20.6 Electric Hazards and the Human Body

High voltages are more dangerous than low ones.

Some high voltages, such as those associated with common static electricity, can be harmless.
It is not the only factor that determines a hazard.
AC shocks are more harmful than DC shocks.
The DC power-distribution system in New York City was set up in the late 1800s by Thomas Edison because he thought AC shocks were more harmful.
The use of AC in early power-distribution systems was the subject of bitter fights between Edison and George Westinghouse.
AC has prevailed because of lower power losses.

You can learn how to make a bulb light using magnets.

There are two dangers of electricity.

There are shocks that range in severity from harmless to lethal.
The various factors affecting them are considered in a quantitative manner.

Electric power can cause heating effects when it is converted to thermal energy at a faster rate than can be safely dissipated.

The insulation on wires leading to an appliance has worn out, allowing the two wires to come into contact.
A short is a contact with a high voltage.
The power dissipated in the short is very large because the resistance is very small.
If 120 V is the case, the power is much greater than that used by a typical household appliance.
The thermal energy delivered at this rate will quickly raise the temperature of the surrounding materials.

A short circuit is a low-resistance path.

The thermal power is created so quickly that it can melt the cord.

A short circuit's resistance may be decreased due to the increase in temperature.

If the short creates ionization, this can happen.
The charged atoms and molecules are free to move.
The power dissipated in the short rises, possibly causing more power.
High voltages, such as the 480-V AC used in some industrial applications, lend themselves to this hazard because they create higher initial power production in a short time.

A less dramatic thermal hazard is when wires supplying power to a user are overloading.

The power dissipated in the supply wires is where the resistance of the wires and the current flowing through them are.
The wires can get hot if either is too large.
A worn appliance cord with some of its braided wires broken may not be what it should be.
The cord is dissipated more than is safe if 10.0 A of current passes through it.
If a wire with a resistance is meant to carry a few Amps, but is instead carrying 100 A, it will explode.
In that case, the power dissipated in the wire.
The circuits are used to limit the currents.
When a sustained current exceeds safe limits, each device opens the circuit.

The one shown here has a bimetallic strip that bends to the right if overheated.

The metal strip is broken by the spring at the points.

When sustained current exceeds desired limits, circuit breakers open like automatic switches.

Circuit breakers for large voltages and currents can be difficult to make, but they are relatively easy to produce.

When a circuit breaker tries to interrupt the flow of electricity, a spark can jump across its points and allow the current to continue flowing.

In power-distribution systems, large circuit breakers use jets of gas to blow out sparks.

Since AC current goes through zero 120 times per second, it's safer than DC.

People produce electrical currents.

It is possible to block back pain with an electrical current.
It is possible to use electrical current to help paraplegics walk.
TV dramatizations in which electrical shocks are used to bring a heart attack victim out of ventricular fibrillation are more common.
Most electrical shock deaths are caused by a current putting the heart into fibrillation.
The heart is stimulated by electrical shocks.
Some fatal shocks do not cause burns, but they can be safely burned off with electric current.
There are consistent explanations for the effects.
The effects of electrical shock depend on 1 factors.
There is a path taken by the current 3.

Table 20.3 shows the effects of electrical shocks as a function of current for a typical accidental shock.

60-hertz power causes a shock that passes through the trunk of the body and has a duration of 1 s.

Muscular contraction can be caused by an electric current.

Those that close their fingers are stronger than those that open them.

Our bodies are good conductors because of the water.

Electric currents preferentially flow through paths in the human body that have a minimum resistance in a direct path to earth, because larger currents will flow through sections with lower resistance.
The earth has an electron sink.
A requirement in many professions is that you wear shoes that provide a large resistance in the path of electrons.

When working with high-power tools, make sure you don't provide a path for current flow through the heart.

Currents that are very small pass harmlessly through the body.

You don't know when this happens.
The threshold of sensation is 1 mA and shocks are harmless for currents less than 5 mA.
The maximum allowed shock is taken by many safety rules.
Nerve impulses can be stimulated by the current at 10 to 20 mA.
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 hand closes involuntarily on the wire because the muscles that close the fingers are stronger than those that open them.
This can prolong the shock for a long time.
The rescuer's hand may close about the victim's wrist, which is a danger to the person trying to rescue the victim.
The best way to help the victim is to blow ajar or throw an insulator at the fist.
Modern electric fences, which are used in animal enclosures, are less lethal than in the past because they are pulsed on and off to let people who touch them get free.

Greater currents can affect the heart.

This condition is fatal due to a lack of blood circulation.
There is a threshold for ventricular fibrillation.
The greater the concentration of current, the greater the chance of burns.

Large currents cause the heart and diaphragm to contract during a shock.

The heart and breathing stop.

Both return to normal after the shock.

The electrical patterns on the heart are completely erased in a way 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 similar to writing on a blackboard.
Large paddles are shown in TV dramatizations of the electric shock used to save a heart attack victim.
The spread out current is used to reduce the chance of burns.

The table and preceding discussion show that current is the main factor in determining shock severity.

The severity of the shock depends on the combination of resistance and voltage.
A person with dry skin has a resistance of about.
A current does not pass through him if he comes into contact with 120-V AC.

