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What
do you understand by inductance? |
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Inductance
is usually refer to a coil. It is defined as the total flux linkage per
unit current. |
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| b. |
By
considering the growth of current in an inductor connected across a battery,
show that energy is stored in an inductor that carries a current. |
5
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The p.d. across an inductor when the current
keeps changing is |
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Thus, the electrical power transferred to the inductor
is |
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The electrical energy is stored up as magnetic energy.
When the current reaches the maximum value, the magnetic energy stored
in the inductor is |
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| c. |
Discuss,
in detail, how the current through a d.c. motor driven by a battery varies
with time if the motor is switched on with
i)
no mechanical load |
10
marks |
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The back e.m.f. of a motor with angular speed w
is |
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The maximum angular speed occurs when the back e.m.f. is
equal to the e.m.f. of the power supply. |
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When this occurs, the current in the circuit is zero. |
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When the motor is started, the current rises to a maximum
and then falls to zero. |
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ii)
a mechanical load.
State and explain
the condition for obtaining the maximum mechanical power output from a
d.c. motor. |
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Suppose the resistance of the motor is R. At a certain
speed less than the maximum speed, the current is given by |
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The heavier is the load, the lower is the speed and the
higher is the current. |
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From equation (7), |
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We can regard the circuit as a cell of e.m.f.
xo and internal resistance R.
Thus, the maximum power output is when |
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The angular speed for this to occur is |
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