Shortwave diathermy in rehabilitative medicine
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Shortwave diathermy in rehabilitative medicine
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[post_author] => 12815
[post_date] => 2025-01-09 11:14:04
[post_date_gmt] => 2025-01-09 16:14:04
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
In physiotherapy settings, low-frequency electromagnetic (EM) waves are used for diagnosis and therapy in diathermy, which is the use of electricity or electromagnetic energy to produce heat. One of the major types of diathermy is shortwave, which is considered a non-ionizing form of radiation. These waves can speed up the healing of tissues by providing deep heat to a large area. On the microscopic level, three basic mechanisms underlie the interactions: the displacement of electrons and ions, orientation of existing dipoles, and polarization of atoms and molecules.
Figure 1. Electric dipole (p) in an uniform electric field.
The two modalities, the condenser and the inductive coil methods, are depicted in Figure 2. For the former, the patient becomes part of the circuit and is placed between the two electrodes, which act like the plates of a capacitor. Thus, the tissue that provide the most resistance to current flow develops the most heat. For the latter, the patient is circumferentially wrapped around with a coil or has an electrode with an insulated coiled within placed onto the target tissue. Heating is due to the eddy currents (small circular electric fields) generated by the alternating electric current through the coil. In this case, the patient is not part of the circuit but a “parallel” circuit where the current will flow through the tissues that provide the least amount of resistance.
Figure 2. Circuit depiction of the condenser (left) and the inductive coil (right) method in shortwave diathermy
When an EM wave interacts with matter, there is absorption, and each type of EM wave has its own characteristic interaction. Ionizing radiations like X and gamma rays are dangerous because such high-energy photons cause an electron to be ejected from its orbital. However, concerns raised about exposure to non-ionizing radiation have prompted governments to set guidelines for exposure, which is quantified by the specific absorption rate (SAR), the rate at which energy is imparted to a given mass of tissue measured in watts per kilogram:
Equation 1. Specific Absorption Rate (SAR)
where σ is tissue conductivity, ρ is tissue density, t is time, T is temperature, E is electric field, and c is specific heat.
[post_title] => Shortwave diathermy in rehabilitative medicine
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[post_modified] => 2025-01-09 11:15:03
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[question] => Which of the following tissues would experience the greatest increase in temperature in a capacitive circuit?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
The passage states that in a capacitive circuit heating is due to the rapid rotation of the dipoles, and the rapid rotation is due to the oscillations of the alternating current.
The passage also states that the tissue acts like a dielectric between the plates of a capacitor. Tissues that have a greater number of dipoles have a greater capacitance to store electrical charge.
For such tissues, increased current is needed to cause dipole motion in these tissues, decreasing the heating effect since it is due to rapid rotation of the dipoles.
Skin and muscle have the largest number of dipoles. Adipose tissue has the least number of dipoles and has the least capacitance to store energy, increasing its potential to be heated.
The passage states that the tissues that provide the most resistance inhibit the amount of heat transfer to the body, so areas of the body that have lower amounts of body fat tend to receive better electrical transmission.
Skin and muscle have the largest number of dipoles.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Adipose
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[1] => Array
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[each_answer] => B. Skin
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[each_answer] => C. Muscle
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[3] => Array
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[each_answer] => D. Bone
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => According to the equation for SAR given in the passage, which of the following expressions has units equivalent to SAR?
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
The units for SAR are 
which represents power divided by mass or 
.
Q has the same units as
. This is the first law of Thermodynamics, q = mc∆T.
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[answers] => Array
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[0] => Array
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[each_answer] => A.
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[1] => Array
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[each_answer] => B.
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[2] => Array
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[each_answer] => C.
)
[3] => Array
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[each_answer] => D.
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => The water molecule, in the gaseous state, has three types of transitions that can give rise to the absorption of electromagnetic radiation. Which of the following transitions could be due to the absorption in the UV range?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
Each type of EM wave has a different characteristic interaction with matter. In order for a wave to interact at all, the energy of the incident EM wave has to match the available quantized energy levels with spacings. Otherwise, the substance will be transparent to that particular EM wave.
