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MCAT-Section-3-Physical-Sciences – Section Three : Physical Sciences

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Showing 4–6 of 15 questions

Question 4

Historically, two different methods have been used to estimate the fluid pressure in capillary beds.

Method 1

A glass pipette is inserted into the capillary. The level of blood rising in the pipette is measured and used to calculate the pressure. Alternatively, an inert fluid of density ρ can be placed in the pipette and its height h can be measured. The pressure in the capillary is given by ρgh, where g is the acceleration due to gravity.

Figure 1

Method 2

The pressure can be measured indirectly in the following way. A section of gut tissue is removed from a specimen and placed on a beam balance. Blood is circulated through the tissue by a pump. The arterial pressure is then decreased. This leads to a decrease in the capillary hydrostatic pressure in the gut capillaries. The constant osmotic pressure of plasma proteins in the capillary causes absorption of fluid from the gut section which will decrease its weight. To prevent a change in the weight of the gut section, the venous pressure is increased. This tends to increase the capillary pressure, reducing the flow of fluid from the gut tissue into the capillaries. The capillary pressure is thus held constant (and the balance kept level) as the arterial pressure is decreased and the venous pressure increased. The arterial and venous pressures meet at the capillary pressure being measured.

(Π = MRT, where Π is the osmotic pressure, M the molarity of the solutes, R the universal gas constant, and T the temperature in Kelvin.)

Figure 2

Assume that the beam balance of Method 2 is initially level. If the arterial pressure is decreased to a lower level while everything else is held constant, which graph best represents the change in the mass of the gut following the decrease in arterial pressure?

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  • ○

    A
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    B
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    C
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    D
Question 5

One of the most common methods that scientists use to determine the age of fossils is known as carbon dating. 14C is an unstable isotope of carbon that undergoes beta decay with a half-life of approximately 5,730 years. Beta decay occurs when a neutron in the nucleus decays to form a proton and an electron which is ejected from the nucleus.

14C is generated in the upper atmosphere when 14N, the most common isotope of nitrogen, is bombarded by neutrons. This mechanism yields a global production rate of 7.5 kg per year of 14C, which combines with oxygen in the atmosphere to produce carbon dioxide. Both the production and the decay of 14C occur simultaneously. This process continues for many half-lives of 14C, until the total amount of 14C approaches a constant.

A fixed fraction of the carbon ingested by all living organisms will be 14C. Therefore, as long as an organism is alive, the ratio of 14C to 12C that it contains is constant. After the organism dies, no new 14C is ingested, and the amount of 14C contained in the organism will decrease by beta decay. The amount of 14C that must have been present in the organism when it died can be calculated from the amount of 12C present in a fossil. By comparing the amount of 14C in the fossil to the calculated amount of 14C that was present in the organism when it died, the age of the fossil can be determined.

In determining the age of the galaxy, a technique similar to carbon dating is used on stars with the radioactive isotope 232Th, which has a half-life of 1010 years. 14C is less suitable for this application because:

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    its half-life is too long.
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    14C is more abundant than 232Th is in stars.
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    14C is unstable.
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    its half-life is too short.
Question 6

There are two opposing theories of light: the particle theory and the wave theory. According to the particle theory, light is composed of a stream of tiny particles that are subject to the same physical laws as other types of elementary particles. One consequence of this is that light particles should travel in a straight line unless an external force acts on them. According to the wave theory, light is a wave that shares the characteristics of other waves. Among other things, this means that light waves should interfere with each other under certain conditions.

In support of the wave theory of light, Thomas Young’s double slit experiment proves that light does indeed exhibit interference. Figure 1 shows the essential features of the experiment. Parallel rays of monochromatic light pass through two narrow slits and are projected onto a screen. Constructive interference occurs at certain points on the screen, producing bright areas of maximum light intensity. Between these maxima, destructive interference produces light intensity minima. The positions of the maxima are given by the equation dsinθ = nλ, where d is the distance between the slits, θ is the angle shown in Figure 1, the integer n specifies the particular maxima, and λ is the wavelength of the incident light. (Note: sin θ≈ tan θ ≈ θ for small angles.)

Figure 1

Light waves can be described in terms of frequency f and wavelength λ or in terms of wave number k and angular frequency ω. These quantities are related by the following equations: k = 2π/λ and ω = 2πf

Which equation below accurately describes the speed of the wave v in terms of k and ω?

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    v = fλ
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    v = ω + k
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    v = ω/k
    Choice A, while a correct relation, is not the right answer because it does not express v in terms of ω and k. Choices B and D can be eliminated on the basis of dimensional analysis because they do not have the units of speed, m/s. k has units of m-1 and ω has units of S-1, so choice B is nonsensical and choice D has units of 1/(ms).
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    v = ωk