2
Calibration
You do not need to calibrate the Charge Sensor. We have set the sensor to
match our stored calibration before shipping it. You can simply use the
appropriate calibration file that is stored in your data-collection program from
Vernier.
Stored Calibration Values
±2 V setting in Volts
slope: 1.0
intercept: –2.5
±2 V setting in nC
slope: 10.0
intercept: –25.0
±10 V setting in Volts
slope: 3.3
intercept: –8.25
±10 V setting in nC
slope: 33.0
intercept: –82.5
Specifications
Ranges:
±2 V (±20 nC)
±10 V (±100 nC)
Maximum input:
±150 V
Typical bias input current:
0.005 pA
Instrument time constant:
0.1 s
This sensor is equipped with circuitry that supports auto-ID. When used with
LabQuest 2, LabQuest, LabQuest Mini, LabPro, Go! Link, SensorDAQ, TI-
Nspire™ Lab Cradle, EasyLink, or CBL 2™, the data-collection software
identifies the sensor and uses pre-defined parameters to configure an experiment
appropriate to the recognized sensor.
Care and Maintenance
Do not wrap the cable tightly around the sensor for storage. Repeatedly doing
so can irreparably damage the wires and is not covered under warranty.
How the Sensor Works
The Charge Sensor is an extremely high impedance voltage sensor with a
0.01 μF capacitor in series with the input. The capacitor will accumulate charge
until the source’s voltage is reached, i.e. equilibrium is achieved. Small amounts
of charge can be measured even though their initial potential is higher than the
input range of the sensor. The input circuit also includes a 1 MΩ resistor in
series with the capacitor to protect the unit from large current surges. When used
with a data-collection interface, connected by USB to a computer, or to an
Experiment Ideas
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Use the sensor with a Faraday pail to investigate charging by induction.
Bring a charged object near the pail. What charge is measured? Ground the
pail and remove the charged object. What charge is measured?
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Use a Faraday pail to investigate charging by contact. Do this by dropping a
charged object into the can. All of the charge on the object will be
transferred to or induced in the can.
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Without a grounding strap, scuff your feet on carpet or pull off a sweater.
Hold your hand near a Faraday pail. Do you induce a charge? What sign?
Does a ground strap remove or reduce this effect?
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Charge various objects and determine the sign of the charge.
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Measure how quickly objects lose charge. Plot the charge as a function of
time; this will take some minutes on a dry day.
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Use the Charge Sensor and Faraday pail to observe the separation of charge
when two strips of invisible tape are pulled apart. On two 3 inch pieces of
tape, make a tab by folding over the top of each. Stick the combination to
the table top. Pull the combination off the table and run your thumb or
finger along the smooth side of the tape to neutralize the combination. Then
pull the top strip off the bottom strip. Individually insert each strip into the
Faraday pail to measure the charge. Simultaneously insert them in the pail to
measure the charge.
l
Charge a Faraday pail by contact on the inside; add more charge. How much
charge can you add from the inside of the can? Can you add as much from
the outside? More? Less? Investigate.
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Use a second pail (not connected to the red lead as a detector) and charge it
by induction. Do this by charging an insulating object, holding it inside the
pail (inducing a charge on the outside of the can) and then briefly grounding
the pail. Remove the charged insulator, and you’ve got a charged pail.
Measure the charge by testing it by induction, or by touching it to the inside
of the detector pail.
Many of these tips are based on suggestions from Robert Morse, Ph.D.
Videos
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