MCAT Physical : Capacitors and Dielectrics

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

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Example Question #51 :Circuits

Batteries and AC current are often used to charge a capacitor. A common example of capacitor use is in computer hard drives, where capacitors are charged in a specific pattern to code information. A simplified circuit with capacitors can be seen below. The capacitance of C1is 0.5 μF and the capacitances of C2and C3are 1 μF each. A 10 V battery with an internal resistance of 1 Ω supplies the circuit.

PretextQuestion_2

Capacitors in series share the same __________, while capacitors in parallel share the same __________.

Possible Answers:

Charge . . . current

Voltage . . . current

Current . . . charge

Current . . . voltage

Correct answer:

Current . . . voltage

Explanation:

As with resistors, capacitors in series share the same current. This is a re-statement of the law of conservation of charge. In contrast, capacitors and resistors in parallel share the same voltage.

Example Question #52 :Circuits

Batteries and AC current are often used to charge a capacitor. A common example of capacitor use is in computer hard drives, where capacitors are charged in a specific pattern to code information. A simplified circuit with capacitors can be seen below. The capacitance of C1is 0.5 μF and the capacitances of C2and C3are 1 μF each. A 10 V battery with an internal resistance of 1 Ω supplies the circuit.

PretextQuestion_2

How much total charge is stored by the capacitors of the circuit?

Possible Answers:

25μC

10μC

20μC

15μC

Correct answer:

10μC

Explanation:

We are asked how much charge is stored in total on the circuit. We can use the equivalent capacitance and the voltage supplied by the battery to calculate the charge. Remember that Q = CV, where Q is the total charge, C is the equivalent capacitance, and V is the voltage. We must first solve for equivalent capacitance.

C2and C3are capacitors in series, while C1is in parallel.

C23= 0.5μF

Ceq= C23+ C1= 0.5μF + 0.5μF = 1μF

Now we can plug in the Ceqand battery voltage to find the charge.

Q = (1μF)(10V) = 10μC

Example Question #53 :Circuits

Batteries and AC current are often used to charge a capacitor. A common example of capacitor use is in computer hard drives, where capacitors are charged in a specific pattern to code information. A simplified circuit with capacitors can be seen below. The capacitance of C1is 0.5 μF and the capacitances of C2and C3are 1 μF each. A 10 V battery with an internal resistance of 1 Ω supplies the circuit.

PretextQuestion_2

While the capacitor is charging, does the capacitor generate a magnetic field?

Possible Answers:

No

Yes

Cannot be determined

Correct answer:

Yes

Explanation:

This question asks us to consider electromagnetism and what happens when a capacitor is charging. When a capacitor is charging, charge is accumulating on the surface over a period of time. Given that the electric field due to a capacitor is given by the formula E = σ/ε0, where σ is the charge per unit area and ε0is the constant of permeability of free space, we can see that E is directly proportional to σ; therefore, the more charge that builds up, the higher the resulting E field.

Because σ is changing during charging, and thus E is changing, we also know that a magnetic field, B, must be created.

Remember the principle—a changing electric field induces a changing magnetic field, and a changing magnetic field induces a changing electric field.

Example Question #54 :Circuits

Batteries and AC current are often used to charge a capacitor. A common example of capacitor use is in computer hard drives, where capacitors are charged in a specific pattern to code information. A simplified circuit with capacitors can be seen below. The capacitance of C1isand the capacitances of C2and C3areeach. Abattery with an internal resistance ofsupplies the circuit.

PretextQuestion_2

How much potential energy is stored by the capacitors of the circuit?

Possible Answers:

Correct answer:

Explanation:

First, we need to determine the type of energy being stored by the capacitors in the circuit. As this is an electric circuit, electric energy is being stored. Thus, we can use the formula for potential energy stored in a capacitor, U = ½CV2. We must first find the equivalent capacitance.

C2and C3are capacitors in series, while C1is in parallel.

C23= 0.5μF

Ceq= C23+ C1= 0.5μF + 0.5μF = 1μF

U = ½CV2= ½(1μF)(10V)2= 5 * 10-5J

Example Question #55 :Circuits

Which of the following arrangements of parallel conducting plates would store the most charge when connected in series with a battery of voltage?

Assume that in each arrangement, each plate has area.

Possible Answers:

Plates separated of; glass fills the space between them

Plate separation of; air fills the space between them

Plate separation of; glass fills the space between them

Plate separation of; air fills the space between them

Plate separation of; air fills the space between them

Correct answer:

Plate separation of; glass fills the space between them

Explanation:

Relevant equations:

The first equation shows that chargeis directly proportional to capacitance, so we want to maximizein order to find the maximum charge.

The second equation shows thatis directly proportional to the dielectric constant, and inversely proportional to distance. To increase capacitance, we needto be large andto be small.

A smaller distanceand a larger dielectric constant (choosing glass instead of air) will lead to a large stored charge.

Example Question #56 :Circuits

An RC circuit is assembled by connecting a voltage source, a resistor, and a capacitor in series. The capacitor in the circuit has a potential difference of. After discharging the capacitor for two seconds, the potential difference of the capacitor drops to. What are the approximate capacitance and time constant of this circuit if the resistor has a resistance of?

Possible Answers:

Correct answer:

Explanation:

To solve this question, we need to use the equation that describes voltage decay during capacitor discharging:

Here,is the voltage after a certain amount of time,is the initial voltage,is the time elapsed,is the resistance of the resistor, andis the capacitance of the capacitor. We can rearrange this equation in such a way that we solve for capacitance,:


To remove the exponentialfrom the equation, we need to take the natural logarithm of both sides:

Solving forgives us:

The time constant,, of an RC circuit is the product of the resistance and the capacitance; therefore, the time constant of this circuit is:

Example Question #57 :Circuits

During discharge, the decay of voltage in a capacitor is an example of__________decay, the decay of current in a capacitor is an example of__________decay, and the decay of charge in a capacitor is an example of__________decay.

Possible Answers:

exponential . . . linear . . . linear

exponential . . . exponential . . . exponential

linear . . . linear . . . linear

linear . . . exponential . . . exponential

Correct answer:

exponential . . . exponential . . . exponential

Explanation:

A capacitor is an electrical device consisting of two parallel conducting plates that store charge. A capacitor undergoes two processes: charging and discharging. During charging, a capacitor accumulates and stores charge between the two plates. During discharge, a capacitor loses the stored charge. Since there is a decrease in the amount of charge during discharge, there is also a decrease (or decay) in current and voltage in a discharging capacitor. The decay of all these parameters is characterized by the equation:

Here,anddenote the variable (voltage, current, or charge),denotes the time elapsed,denotes the resistance of the resistor, anddenotes the capacitance of the capacitor. This is an equation for exponential decay; therefore, all the variables undergo exponential decay in a discharging capacitor.

Example Question #58 :Circuits

A capacitor has a capacitance ofand is connected in series with a resistor to form an RC circuit. At, the capacitor has a potential difference of. At, the potential difference has dropped to.

What is the resistance of the resistor?

Possible Answers:

Correct answer:

Explanation:

问题,潜在的差异dropped fromtoin. The percentage of voltage drop is:

This means that there was avoltage drop in. The definition of time constant is the time it takes for a capacitor to discharge toof its original value, or have adrop; therefore, the time constant for this circuit is.

The time constant, tau, for an RC circuit is the product of the resistance of the resistor and the capacitance of the capacitor:

We can use this equation to solve for the resistance of the resistor:

Solving for resistance gives us:

Notice that you could have used the voltage decay equation to solve this problem; however, knowing the definition of time constant simplifies the problem and reduces the amount of calculations.

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