Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 351:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cation exchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  

  Figure 2.

  Solution X boils at 100.26°C and solution Y boils at 101.04°C. Both solutions are at atmospheric pressure and contain the same solute concentration. Which of the following conclusions can be drawn?

  A. The freezing point of solution X is lower than that of solution Y.

  B. The vapor pressure of solution X is higher than that of solution Y at 100.26°C.

  C. C. Solution X and solution Y are immiscible.

  D. The vapor pressure of solution X is lower than that of solution Y at 100.26°C.

  Correct Answer: B

  This question deals with the colligative properties of solutions. It brings up several critical points about solutions. One is that the presence of a solute in a solution ALWAYS raises the boiling point and lowers the freezing point, compared to the boiling and freezing points of the pure solvent. The more concentrated the solution, the higher the boiling point and lower the freezing point will be. Thus it is clear that choice A is incorrect; since Solution Y's boiling point is higher than Solution X's, its freezing point must be lower as well. Since there isn't actually enough information to say that this answer is right or wrong since you don't know the original freezing point or freezing point depression constant for either solution, there must be a better answer. The second important concept is that of vapor pressure, which is the topic of both answers B and D. A solution boils when its vapor pressure is equal to the atmospheric pressure, solution Y must have had a lower vapor pressure to begin with. Choice B is therefore correct, and choice D incorrect. Choice C concerns the solubility of the two solutions in each other. Again, we don't have enough information to say that this answer is correct. After all, we know nothing about the two solvents of solutions X and Y, only that they have the same concentration of solute in them and what the resultant boiling points are.

• Question 352:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cationexchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  Figure 2.

  

  Which of the following would probably NOT displace + in soil micelles?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  The passage tells you that in soil micelles, cations like A13 and Si4+ are replaced by cations with lower valences. That is what gives the micelles their negative charge. When an A13 cation is displaced by a cation with a +2 charge, the micelle gains a ? charge. So the leaching of these actions as depicted in Table 1 gives an indication of the permanent charge as we saw in the earlier questions. Knowing this, it is easy to see that Si4+, with a valence of +4, is the cation that could not displace the A13+, since Na+, Mg2+, and Cr2+ all have lower valences than the aluminum's +3 and would give a net negative charge to the micelle.

• Question 353:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cationexchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  

  Figure 2.

  Anaerobic organisms are able to denitrify wet soils by the following metabolic pathway.

  

  If all the oxygen in the nitric acid is converted to water, how many additional equivalents of acid will be consumed during the production of 5 M of nitrogen?

  A. 20

  B. 30

  C. 40

  D. 50

  Correct Answer: D

  Each mole of nitrogen is a bimolecular molecule. Therefore, to produce five moles of nitrogen you would need ten moles of nitric acid. Ten moles of nitric acid contain thirty moles of oxygen. To convert this oxygen into water, you would need sixty moles of hydrogen ions. The ten moles of nitric acid provides only ten moles of hydrogen ions; therefore you would need another fifty moles, which is the same as fifty equivalents of acid, choice D.

• Question 354:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cation exchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  Figure 2.

  

  The amount of soil on a particular one-acre field down to a depth of one furrow slice weighs 9 ?105 kilograms. Based on Figure 2, how many kilograms of CaCO3 would have to be added to this field to raise the pH from 5 to 6?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  Beginning at pH 5, the graph tells you how much of a change in pH is produced by adding a given number of milliequivalents of acid or base to 10 grams of soil. You can see by examining the graph that 0.2 milliequivalents of base would raise the pH of the sample from 5 to 6. Since one equivalent equals a thousand milliequivalents, and one kilogram equals a thousand grams, you can convert this ratio from 0.2 milliequivalents per 10 grams, to 0.2 equivalents per 10 kilograms, or

  0.02 equivalents per one kilogram. Since one mole of carbonate can take up two moles of hydrogen ions, a mole of calcium carbonate acts as two equivalents of base. Remember that as the concentration of hydrogen ions goes down, the pH of the solution goes up. The molecular weight of calcium carbonate is 100, so 100 grams of calcium carbonate will provide 2 equivalents of base. We have determined that you need 0.02 equivalents of calcium per kg of soil. Since 2 equivalents are provided by 100 grams of calcium carbonate, 0.02 equivalents are provided by 1 gram. So you need 1 gram of calcium carbonate for each kilogram of soil in the acre furrow slice. This comes to 9 105 grams of calcium carbonate, or 900 kilograms, choice A.

• Question 355:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cationexchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  Figure 2.

  

  What would be the effect of leaching the three soil samples in Table 1 with a buffered BaCl2 solution at pH 9.5 instead of 8.3?

