Chapter 19, Problem 94RPP

BIO Effect of electric current on human body Nerve impulses are initiated at the input end of a nerve cell, travel along a relatively long axon (cable), and then cause an effect at the output end of the cell—for example, the initiation of a muscle contraction in a muscle cell. The nerve impulse is initiated by a stimulus that lowers the potential difference from outside the cell to inside from its normal -70 mV to about -50mV.

A potential difference across two parts of the body (for example, the 120-V potential difference from a wall socket from one hand to the other or from the hands to the feet) can initiate an electric current in the body that stimulates nerve endings and triggers nerve signals that cause muscular contraction. Even worse, the current in the body can upset the rhythmic electrical operation of the heart. The heart muscles might be stimulated randomly in what is called ventricular fibrillation—a random contraction of the ventricles, which can be deadly. A rough guide to the effects of electric current on the body at different current levels is provided in Figure 19.39. Under dry conditions, human skin has high electrical resistance. Wet skin dramatically lowers the body’s resistance and makes electrocution more likely to occur.

Occasionally, the electric circuit that produces a coordinated pumping of blood from the four chambers of the heart becomes disturbed. Ventricular fibrillation can occur—random muscle contractions that produce little or no blood pumping. To stop the fibrillation, two defibrillator pads are placed on the chest and a large current (about 14 amps) is sent through the heart, restarting its normal rhythmic pattern. The current lasts 10 ms and transfers 140 J of electric energy to the body. Which answer below is closest to the potential difference between the defibrillator pads?

a. 100 V

b. 400 V

c. 1000 V

d. 5000 V

e. 10,000 V