All MCAT Physical Resources
Example Questions
Example Question #1 : Capacitors And Dielectrics
Which of the following statements about dielectrics is false?
A dielectric material increases voltage by decreasing stored charge
A dielectric material is non-conducting
A dielectric material increases capacitance by increasing stored charge
A dielectric material increases capacitance by decreasing voltage
A dielectric material increases voltage by decreasing stored charge
Dielectrics are put between two parallel plates of a capacitor to increase capacitance. This is done by holding voltage constant and increasing charge, or decreasing voltage and holding charge constant.
This can be understood using the equation . Either an increase in charge or a decrease in voltage will increase the capacitance.
Example Question #611 : Mcat Physical Sciences
Two capacitors are fully charged and connected in series in a circuit powered by a battery. Find the charge stored.
First, we need to find the equivalent capacitance for the capacitors in series.
Now we can find the charge using the equation .
Example Question #41 : Circuits
Two parallel plate capacitors C1 and C2 are arranged in parallel within a 10V circuit. If C1 stores 2C of charge and C2 stores 5C of charge, each over 3 seconds, what is the total capacitance of the circuit?
1.8 F
0.7 F
2.4 F
0.2 F
0.7 F
The capacitance of any capacitor can be calculated with the formula
Since the capacitors are arranged in parallel, the voltage drop across each will be equal, and in this case 10V. Adding capacitors in parallel is the same as adding resistors in series, where the total capacitance of the circuit is equal to the sum of all individual capacitors.
Example Question #11 : Capacitors And Dielectrics
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 C1 is 0.5 μF and the capacitances of C2 and C3 are 1 μF each. A 10 V battery with an internal resistance of 1 Ω supplies the circuit.
Capacitors in series share the same __________, while capacitors in parallel share the same __________.
Current . . . charge
Voltage . . . current
Charge . . . current
Current . . . voltage
Current . . . voltage
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 #82 : Electricity And Magnetism
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 C1 is 0.5 μF and the capacitances of C2 and C3 are 1 μF each. A 10 V battery with an internal resistance of 1 Ω supplies the circuit.
How much total charge is stored by the capacitors of the circuit?
10μC
25μC
15μC
20μC
10μC
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.
C2 and C3 are capacitors in series, while C1 is in parallel.
C23 = 0.5μF
Ceq = C23 + C1 = 0.5μF + 0.5μF = 1μF
Now we can plug in the Ceq and battery voltage to find the charge.
Q = (1μF)(10V) = 10μC
Example Question #611 : Mcat Physical Sciences
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 C1 is 0.5 μF and the capacitances of C2 and C3 are 1 μF each. A 10 V battery with an internal resistance of 1 Ω supplies the circuit.
While the capacitor is charging, does the capacitor generate a magnetic field?
Yes
Cannot be determined
No
Yes
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 ε0 is 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 #12 : Capacitors And Dielectrics
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 C1 is and the capacitances of C2 and C3 are each. A battery with an internal resistance of supplies the circuit.
How much potential energy is stored by the capacitors of the circuit?
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.
C2 and C3 are capacitors in series, while C1 is in parallel.
C23 = 0.5μF
Ceq = C23 + C1 = 0.5μF + 0.5μF = 1μF
U = ½CV2 = ½(1μF)(10V)2 = 5 * 10-5J
Example Question #12 : Capacitors And Dielectrics
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 .
Plate separation of ; air 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
Plates separated of ; glass fills the space between them
Plate separation of ; glass fills the space between them
Relevant equations:
The first equation shows that charge is directly proportional to capacitance , so we want to maximize in order to find the maximum charge.
The second equation shows that is directly proportional to the dielectric constant , and inversely proportional to distance . To increase capacitance, we need to be large and to be small.
A smaller distance and 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 ?
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, and is the capacitance of the capacitor. We can rearrange this equation in such a way that we solve for capacitance, :
To remove the exponential from the equation, we need to take the natural logarithm of both sides:
Solving for gives 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 #611 : Mcat Physical Sciences
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.
linear . . . linear . . . linear
linear . . . exponential . . . exponential
exponential . . . exponential . . . exponential
exponential . . . linear . . . linear
exponential . . . exponential . . . exponential
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, and denote the variable (voltage, current, or charge), denotes the time elapsed, denotes the resistance of the resistor, and denotes the capacitance of the capacitor. This is an equation for exponential decay; therefore, all the variables undergo exponential decay in a discharging capacitor.
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