AP Physics 2 : AP Physics 2

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #3 : 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.

PretextQuestion_2

How long does it take to fully charge the capacitors of the circuit?

Possible Answers:

1 * 103s

1 * 104s

1 * 105s

1 * 106

Correct answer:

1 * 106

Explanation:

In order to determine the time, we need to know the total charge stored on the capacitors. Remember that Q = CV, where Q is the total charge, C is the equivalent capacitance, and V is the voltage. 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

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

Q = (1μF)(10V) = 10μC

Additionally, we need to know the current the battery can provide (the charge per unit time). Knowing both the total charge and current will allow us to calculate the time. We can use V = IR to determine the current.

I = V/R = 10V/1Ω = 10A = 10C/sec

We can equate charge and current to determine time.

10μC = 10 C/t

t = 10 C/10 μC = 1 * 106s or 11.6days

Example Question #3 : 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.

PretextQuestion_2

Instead of air, assume that we insert a dielectric material with a dielectric constant k between the capacitor plates. How would the total capacitance of the circuit change?

Possible Answers:

Remain the same

Increase

Decrease

Correct answer:

Increase

Explanation:

In this question, we are asked how the total charge stored on the surface of the capacitors would change if we inserted a dielectric between the parallel plates. As we can see in the equation for capacitance based on physical properties, , where k is the dielectric constant (k = 1 for air; k > 1 for all dielectric materials), ε0 is the constant of the permeability of free space, A is the area of the plates, and d is the distance between the plates.

If we insert a dielectric material, k > 1, the value of C increases, thus the overall capacitance of the circuit increases.

Example Question #4 : Capacitors And Dielectrics

Which of the following statements about dielectrics is false?

Possible Answers:

A dielectric material increases capacitance by increasing stored charge

A dielectric material increases voltage by decreasing stored charge

A dielectric material increases capacitance by decreasing voltage

A dielectric material is non-conducting

Correct answer:

A dielectric material increases voltage by decreasing stored charge

Explanation:

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 #1 : Capacitors And Dielectrics

Two capacitors are fully charged and connected in series in a circuit powered by a  battery. Find the charge stored.

Possible Answers:

Correct answer:

Explanation:

First, we need to find the equivalent capacitance for the capacitors in series.

Now we can find the charge using the equation .

Example Question #1 : Circuits

A capacitor attached to a 50 V battery aquires a charge of 10 C over a period of 5 seconds. How much energy is required to charge the capacitor?

Possible Answers:

1250 J

50 J

250 J

500 J

Correct answer:

1250 J

Explanation:

The power consumed by a capacitor is given by

\dpi{100} \small P=\frac{1}{2}qV

After finding power, we can calculate the energy used as we are told that the capacitor took 5 seconds to charge. The units of power are watts which can be further broken down into joules per second. By multiplying the time taken to charge the capacitor by the power, we can find the total energy required to charge the capacitor.

Example Question #3 : Circuits

A low resistance light bulb and a high resistance light bulb are connected in parallel with each other. This parallel combination is in series with a  battery. Which bulb is brighter?

Possible Answers:

Only the high resistance bulb will be lit

Only the low resistance bulb will be lit

Both bulbs are equally bright

The low resistance bulb

The high resistance bulb

Correct answer:

The low resistance bulb

Explanation:

The brightness of each bulb depends on its power, or how much energy it dissipates per unit time. This can be calculated using the equation:

If the bulbs are in parallel with each other, the bulb with lower resistance will receive more current, since current will tend toward the path of least resistance. Current is squared in the power equation, so it is of greater importance in determining the power. The bulb with greater current and lower resistance will thus have more power and will shine more brightly.

Example Question #1 : Voltage, Energy, And Power

During the cold winter months, some gloves have the ability to provide extra warmth due to an internal heating source. A simplified circuit, similar to those in electric gloves, is comprised of a 9V battery with no internal resistance and three resistors as shown in the image below. 

Screen_shot_2013-10-09_at_11.08.15_pm

How much power does the circuit provide?

Possible Answers:

Correct answer:

Explanation:

This question asks us about the power of the circuit, meaning the amount of energy per unit time. The power equation is P = IV. V = IR can be substituted in to allow P to be calculated from a number of parameters.

P = IV = I2R = V2/R

To solve for P, we first need the current supplied by the battery.

We can use the formula V = IR because we have the voltage drop across the circuit (9V) and can calculate the equivalent resistance.

By taking the inverse of the equation, we can see that RA4 is equal to 2Ω.

Req = RA4 + R1 = 2Ω + 2Ω = 4Ω

Now, using V=IR, we can solve for the current.

V = IR

I = V/R = 9V/4Ω = 2.25A

Now, we can use the current we calculated and the voltage that is dissipated across the circuit to calculate the power.

P = IV = (2.25 A)(9V) = 20.25W

Example Question #35 : Circuits

If the peak voltage across a  resistor in an AC circuit is , what is the rms current across it?

Possible Answers:

Correct answer:

Explanation:

First, find the rms voltage across the resistor with the equation:

The pure voltage is , allowing us to solve for the rms voltage.

Now we can use Ohm's law to find the rms current.

Example Question #6 : Circuits

Household voltage in the United States is , supplied by an alternating current (AC). What is the maximum voltage at the outlet?

Possible Answers:

Correct answer:

Explanation:

Alternative current (AC) voltage is governed by the RMS (root-mean square) voltage law. This means that the RMS voltage of the circuit will be , but the actual voltage will fluctuate between two values. These values are determined by the equation:

Using the values from our question, we can find the maximum voltage value.

 

 

Example Question #21 : Light

Which color of the visible spectrum of light has the highest energy of the answer choices?

Possible Answers:

Yellow

Orange

Green

Blue

Red

Correct answer:

Blue

Explanation:

The highest energy of any visible light belongs to violet. The greater the wavelength, the lower the energy of the light. The greater the frequency, the higher the energy of the light. This is why ultraviolet light ("ultra" meaning "beyond" violet) is so damaging to DNA. Out of the answer choices, blue light has the lowest wavelength and greatest frequency, making it the highest energy.

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