  A. The measured permanent charge would be greater.

  B. The measured pH-dependent charge would be greater.

  C. The measured permanent charge would be smaller.

  D. The measured pH-dependent charge would be smaller.

  Correct Answer: B

  You are told in the passage that the pH-dependent charge of soil is created when hydrogen dissociates from hydroxyl moieties found in organic matter and on the surface of the soil micelles. Remember that hydrogen does not normally dissociate from a hydroxyl or carboxyl group under acidic or neutral conditions. Under alkaline conditions, the hydrogen from a hydroxyl or carboxyl group may be pulled away from the oxygen by hydroxide ions in the solution. Thus, the more basic the leaching solution, the more hydrogen is likely to be released. This will result in a larger reading for the pH-dependent charge.

• Question 356:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cation exchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  Figure 2.

  

  Which soil from Table 1 most likely has the highest percentage of organic matter?

  A. Sample I

  B. Sample II

  C. Sample III

  D. Cannot be determined

  Correct Answer: A

  According to the passage, soils that are high in organic matter contain primarily pH-dependent charges. These are formed when hydrogen dissociates from the hydroxyl moieties of organic acids or alcohols. The pH-dependent charge is represented in the table by the column for H+, which was removed by leaching with a basic solution. When looking at the table, we see that sample I has far more than half its ion exchange capacity accounted for by pH-dependent charges. Samples II and III have roughly half of their total ion exchange capacity accounted for by pH-dependent charges. Therefore, sample I probably has the highest percentage of organic matter in it, choice A.

• Question 357:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cationexchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  Figure 2.

  

  What percentage of the cation exchange capacity of Sample I is base-saturated?

  A. 4%

  B. 6%

  C. 29%

  D. 40%

  Correct Answer: A

  The percentage base saturation consists of the number of milliequivalents of basic ions divided by the entire cation exchange capacity of the soil. For Sample I, that is the third column in Table 1 divided by the fifth, or 1.9 divided by 47.6. If you multiply 1.9/47.6 by 2/2, you can estimate this ratio to be approximately 4/96, which is about 4%, choice A.

• Question 358:

  The periodic beating of the heart is controlled by electrical impulses that originate within the cardiac muscle itself. These pulses travel to the sinoatrial node and from there to the atria and the ventricles, causing the cardiac muscles to contract. If a current of a few hundred milliamperes passes through the heart, it will interfere with this natural system, and may cause the heart to beat erratically. This condition is known as ventricular fibrillation, and is life-threatening. If, however, a larger current of about 5 to 6 amps is passed through the heart, a sustained ventricular contraction will occur. The cardiac muscle cannot relax, and the heart stops beating. If at this point the muscle is allowed to relax, a regular heartbeat will

  usually resume.

  The large current required to stop the heart is supplied by a device known as a defibrillator. A schematic diagram of a defibrillator is shown below. This device is essentially a "heavy-duty" capacitor capable of storing large amounts of energy. To charge the capacitor quickly (in 1 to 3 seconds), a large DC voltage must be applied to the plates of the capacitor. This is achieved using a step-up transformer, which creates an output voltage that is much larger than the input voltage. The transformer used in this defibrillator has a step-up ratio of 1:50.

  

  The AC voltage that is obtained from the transformer must then be converted to DC voltage in order to charge the capacitor. This is accomplished using a diode, which allows current flow in one direction only. Once the capacitor is fully charged, the charge remains stored until the switch is moved to position B and the plates are placed on the patient's chest. To cut down the resistance between the patient's body and the defibrillator, the electrodes are covered with a wetting gel before use. Care must be taken to insure that the patient is not in electrical contact with the ground while the defibrillator is in use.

  If a dielectric was inserted between the plates of the capacitor in the defibrillator when the switch is in position A:

  A. the energy stored in the capacitor would increase.

  B. the energy stored in the capacitor would decrease.

  C. the electric field between the plates would increase.

  D. the electric field between the plates would decrease.

  Correct Answer: A

  Since the capacitor is, in effect, connected to a constant voltage source, the potential difference doesn't change when a dielectric is inserted between the plates. Therefore, the charge on the plates must increase to compensate for the effect of the polarization of charge in the dielectric. The polarization of charge in the dielectric sets up an electric field opposite in direction and weaker in magnitude than the electric field due to the charge on the capacitor plates. This would decrease the net electric field between the plates if the capacitor were not connected to a constant voltage source. However, since it is connected to a constant voltage source when the dielectric is inserted, additional charge builds up on the plates to keep the potential difference constant. Therefore, the net electric field between the plates with the dielectric in place is equal to the net electric field between the plates with no dielectric and choices C and D are wrong. Since the charge on the plates increases, the energy stored will increase because the energy stored is given by (1/2) QVk, where Q is the charge on the plates and V is the potential difference between the plates. Therefore, choice A is correct.

