Unit 8 Problems

CC2404 Applied Physics and Instrumentation in Health Care Problems Unit 8 (2008)

1. Which ion rushes into a resting cell when depolarization occurs?

2. What is the normal resting potential across of a human nerve cell? 3. What is the approximate amplitude of a ECG pattern recorded from the skin? 4.

For a typical ECG waveform, what are the meanings of P- , QRS- and T- waves?

During surgery, a current as small as 20.0 µ A applied directly to the heart may cause

5.

ventricular fibrillation. Do you think that special electrical safety precautions are needed?

6. The outer space of a neuron’ s axon membrane (κ =5 thickness d= 1.2x10-8m ) is charged positively, and the inner portion is charged negatively. Assuming that an axon can be treated like a parallel plate capacitor with a plate area of 6x10-6 m2, what is its capacitance ?

7.

(a) During surgery, a current as small as 20.0 µ A applied directly to the heart may cause

ventricular fibrillation. If the resistance of the exposed heart is 300 Ω , what is the smallest voltage that poses this danger? (b) For a time of 10 s, how much charge passed through the heart? (c) What is the energy dissipated during this 10 s?

8. a) What is the peak current through a 500 W room heater that operates on 120 V AC power? (b)What is the peak power? (c) For the same power consumption, if it operates on 220 V AC line, what is the peak current. (5.89A, 1000W, 3.2A)

9. (a) What is the resistance of a 220 V ac short circuit that generates a peak power of 96.8 kW? (b) What would be the average power be if the voltage was 120 V ac?

10. What is the resistance of a copper wire 0.5 mm in diameter and 20 m long? The resistivity of copper is 1.7 x 10-8 Ω·m.

11. Estimate the current and power dissipation on a human being under a voltage of 1 V and 1000 V respectively. We can model the human body into a cylindrical geometry of 30 cm in diameter and 1 m in height. We assume a resistivity of 500 Ω -cm for the tissue.