1. 16.5 Measurement of magnetic fields
• Hall probe
• Current balance
• Search coil
• Cathode ray oscilloscope
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2. 16.5 Measurement of magnetic fields (SB p. 207)
Hall probe
Hall probe - by principle of Hall effect - measure magnetic flux density - use a piece of semiconductor - two wires connected to vertical ends (supply current I) - two wires connected to horizontal edges (connected to millivoltmeter) to measure VH - calibrate the probe by a known B0 and measure V0 - unknown field
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3. 16.5 Measurement of magnetic fields (SB p. 208)
Hall probe
Notes:
1. Hall probe is suitable for measuring steady magnetic field.
2. The plane of the semiconductor of Hall probe should be perpendicular to the magnetic field in measuring.
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4. 16.5 Measurement of magnetic fields (SB p. 208)
Current balance
Current balance - by principle of a force acting on a current-carrying conductor in B-field - use in calibration of ammeters - I through wire PQRS - F acting downwards on QR - place rider of mass m at the other end to balance F - Moment due to F = Moment due to weight of rider BIld = mgx
Note: The current balance is only used in measuring steady magnetic field.
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5. 16.5 Measurement of magnetic fields (SB p. 209)
Search coil
Search coil - change of magnetic flux induces e.m.f. in search coil - B  f
Notes:
1. The search coil is only used in measuring a varying magnetic field.
2. The Earth’s field can be ignored because it is a steady magnetic field.
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6. Measured magnetic field
16.5 Measurement of magnetic fields (SB p. 210)
Search coil
Measuring device
Measured magnetic field
Position of device
1. Hall probe
Steady
Plane of semiconductor of Hall probe perpendicular to magnetic field
2. Current balance
Wire of current balance perpendicular to magnetic field
3. Search coil
Varying
Plane of coil perpendicular to magnetic field
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7. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 210)
Cathode ray oscilloscope
Cathode ray oscilloscope (CRO) - show waveforms of induced e.m.f. in search coil (for measuring B-field)
Y-input
Y-gain
earth terminal
time base
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8. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 211)
Cathode ray oscilloscope
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9. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 211)
Cathode ray oscilloscope
1. As d.c./a.c. voltmeter Calibration: - connect a battery of known e.m.f. (E) to y-input of CRO - measure deflection a cm
(a) As d.c. voltmeter
Voltage measured = Amount of deflection (in cm) x Y-gain value (in V cm-1)
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10. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 211)
Cathode ray oscilloscope
(b) As a.c. voltmeter
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11. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 212)
Cathode ray oscilloscope
2. Studying waveforms (a) Time base - x-plates connected to auxilliary circuit - the circuit generates a saw-tooth voltage - spot of light moves from left to right of screen at AB - then flies back to left quickly at BC
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12. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 213)
Cathode ray oscilloscope
(b) Studying a.c. waveforms - connect applied voltage to y-input and time base on
fx = fy
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13. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 213)
Cathode ray oscilloscope
fx = ½ fy
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14. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 214)
Cathode ray oscilloscope
3. Measuring period and frequency
Period = Wavelength of a complete cycle (in cm) x Time base value (in s cm-1)
Go to
Example 5
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15. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 214)
Cathode ray oscilloscope
4. Comparing frequencies of two alternating voltages - off time base - 2 alternating voltages connected to x- and y-inputs of CRO - Lissajous figures
fy:fx =1:1
fy:fx =2:1
fy:fx =1:2
fy:fx =3:2
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16. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 215)
Cathode ray oscilloscope
5. Measuring phase difference ()
x = x0 sint, y = y0 sin(t +)
(a)  = 00 x = x0 sint, y = y0 sint
straight line
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17. Cathode ray oscilloscope
16.5 Measurement of magnetic fields (SB p. 216)
Cathode ray oscilloscope
5. Measuring phase difference ()
(b)   00 x = x0 sint, y = y0 sin(t +) (i) x = 0, y = y0 sin or -y0 sin AB = 2y0 sin
(i) y = 0, x = x0 sin or -x0 sin CD = 2x0 sin
 = 900 AB = 2y0 , CD = 2x0
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18. End
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19. 16.5 Measurement of magnetic fields (SB p. 214)
Example 5
Q: The figure shows the trace on a CRO when a sinusoidal voltage is applied to the y-input. The Y-gain is 4.00 V per division. What is the root-mean-square value of the applied voltage?
Solution
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20. Example 5 Text Solution: Length of trace = 3 divisions
16.5 Measurement of magnetic fields (SB p. 214)
Example 5
Solution:
Length of trace = 3 divisions
Let V0 = peak voltage, 2 V0 = 3 ×4.00
V0 = 6.00 V
Root-mean-square voltage
Return to
Text
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