• Question 359:

  Many nutrients required by plants exist in soil as basic cations:

  

  A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.

  

  Replacement of + and + by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of ?, to which other cations can become adsorbed.

  

  Figure 1

  A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-

  dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.

  In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges. In a research study,

  three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the

  displaced H+ measured. Table 1 gives results for three soils tested by this method.

  Table 1

  

  Due to the buffering effect of the soil's cation exchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

  Figure 2.

  

  Which column(s) in Table 1 represent(s) the permanent charge of the soil micelles?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  The permanent charge would be represented by the ions that are removed at a neutral pH. The passage tells us that the permanent charge is the net negative charge held in the inner layer of the micelles that is generated when lower valence cations displace other cations like A13+ and Si4+. This part of the soil's charge is not dependent on the pH and will be displaced by the low valence ion K+ in the KC1 solution. Therefore, since A13+ and the basic cations were displaced by the KC1 solution, together they represent the permanent charge.

• Question 360:

  The periodic beating of the heart is controlled by electrical impulses that originate within the cardiac muscle itself. These pulses travel to the sinoatrial node and from there to the atria and the ventricles, causing the cardiac muscles to contract. If a current of a few hundred milliamperes passes through the heart, it will interfere with this natural system, and may cause the heart to beat erratically. This condition is known as ventricular fibrillation, and is life-threatening. If, however, a larger current of about 5 to 6 amps is passed through the heart, a sustained ventricular contraction will occur. The cardiac muscle cannot relax, and the heart stops beating. If at this point the muscle is allowed to relax, a regular heartbeat will usually resume.

  The large current required to stop the heart is supplied by a device known as a defibrillator. A schematic diagram of a defibrillator is shown below. This device is essentially a "heavy-duty" capacitor capable of storing large amounts of energy. To charge the capacitor quickly (in 1 to 3 seconds), a large DC voltage must be applied to the plates of the capacitor. This is achieved using a step-up transformer, which creates an output voltage that is much larger than the input voltage. The transformer used in this defibrillator has a step-up ratio of 1:50.

  

  The AC voltage that is obtained from the transformer must then be converted to DC voltage in order to charge the capacitor. This is accomplished using a diode, which allows current flow in one direction only. Once the capacitor is fully

  charged, the charge remains stored until the switch is moved to position B and the plates are placed on the patient's chest. To cut down the resistance between the patient's body and the defibrillator, the electrodes are covered with a wetting

  gel before use. Care must be taken to insure that the patient is not in electrical contact with the ground while the defibrillator is in use.

  The plates of the capacitor are originally separated by a vacuum. If a dielectric K>1 is introduced between the plates of the capacitor, and the capacitor is allowed to charge up, which of the following statements is/are true?

  I) The capacitance of the capacitor will increase.

  II) The voltage across the capacitor plates will increase.

  III) The charge stored on the capacitor will increase.

  A. I only

  B. I and II only

  C. II and III only

  D. I and III only

  Correct Answer: D

  

  The capacitance of a parallel-plate capacitor is given by , where K is the dielectric constant, 0 is the permittivity of free space, A is the area of overlap of the two the plates, and d is the separation of the two plates. This implies that when a material with a dielectric constant K is introduced between the plates of a capacitor, the capacitance increases by a numerical factor K. In the question we are told that a dielectric with a value of K> 1 is introduced between the plates which means that the capacitance will increase. So statement I is correct and choice C can be ruled out. It is important to note at this point that the voltage across the plates does not increase with the introduction of a dielectric. This is because the voltage across the plates of a fully charged capacitor is equal to the voltage applied across the plates when it was initially charged. Since this is held constant at 8,000 volts, the voltage across the plates will remain at 8,000 volts. So statement II is false and choice B can be eliminated. To choose between choices A and D, look at how the charge on the plates of a capacitor is affected by the introduction of a dielectric. The capacitance of a capacitor is related to the voltage across the plates and the charge stored by the equation C=Q/V, where C is the capacitance, Q is the charge stored, and V is the voltage. As mentioned previously, the voltage across the capacitor remains constant when the dielectric is introduced. This implies that the capacitance of the capacitor is directly proportional to the charge stored. Since this is the case, as the capacitance increases, the charge stored on the plates must also increase. So statement III is true and choice D is the correct answer.