Rotational transitions are caused by absorption of microwaves and produce heat as a result of that motion. UV light would be too energetic to cause rotational transitions.
Vibrational transitions are caused by absorption of infrared waves and also produce heat as it is absorbed. UV light would be too energetic to cause vibrational transitions.
Photoionization is caused by X-rays and gamma rays according to the passage. The higher end of the UV spectrum has sufficient energy to cause photoionization.
Electronic transitions are usually caused by visible light and can produce heat or be released as photons in the visible range. However, water is transparent to visible light, so there must not be any quantized energy levels similar to that of photons in the visible range.
Electronic transitions can also be caused by absorption in the lower end of the UV spectrum. In the case of water, UV light causes electronic transitions.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Electronic transitions
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[1] => Array
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[each_answer] => B. Rotational transitions
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[2] => Array
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[each_answer] => C. Vibrational transitions
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[each_answer] => D. Photoionization
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[quiz_unique_key] => 2261298308
[question] => The image below shows the alternate method of induction diathermy where an induction coil within an electrode is brought close to the surface of the skin, as opposed to a coil being wrapped around a limb as in Figure 2. Which of the following statements most accurately describes the relationship between the current in the coil and the induced current?
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[answer] => 2
[description] => Reason for the Correct Answer:
According to the diagram, the current in the induction coil is moving in a counterclockwise fashion, while the induced current is moving in a clockwise fashion.
To find the magnetic field due to the coil, which can be treated as a solenoid, wrap the fingers in the direction of the current. The thumb should be pointing in the direction of the magnetic field at the center of the coil.
The magnetic field is pointing north through the center of the coil and pointing south through on the outside of the coil.
The magnetic field will induce a current in the tissues such that it produces a magnetic field that opposes the original field. The induced current will produce a magnetic field pointing south.
To double check, to find the magnetic field due to a loop, grab the wire with the thumb pointing in the direction of the current. The fingers or palm indicate the direction of the magnetic field.
The palm should be down, so the magnetic field is pointing southward through the center of the loop, which was determined from the other magnetic field pointing northward.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Both the original current and induced current are moving in a clockwise fashion, which would then create an overlapping magnetic field pointing north through their centers.
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[each_answer] => B. The magnetic field produced from the coil points northward, so the magnetic field produced from the induced current points southward.
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[2] => Array
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[each_answer] => C. The current in the inductive coil moving in a counterclockwise fashion induces the current in the tissue moving in a clockwise fashion.
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[each_answer] => D. The current in the coil creates a magnetic field, which is opposed by the electric field produced by the induced current.
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[quiz_unique_key] => 2377279144
[question] => Figure 1 depicts a dipole in a uniform electric field. Which of the following statements most accurately describes its motion?
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[answer] => 1
[description] => Reason for the Correct Answer:
p is the electric dipole moment vector, which points from the negative end to the positive end of the dipole, and the field lines indicate the direction that a positive charge would move.
Therefore, the negative end of the dipole would experience a force to the left, and the positive end of the dipole would experience a force to the right due to the electric field.
Since the two charges are equal in magnitude, there are equal and opposite forces and no net force on the dipole. As a result, the dipole does not exhibit any translational motion.
Rotationally, due to the electric force, the positive end of the dipole will experience a clockwise or counterclockwise torque by flipping upwards or downwards. Correspondingly, the negative end will experience a clockwise or counterclockwise torque by flipping downwards or upwards.
The two torques are additive, either clockwise or counterclockwise, and the dipole will exhibit circular or rotational motion but no translational motion.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The dipole will exhibit circular motion but no translational motion.
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[each_answer] => B. The dipole will exhibit translational motion.
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[2] => Array
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[each_answer] => C. The dipole will not exhibit circular motion.
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[each_answer] => D. The dipole will not exhibit translational motion.
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