Capacitive circuits

Following are information and questions on capacitive circuits.

Instructions:

For the multi-choice questions:

• choose the correct answer
• write down your answer.

For the calculation questions:

• calculate the unknown value
• specify the appropriate unit
• write down your answer.

Note:

• You can print this page and write your answers onto it.
• You can use a calculator.

 

Safety

Capacitors can retain their electric charge for a long period of time. For this reason they present a personal danger.

Extreme care must be taken with:

• circuits that have been turned off, but contain a charged capacitor
• capacitors that have been removed from a circuit or device.

A resistive bridge circuit should be connected across a capacitor to ensure it is discharged and safe before handling.

Question 1

Multiple choice question

What should you do before handling a capacitor that has been removed from a circuit?

1. Connect the capacitor to a power supply.
2. Check the farad rating of the capacitor.
3. Bridge out the capacitor terminals with a resistive circuit.

Capacitors in parallel

Placing a number of capacitors in parallel has the same effect as increasing the plate area of one capacitor.

Therefore placing capacitors in parallel increases the total capacitance of a circuit.

Calculating parallel capacitance

The following formula is used to calculate total capacitance in a parallel circuit.

C_Total = C₁ + C₂ + C₃

 

Example

For the following circuit diagram:

C_total = C₁ + C₂ +C₃

Ctotal = 3µF + 6µF + 12µF

Ctotal = 21µF

Question 2

Multiple choice question

How will the total capacitance of a circuit change if capacitors are added in parallel to the circuit?

1. Total capacitance will increase
2. Total capacitance will decrease
3. Total capacitance will remain the same
4. It depends on the value of the added capacitor

Question 2

For the following circuit diagram, calculate the total capacitance value of the circuit.

Capacitors in series

Placing a number of capacitors in series has the effect of increasing the distance between plates of a capacitor.

Therefore placing capacitors in series decreases the total capacitance of a circuit.

 

Calculating series capacitance

The following formula is used to calculate total capacitance in a series circuit.

¹/C_T = ¹/C₁+ ¹/C₂ + ¹/C₃

 

Example

For the following circuit diagram:

¹/C_t = ¹/C₁+ ¹/C₂ + ¹/C₃

1/C_t = 1/30 + 1/10 + 1/30

1/C_t = 1/30 +3/30 + 1/30

1/C_t = 5/30

C_t/1= 30/5

Ct = 6µF

Question 3

Multiple choice question

How will the total capacitance of a circuit change if capacitors are added in series to a circuit?

1. Total capacitance will increase.
2. Total capacitance will decrease.
3. Total capacitance will remain the same.
4. It depends on the value of the added capacitor.

Question 4

For the circuit diagram shown, calculate the total capacitance value of the circuit.

Capacitor faults

The following faults can occur to a capacitor:

• short circuit
• open circuit
• capacitance loss.

Short circuit:

• this can occur if there is a breakdown of the dielectric and an electrical connection exists between the plates.

Open circuit:

• an open circuit can occur if there is a break in the lead connecting to either plate of the capacitor.

Capacitance loss:

• caused by dielectric current leakage between the plates. This results in a reduction of capacitance.

Question 5

An open circuit can exist in a capacitor when there is a fault with the dialectic material.

1. True.
2. False.

 

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