26.5 - Interactive checkpoint: energy in a fuel-cell car
In this problem, assume that road friction, air resistance, and electrical resistance use up 20.0% of the energy available, and that the rest of the
electric potential energy stored in the capacitor’s electric field is transformed into the kinetic energy of the car.State your answer in miles per hour (1.00 m/s = 2.24 mph).An FCX fuel-cell car brakes and stops
at a traffic signal. During this time, its
8.00-farad ultracapacitor gets fully
charged by a combined potential
difference of 200 V generated by the
brakes and the fuel cell. When the
light turns green, the 1680-kilogram
car initially uses only energy from the
capacitor to accelerate.
How fast will the car be moving when
the capacitor is fully discharged?
Answer:v = mph
26.6 - Physics in medicine: defibrillator
Although electricity can be harmful if misused, doctors
have used it for medicinal purposes since the days of
the ancient Greeks. For example, early physicians
used discharges from the electric torpedo fish to
relieve aches and pains.
Electricity continues to be used in medicine. If you
have ever watched the television show ER, chances
are you have become well acquainted with a device
called a defibrillator. The heart relies on electrical
impulses to cause its muscles to contract. When the
muscle cells of the heart begin to contract out of
synchronization, or to fibrillate, a defibrillator can be
used to jolt them back into a synchronous rhythm.
Contrary to popular belief, defibrillators are not used to “restart” a heart that has
stopped beating. You may get the impression from TV shows that doctors use
defibrillators as frequently as stethoscopes, but that is just one of the exaggerations of
television. However, these instruments are becoming standard equipment in
ambulances and on airplanes.A conceptual diagram of a defibrillator is shown on the right. The source of potential
difference on the left charges the capacitor. When the switch on the right is closed, the
capacitor rapidly discharges, sending an electrical current through the patient’s heart.
The example problem shows how to calculate the average current when the discharge
time, capacitance and potential difference are known. The current is large, but it lasts
for just a few milliseconds, providing a powerful but brief shock.Cardiac defibrillator.Defibrillator
Capacitor used in heart defibrillator
·Battery charges capacitor to 2500 V
·Discharges in milliseconds, sending
large current through heart(^480) Copyright 2000-2007 Kinetic Books Co. Chapter 26