Chapter 23, Problem 83MSPP

The Defibrillator

A defibrillator is designed to pass a large current through a patient’s torso in order to stop dangerous heart rhythms. Its key part is a capacitor that is charged to a high voltage. The patient’s torso plays the role of a resistor in an RC circuit. When a switch is closed, the capacitor discharges through the patient’s torso. A jolt from a defibrillator is intended to be intense and rapid; the maximum current is very large, so the capacitor discharges quickly. This rapid pulse depolarizes the heart, stopping all electrical activity. This allows the heart’s internal nerve circuitry to reestablish a healthy rhythm.

A typical defibrillator has a 32 μF capacitor charged to 5000 V. The electrodes connected to the patient are coated with a conducting gel that reduces the resistance of the skin to where the effective resistance of the patient’s torso is 100 Ω.

83. For the values noted in the passage above, what is the time constant for the discharge of the capacitor?

A. 3.2 μs

B. 160 μs

C. 3.2 ms

D. 160 ms