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A
cathode ray tube consists of an electron gun, a pair of deflecting plates
and a fluorescent screen. Electrons from the electron gun pass at right
angles through the uniform electric field set up by the deflecting plates.
Show that the electrons leave the electric field in a direction as if they
had come straight from the center of the field. Hence, explain why the
deflection of the spot on the fluorescent screen is proportional to the
voltage applied across the deflecting plates. |
8
marks |
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1 |
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Suppose the horizontal speed of electrons in
the E-field is vx. |
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The time required for an electron to move across
the field is |
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(It is independent of the field strength) |
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Suppose the maximum vertical deflection of
the electrons is D and the vertical component of the velocity of
electrons leaving the plate system is vy. |
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Putting (1) into (2), we have |
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The direction of path at which the electrons
leave the E-field is given by |
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where q is the angle
of the path made with the horizontal as shown in Fig.(c). |
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Combining (3) and (4),
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It is clear that tan q
represetns the slope of the line joining the centre of the E-field and
the point where the electrons emerge from the field. Thus, the cathode
ray seems to come from the centre of the E-field. |
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Let a be the acceleration of the electrons
in the E-field. |
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The maximum deflection of electron in the E-field
is therefore |
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where k is a constant. |
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Thus, the deflection is proportional to the
applied p.d. VP. |
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As the electrons move in a straight line on
leaving the field, the deflection Y on the screen is proportional
to the applied p.d. VP. |
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| b. |
Draw
a clear labelled diagram showing the essential features of a single-beam
oscilloscope. Explain briefly
i)
how the brightness of the trace can be controlled |
8
marks |
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2 |
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The brightness of the trace depends on the
rate of electrons hitting the screen. The more electrons hitting per second,
the brighter is the trace. |
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The amount of electrons leaving the electron
gun depends on the potential of the grid relative to the cathode. If the
grid is set at a more negative potential than the cathode, only the energetic
electrons can pass through the grid. |
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ii)
how the trace can be shifted up and down by the Y-shift control |
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The Y-shift is a variable voltage supply that add to the
Y-input after amplification. Increasing the voltage of the Y-shift will
shift the electron beam upwards. |
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iii)
how a static waveform could be obtained. |
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A static waveform is produced by applying a
suitable sawtooth waveform across the X-plates. The electrons are swept
at a uniform speed from left to right and then quickly flyback to the left.
The process is repeated on and on. |
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