PDF Physics of the Human Body

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Identify different mechanisms by which an engineering system may be controlled. Describe some aspects of control engineering applied to the human body. Predict how heat is generated and lost by the human body in different circumstances. Explain how the body responds to hot and cold conditions. Choose appropriate methods of measuring physiological temperatures.

Describe the physical operation of sensory systems. Identify common problems in sensing and how these are mitigated. Explain how pressure, volume and flow relate to each other in the human body. Describe the haemodynamics of the brain. Teaching and exams Teaching will consist of: Lectures, hours.

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Required written work essays, problem sheets , 40 hours. Private reading, 60 hours. A classic example of this is the short circuit , a low-resistance path between terminals of a voltage source. An example of a short circuit is shown in Figure Insulation on wires leading to an appliance has worn through, allowing the two wires to come into contact. Such an undesired contact with a high voltage is called a short. Thermal energy delivered at this rate will very quickly raise the temperature of surrounding materials, melting or perhaps igniting them. One particularly insidious aspect of a short circuit is that its resistance may actually be decreased due to the increase in temperature.

This can happen if the short creates ionization. High voltages, such as the V AC used in some industrial applications, lend themselves to this hazard, because higher voltages create higher initial power production in a short. Another serious, but less dramatic, thermal hazard occurs when wires supplying power to a user are overloaded with too great a current.

If Similarly, if a wire with a 0.

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Fuses and circuit breakers are used to limit excessive currents. See Figure Each device opens the circuit automatically when a sustained current exceeds safe limits. Fuses and circuit breakers for typical household voltages and currents are relatively simple to produce, but those for large voltages and currents experience special problems. For example, when a circuit breaker tries to interrupt the flow of high-voltage electricity, a spark can jump across its points that ionizes the air in the gap and allows the current to continue.

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Large circuit breakers found in power-distribution systems employ insulating gas and even use jets of gas to blow out such sparks. Here AC is safer than DC, since AC current goes through zero times per second, giving a quick opportunity to extinguish these arcs. Electrical currents through people produce tremendously varied effects.

An electrical current can be used to block back pain. The possibility of using electrical current to stimulate muscle action in paralyzed limbs, perhaps allowing paraplegics to walk, is under study. TV dramatizations in which electrical shocks are used to bring a heart attack victim out of ventricular fibrillation a massively irregular, often fatal, beating of the heart are more than common. Yet most electrical shock fatalities occur because a current put the heart into fibrillation. A pacemaker uses electrical shocks to stimulate the heart to beat properly.

Some fatal shocks do not produce burns, but warts can be safely burned off with electric current though freezing using liquid nitrogen is now more common. Of course, there are consistent explanations for these disparate effects. The major factors upon which the effects of electrical shock depend are. Table The effects are for a shock that passes through the trunk of the body, has a duration of 1 s, and is caused by Hz power.

Our bodies are relatively good conductors due to the water in our bodies.

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Given that larger currents will flow through sections with lower resistance to be further discussed in the next chapter , electric currents preferentially flow through paths in the human body that have a minimum resistance in a direct path to Earth. The Earth is a natural electron sink. Wearing insulating shoes, a requirement in many professions, prohibits a pathway for electrons by providing a large resistance in that path.

Thermal transfers

Whenever working with high-power tools drills , or in risky situations, ensure that you do not provide a pathway for current flow especially through the heart. Very small currents pass harmlessly and unfelt through the body.


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This happens to you regularly without your knowledge. The threshold of sensation is only 1 mA and, although unpleasant, shocks are apparently harmless for currents less than 5 mA. A great number of safety rules take the 5-mA value for the maximum allowed shock. At 10 to 20 mA and above, the current can stimulate sustained muscular contractions much as regular nerve impulses do.

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People sometimes say they were knocked across the room by a shock, but what really happened was that certain muscles contracted, propelling them in a manner not of their own choosing. The muscles that close the fingers are stronger than those that open them, so the hand closes involuntarily on the wire shocking it. This can prolong the shock indefinitely. It can also be a danger to a person trying to rescue the victim, because the rescuer's hand may close about the victim's wrist. Modern electric fences, used in animal enclosures, are now pulsed on and off to allow people who touch them to get free, rendering them less lethal than in the past.

Greater currents may affect the heart. The threshold for ventricular fibrillation is between and mA. At about mA and above, the shock can cause burns, depending on the concentration of current—the more concentrated, the greater the likelihood of burns.

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Very large currents cause the heart and diaphragm to contract for the duration of the shock. Both the heart and breathing stop. Interestingly, both often return to normal following the shock. The electrical patterns on the heart are completely erased in a manner that the heart can start afresh with normal beating, as opposed to the permanent disruption caused by smaller currents that can put the heart into ventricular fibrillation.

The latter is something like scribbling on a blackboard, whereas the former completely erases it.