5.11 Bob is building a phonocardiograph, a device that records heart sounds, for

Question

5.11 Bob is building a phonocardiograph, a device that records heart sounds, for his bioengineering class. When he is finished, he plugs the device into an elec. trical outlet, which delivers a maximum voltage of 120 V Unfortunately, the plug breaks. Assume the body follows Ohm's law. (Note: Given resistances are approximate and should NOT be tested.) (a) Bob puts the palm of his hand over the electrical outlet, so that the alter- nating current is being delivered to his palm. A dry human palm has a re sistance of about 5 kn. Analysis of several accidents has shown that a (b) person feels pain to 3 Will Bob feel pain? than 5 A. it also been shown that tissue is burned if exposed to more (c) Will the tissue in the palm of Bob's hand be from friend Edna to help Bob out by removing the plug pieces the the electrical outlet. She takes one plug piece in each hand, causing of current to flow through her arms and chest. Assume that the resistance is each arm 750 n, resistance of the chest is 500 n, and resistance uniform across the chest. It has been shown that the heart stops if ex posed to 4 A. Will Edna's heart stop?

Explanation / Answer

a) Given max voltage, Vmax = 120 volts

Human palm resistance, R=5k ohms = 5000 ohms

current through the palm, i=V/R = 120/5000 = 24 mA

which is more than 3mA, so Bob will feel pain.

b) As per the calculation , i=24mA, which less than the burnt current of 5A, so tissue in the palm of Bob's hand will not burned.

c) Edna arm resistace, Ra= 750 ohms

for two hands in series, total resistance of the arms = 750+750 = 1500 ohms

resistance of the chest, Rc= 500 ohms

Total resistance of chest and arms, R= 1500+500 = 2000 ohms

voltage , V= 120 volts

current throught the chest of Edna, i= V/R = 120/2000 = 60 mA

Heart stops for a current of 4A,

so, 60mA is less than the 4A, heart of Edna will not stops.

d) current for fibrillate = 75 mA

60mA is less than 75mA, so heart will not fibrillate.

e) resistance of gloves= 20M ohms = 20000000 ohms

voltage, v=120 volts

current through the gloves, i = 120 / 20000000 = 6 micro Amps.

as this current is very very less, no parts of the Doris body will effect.

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