electric current
Galvanometer
Ammeter, Voltmeter, Watt meter, Energy Meter
Cathode Ray Oscilloscope
Electron Gun
Cathode Ray Tube
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Right Hand Thumb Rule or Curl Rule[1]:
If a current carrying conductor is imagined to be held in the
right hand such that the thumb points in the direction of the
current, then the tips of the fingers encircling the conductor
will give the direction of the magnetic lines of force.
I
B
Magnetic Effect of Electric Current
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KE
TIP:
When we look at any end of the coil carrying current, if the current is in anti-
clockwise direction then that end of coil behaves like North Pole and if the
current is in clockwise direction then that end of the coil behaves like South Pole.
B
I I
Magnetic Field due to a Current in a Solenoid[2]
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KE
I I
Electromagnet[3]
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6
Measuring current
If you want to measure current "somewhere" in your
circuit, keep a current-measuring-device (ammeter) inthe current path
A
I I
Ideally, the presence of the ammeter should not
influence what is going on in your circuit Should not change the current or the voltages
The ideal ammeter has zero resistance
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Galvanometer
The galvanometer is the "classic" device to
measure current
Permanent Magnet Moving Coil (PMMC)
Instrument
Based on the magnetic effect of current coil
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Working of Galvanometer[4]
Current Coil Produces Magnetic FiledProportional to the current
This current coil is placed in apermanent magnetic field
When current flows, the coilexperiences a torque proportional tothe current
The movement of the coil is"opposed" by a spring
The deflection of the needle isproportional to the current
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Galvanometer (cont.)
A typical galvanometer has a "full-scale-
current" (Ifs) of 10 A to 10 mA
The resistance of the coil is typically 10 to
1000 .
How can we use a galvanometer to measure
currents higher than its full scale current?
1. Divide the current, so that only a well
understood fraction goes through the coil2. Measure how much goes through the coil
3. Rescale by the known fraction
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Rsh = "shunt" resistance
The current I divides itself between the
coil and the shunt I = IC + Ish
10
By Ohms's law, Vab = IC RC = Ish Rsh
Ish = IC (RC/Rsh) I = IC + Ish = IC (1 + RC/Rsh)
If RC and Rsh are known, measuring IC is equivalent tomeasuring I
Furthermore, I is still proportional to IC, which isproportional to the deflection of the needle
Thus, by "switching in" different shunt resistances Ican effectively change the "range" of my currentmeasurement
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Example Galvanometer, RC=10, Ifs=1 mA What shunt resistance should I use to make a
voltmeter with full scale deflection of the needleVfs = 10 V?
IC = Vab/(RC + Rsh)
RC + Rsh = Vfs/Ifs = 10 V / 1 mA = 104
Rsh = 9,990
Bonus: RC and Rsh in series Equivalent resistance of voltmeter = RC + Rsh = 10
4 (large!)
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Energy Meter
Energy = Power X Time
To measure energy in a circuit, spring in the
wattmeter is removed
Total deflection in the needle is proportional to
energy dissipated in the circuit
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Review
Magnetic Effect of Electric Current
Quantitative Measurement of
Current : Ammeter
Galvanometer with Shunt Resistance
Voltage : Voltmeter
Galvanometer with Series Resistance
Power : Wattmeter
Energy : Energy-meter
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Cathode Ray Oscilloscope
Qualitative Measurement of Voltage Signals
Components
Electron Gun
Fluorescent Screen
Control Circuits
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Electron Beam
Electron Beam Passing Through Phosphorus
Vapour[7]
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CRO Block Diagram[9]
The x and y plates can apply an external field to deflect thisbeam.
The beam then hits a fluorescent screen
produces a bright spot
saw-tooth wave is applied to x plate .
Bright spot moves horizontally and shows a horizontal line on screen
External signal is applied to y plate Shows the signal in the screen
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CRO Screen[9]
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Measuring the Waveform[9]
What are the Maximum and Minimumvoltages
What is the frequency of the signal
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References
1) http://hyperphysics.phy-
astr.gsu.edu/hbase/magnetic/magfor.html
2) http://farside.ph.utexas.edu/teaching/316/le
ctures/node76.html
3) http://www.coolmagnetman.com/magelect.
htm
4) http://www.buzzle.com/articles/galvanomet
er-how-it-works.htmlContd
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.htmlhttp://farside.ph.utexas.edu/teaching/316/lectures/node76.htmlhttp://farside.ph.utexas.edu/teaching/316/lectures/node76.htmlhttp://www.coolmagnetman.com/magelect.htmhttp://www.coolmagnetman.com/magelect.htmhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.buzzle.com/articles/galvanometer-how-it-works.htmlhttp://www.coolmagnetman.com/magelect.htmhttp://www.coolmagnetman.com/magelect.htmhttp://farside.ph.utexas.edu/teaching/316/lectures/node76.htmlhttp://farside.ph.utexas.edu/teaching/316/lectures/node76.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html -
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5) http://www.tpub.com/neets/book16/68g.ht
m
6) http://advressys.com/analytic/sem.htm
7) http://www.physics2000.com/Pages/Non-
CalcTexts.html
8) http://commons.wikimedia.org/wiki/File:Cat
hode_ray_tube_-_neutral.png
9) http://www.cmccord.co.uk/Radio/oscilloscop
e.htm
http://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://commons.wikimedia.org/wiki/File:Cathode_ray_tube_-_neutral.pnghttp://commons.wikimedia.org/wiki/File:Cathode_ray_tube_-_neutral.pnghttp://www.cmccord.co.uk/Radio/oscilloscope.htmhttp://www.cmccord.co.uk/Radio/oscilloscope.htmhttp://www.cmccord.co.uk/Radio/oscilloscope.htmhttp://www.cmccord.co.uk/Radio/oscilloscope.htmhttp://commons.wikimedia.org/wiki/File:Cathode_ray_tube_-_neutral.pnghttp://commons.wikimedia.org/wiki/File:Cathode_ray_tube_-_neutral.pnghttp://commons.wikimedia.org/wiki/File:Cathode_ray_tube_-_neutral.pnghttp://commons.wikimedia.org/wiki/File:Cathode_ray_tube_-_neutral.pnghttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.htmlhttp://www.physics2000.com/Pages/Non-CalcTexts.html