High School Physics : Capacitors

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #1 : Capacitors

Which of these is the correct SI unit for capacitance?

Possible Answers:

Volts

Farad-Joules

Farads

Ohms

Newtons

Correct answer:

Farads

Explanation:

The correct unit for capacitance is the Farad, named after the English physicist Michael Faraday.

Newtons are used to measure force. Ohms measure resistance and volts measure electrical energy or potential. Joules are a unit of energy, but Farad-Joules are not a recognizable unit.

Example Question #1 : Understanding Capacitance

By using a dielectric, it is possible to store more energy in a capacitor. Why is this? 

Possible Answers:

The dielectric can store excess energy

The dielectric provides a direct energy source to the capacitor

The dielectric changes the distance between the two plates of the capacitor

The dielectric replaces air, which breaks down when the electric fields get strong enough

Correct answer:

The dielectric replaces air, which breaks down when the electric fields get strong enough

Explanation:

Dielectrics are insulating materials that can be polarized, and can be used to increase the separability of the two plates of a capacitor. When a dielectric is absent, air is present between the capacitor plates. At high voltages, when the electric field between the plates is incredibly strong, air begins to break down and becomes unable to prevent the flow of electricity between the capacitor plates. When this occurs, the capacitor cannot store energy, nor can it properly function. The presence of a dielectric eliminates this problem, allowing the capacitor to store much more energy.

Example Question #2 : Capacitors

Three capacitors are in series. They have capacitances of \(\displaystyle 3F\)\(\displaystyle 2F\), and \(\displaystyle 8F\), respectively. What is their total capacitance?

Possible Answers:

\(\displaystyle 0.958F\)

\(\displaystyle 1.11F\)

\(\displaystyle 13F\)

\(\displaystyle 1.04F\)

\(\displaystyle 0.077F\)

Correct answer:

\(\displaystyle 1.04F\)

Explanation:

For capacitors in series the formula for total capacitance is:

\(\displaystyle \frac{1}{C_{eq}}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}...\)

Note that this formula is similar to the formula for total resistance in parallel. Using the values for each individual capacitor, we can solve for the total capacitance.

\(\displaystyle \frac{1}{C_{eq}}=\frac{1}{3F}+\frac{1}{2F}+\frac{1}{8F}\)

\(\displaystyle \frac{1}{C_{eq}}=0.958F\)

\(\displaystyle C_{eq}=1.04F\)

Example Question #43 : Electric Circuits

Three capacitors are in parallel. They have capacitance values of \(\displaystyle 3F\)\(\displaystyle 2F\), and \(\displaystyle 8F\). What is their total capacitance?

Possible Answers:

\(\displaystyle 1.09F\)

\(\displaystyle 1.04F\)

\(\displaystyle 13F\)

\(\displaystyle 0.958F\)

\(\displaystyle 0.077F\)

Correct answer:

\(\displaystyle 13F\)

Explanation:

For capacitors in parallel the formula for total capacitance is:

\(\displaystyle C_{eq}=C_1+C_2+C_3...\)

Note that this formula is similar to the formula for total resistance in series. Using the values for each individual capacitor, we can solve for the total capacitance.

\(\displaystyle C_{eq}=3F+2F+8F\)

\(\displaystyle C_{eq}=13F\)

Example Question #3 : Capacitors

Three capacitors, each with a capacity of \(\displaystyle 4F\) are arranged in parallel. What is the total capacitance of this circuit?

Possible Answers:

\(\displaystyle 1.33F\)

\(\displaystyle 0.08F\)

\(\displaystyle 64F\)

\(\displaystyle 12F\)

\(\displaystyle 0.75F\)

Correct answer:

\(\displaystyle 12F\)

Explanation:

The formula for capacitors in parallel is:

\(\displaystyle C_{eq}=C_1+C_2+C_3...\)

Our three capacitors all have equal capacitance values. We can simply add them together to find the total capacitance.

\(\displaystyle C_{eq}=4F+4F+4F\)

\(\displaystyle C_{eq}=12F\)

Example Question #4 : Calculating Total Capacitance

What is the total capacitance of a series circuit with capacitors of \(\displaystyle 5F\)\(\displaystyle 1.1F\), and \(\displaystyle 7.5F\)?

