Basic Electrical Sciences Module 4

7/28/2019 Basic Electrical Sciences Module 4

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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

8


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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

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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

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Basic Electrical Sciences Module 4