How to Find Charge & Voltage Across Capacitors

Written by bert markgraf Google
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How to Find Charge & Voltage Across Capacitors
The voltage across a capacitor is proportional to its charge. (Hemera Technologies/ Images)

If you measure the voltage across a capacitor, you can calculate the charge. Capacitors consist of two conducting surfaces with an insulating dielectric between them. When a voltage is applied across the plates, electrons migrate from one plate to the other, setting up an electric field across the dielectric. The charge made up of the displaced electrons is proportional to the voltage applied. When the voltage applied to the capacitors varies, the charge must vary as well, so that a current flows between the capacitor plates and can supply power in a circuit.

Skill level:

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Things you need

  • Multimeter
  • Capacitors
  • Wire
  • Battery
  • Potentiometer

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  1. 1

    Connect the multimeter to the capacitor leads and then connect the battery across the capacitor. Measure the voltage with the multimeter. The voltage is steady and equal to the battery voltage after a very short time, during which the capacitor is charging. Calculate the charge with the formula Charge Q in coulombs equals the capacitor capacitance C in farads times the voltage V in volts.

  2. 2

    Disconnect the battery. Connect the leads of a second, third and fourth capacitor to the first so that all four leads from one end of each capacitor are connected. The capacitors are connected in parallel. Connect the battery to the capacitor leads and measure the voltage. It will again be steady after a slightly longer time while the larger number of capacitors is charging. Calculate the charge of parallel capacitors by adding the capacitances and multiplying by the voltage as before.

  3. 3

    Disconnect the battery and connect the capacitors so that one lead from the first is connected to one lead of the second while the other lead of the second capacitor is connected to one lead of the third and so on. These capacitors are now connected in series. Connect the multimeter to the free leads of the first and fourth capacitor. Connect the battery to the free leads of the first and fourth capacitor. Measure the voltage. It is steady after a short time while the capacitors charge. Calculate the charge by combining the capacitances of the four capacitors according to the formula 1/C = 1/C1 + 1/C2 + 1/C3 +1/C4 where C1 to C4 are the capacitances of the four capacitors. Multiply by the voltage and note that C, for series capacitors, is now less than any of the individual capacitance values and the charge is less as well.

  4. 4

    Disconnect one side of the battery and connect it to the potentiometer. Connect the other side of the potentiometer to the free end of the first capacitor. Turn the potentiometer to zero and measure the voltage. The voltage will be steady and the same as for step 3. Turn the potentiometer up and down continuously. The voltage will also vary continuously. Use the multimeter to measure the current in the wire leading to the battery. The current is also varying continuously as the charge flows in and out of the capacitors according to the level of the voltage. When the voltage varies continuously, the charge does as well.

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