Possible Answers:

\(\displaystyle 1.2F\)

\(\displaystyle 0.07F\)

\(\displaystyle 4.5F\)

\(\displaystyle 13.6F\)

\(\displaystyle 0.8F\)

Correct answer:

\(\displaystyle 0.8F\)

Explanation:

The total capacitance of a series circuit is \(\displaystyle \frac{1}{C_{eq}}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}...\)

Plug in our given values.

\(\displaystyle \frac{1}{C_{eq}}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}...\)

\(\displaystyle \frac{1}{C_{eq}}=\frac{1}{5F}+\frac{1}{1.1F}+\frac{1}{7.5F}\)

\(\displaystyle \frac{1}{C_{eq}}=1.24F\)

\(\displaystyle C_{eq}=0.8F\)

Example Question #1 : Capacitors

Calculate the total capacitance of a circuit with the following three capacitors in parallel.

\(\displaystyle C_1=10F,\ C_2=15F,\ C_3=5F\)

Possible Answers:

\(\displaystyle 3.21F\)

\(\displaystyle 2.73F\)

\(\displaystyle 5F\)

\(\displaystyle 30F\)

\(\displaystyle 15F\)

Correct answer:

\(\displaystyle 30F\)

Explanation:

To calculate the total capacitance for capacitors in parallel, simply sum the value of each individual capacitor.

\(\displaystyle C_{eq}=C_1+C_2+C_3\)

\(\displaystyle C_{eq}=10F+15F+5F=30F\)

Example Question #4 : Calculating Total Capacitance

Calculate the total capacitance of a circuit with the following three capacitors in series.

\(\displaystyle C_1=10F,\ C_2=15F,\ C_3=5F\)

Possible Answers:

\(\displaystyle 2.73F\)

\(\displaystyle 3.12F\)

\(\displaystyle 25F\)

\(\displaystyle 0.367F\)

\(\displaystyle 30F\)

Correct answer:

\(\displaystyle 2.73F\)

Explanation:

To find the total capacitance for capacitors in series, we must sum the inverse of each individual capacitance and take the reciprocal of the result.

\(\displaystyle \frac{1}{C_{eq}}=\frac{1}{C_{1}}+\frac{1}{C_{2}}+\frac{1}{C_{3}}\)

\(\displaystyle \frac{1}{C_{eq}}=\frac{1}{10F}+\frac{1}{15F}+\frac{1}{5F}\)

\(\displaystyle \frac{1}{C_{eq}}=\frac{11}{30}=0.367\)

Remember, you must still take the final reciprocal!

\(\displaystyle C_{eq}=\frac{30}{11}=2.73F\)

Example Question #1 : Using Capacitor Equations

Three capacitors in parallel have a capacity of \(\displaystyle 2F\)\(\displaystyle 12F\), and \(\displaystyle 8F\), respectively. What is the total capacitance?

Possible Answers:

\(\displaystyle 4.23F\)

\(\displaystyle 22F\)

\(\displaystyle 7.33F\)

\(\displaystyle 0.045F\)

\(\displaystyle 1.41F\)

Correct answer:

\(\displaystyle 22F\)

Explanation:

The formula for capacitors in parallel is:

\(\displaystyle C_{eq}=C_1+C_2+C_3...\)

Plug in our given values:

\(\displaystyle C_{eq}=2F+12F+8F\)

\(\displaystyle C_{eq}=22F\)

Example Question #52 : Electric Circuits

Three capacitors in series have a capacity of \(\displaystyle 2F\)\(\displaystyle 12F\), and \(\displaystyle 8F\), respectively. What is the total capacitance?

Possible Answers:

\(\displaystyle 7.33F\)

\(\displaystyle 0.45F\)

\(\displaystyle 0.71F\)

\(\displaystyle 22F\)

\(\displaystyle 1.41F\)

Correct answer:

\(\displaystyle 1.41F\)

Explanation:

The formula for capacitors in series is:

\(\displaystyle \frac{1}{C_{eq}}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}...\)

Plug in our given values:

\(\displaystyle \frac{1}{C_{eq}}=\frac{1}{2F}+\frac{1}{12F}+\frac{1}{8F}\)

\(\displaystyle \frac{1}{C_{eq}}=0.70833\frac{1}{F}\)

\(\displaystyle C_{eq}=1.41F\)

 

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