Jeulin 281 265 Mode D'emploi page 12

Electricity
Charging a capacitor at
constant voltage
Ref :
281 265
ENGLISH
3.3 Manual reading of voltages
The device is designed to charge (and discharge) a capacitor long enough
so that the students can observe the variations in the voltage U
also observe the variation and sign of the current i(t) depending on whether
the capacitor is charging or discharging.
Thus it is advisable to select a capacitor with the highest capacitance
(1000 µF) and a resistance of 10 kΩ (or C = 100 µF and R = 100 kΩ).
In these conditions, the time constant τ = RC = 104.10
charging of the capacitor (at 98%) is done at Δt = 5τ, the charging is done in
about 50 s (i.e. of the order of a minute).
During this time, the students can easily observe that the voltage U
the beginning, increases quickly then more slowly until the value of the
voltage E is reached: the charging is then complete. At the same time, we
can observe a sharp decrease in the current from a maximum value tending
asymptotically towards zero when the charging is complete.
During discharge of the capacitor (reversing switch K in 2), the voltage U
decreases rapidly from the value E and tends towards zero. During the same
period, the current becomes negative and decreases from the same
maximum value as during charging, then tends asymptotically towards zero
when the discharging is complete.
3.4 Computerized input of the voltage U
The charging time is not as important as it is for a manual experiment.
The choice of the capacitor and the charging current is therefore left to the
student who can make various comparisons depending on the choice of the
capacitor, the resistance and voltage E.
The synchro output triggers the acquisition system 0.1 s before the start of
the charging or discharging of the capacitor. Thus, the beginning of the
phenomenon is systematically viewed even though it lasts for a very short
duration.
To use this function, it is advisable to connect the terminals of the synchro
output located at the back of the box to the two synchro sockets of the
®
ESAO console. For a study with Généris, the "external synchro triggering of
acquisitions" mode must be selected.
11
. They can
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