A person who is soaking wet may have a resistance of 120 V and a current greater than the "can't let go" threshold.

The body's resistance is in its dry skin.

Salts go into ion form when wet.
The interior of the body has a lower resistance than dry skin because of all the ionic solutions and fluids it contains.
The currents listed in Table 20.3 produce similar effects.
Currents as small as possible can be used during openheart surgery.
The doubly disadvantaged microshock-sensitive patient is related to the Stringent electrical safety requirements in hospitals.
An average male is shocked through the trunk of his body for 1 s. 80% of those listed have values for females.

The graph shows average values for the threshold of sensation and the "can't let go" current.

The more sensitive the body is, the lower the value is.

The path, duration, and AC frequencies are other factors that affect the severity of a shock.

There are consequences to the path.
The heart is unaffected by an electric shock through the brain, which may be used to treat manic depression.
The longer a shock lasts, the greater its effects.
The curves show the minimum current for two different effects.
The more sensitive the body is, the lower the current needed.
In common use, the body is most sensitive to the 50- or 60-hertz frequencies.

The body becomes less sensitive to effects that involve nerves at higher frequencies.

The maximum rates at which nerves can fire is related to this.
A wart can be burned off without causing the heart to stop.
High frequencies and low currents are used in some of the spectacular demonstrations of electricity, in which high-voltage arcs are passed through the air and over people's bodies.
In Electrical Safety: Systems and Devices, electrical safety devices and techniques are discussed.

The answer depends on how much power is involved.

Electric currents in our body allow us to sense the world, control parts of our body, and think.

There are three major functions of nerves.
The central nervous system consists of the brain and spine.
Nerves carry messages from the central nervous system to muscles.
Nerves communicate with the central nervous system.
The sheer number of nerve cells and the number of connections between them make this system a subtle wonder.

Nerve cells have tendrils that connect them with other cells and look different than other cells.

The signals arrive at the cell body across the dendrites, stimulating the neuron to send its own signal to other nerve or muscle cells.
Signals may arrive from many other locations and be transmitted to others, giving the system its complexity and ability to learn.

The electric currents that reach the cell body through dendrites and across the axon cause the neuron to send its own signal down the axon.

The number of connections can be much larger than shown here.

The method by which these electric currents are generated and transmitted is more complex than the simple movement of free charges in a conductor, but it can be understood with principles already discussed in this text.

The Coulomb force is the most important.

Figure 20.28 shows how a potential difference is created in a neuron in its resting state.

Free ion will diffuse from a high concentration to a low concentration.

The force of the Coulomb force makes it impossible for the ion to diffuse across.

The attraction of unlike charges prevents more from leaving either side once the charge layer has built up.
The two layers of charge are balanced by the Coulomb force.
A tiny fraction of the charges move across and the fluids remain neutral, while a separation of charge and a voltage have been created across the membranes.

There are different concentrations of ion inside and out of a cell's semipermeable membrane.

The Coulomb force stops further transfer when Diffusion moves in the direction shown.
It is impermeable to.

There is an action potential inside a nerve cell.

It is caused by the movement of the ion across the cell.
Depolarization happens when a stimulation makes the membrane permeable to ionized matter.
The membrane becomes impermeable to and moves from high to low concentration.
In the long term, active transport slowly maintains the concentration differences, but the cell may fire hundreds of times in rapid succession without seriously deplete them.

A potential difference of 70 to 90 mV is created by the separation of charge.

The resting potential of the interior of a neuron is -90 mV if the exterior is 0 V. Nerve and muscle cells are the largest types of animal cells with such voltages.
25% of the energy used by cells is used to create and maintain potentials.

Electric currents are created by any change in the cell's permeability.

The Coulomb force causes the membrane to rush in, as it becomes temporarily permeable to.

The inrush of first neutralizes the inside, or depolarizes it, and then makes it slightly positive.

The movement of the cell quickly returns it to its resting potential, or repolarizes it, after the depolarization.
The action potential is the result of this sequence of events.

The concentration differences that create bioelectricity need to be maintained.

This pump is an example of active transport, where cell energy is used to move ion across the membranes.

There is an action potential at one location.

Changing voltage and electric fields affect the permeability of the adjacent cell, so that the same process takes place there.
The action potential stimulated at one location causes a nerve impulse to move slowly.

A nerve impulse is the propagation of an action potential.

The action potential at one location is caused by a change in the permeability of the adjacent membrane.
The action potential moves slowly along the cell membrane because this affects it further down.
A wave of charge moving along the outside and inside of the membrane is what the impulse is due to.

The axon has a number of interesting properties.

Cross talk is prevented by the fact that myelin is an Insturment.
The myelinated regions transmit electrical signals at a very high speed.
Cell energy is not used in the myelinated regions.
There is a signal loss in the myelin, but the signal is regenerated in the gaps.

Cross talk and slow signal transmission are characteristics of the normal operation of axons, which are not myelinated.

The myelin sheaths that surround the nerve fibers can be damaged if they are destroyed.

Multiplesclerosis is caused by the body's own immune system attacking the myelin in the central nervous system.
Weakness of arms and legs, fatigue, vision problems, and loss of balance are some of the symptoms of Multiplesclerosis.
Younger adults are more likely to be struck by it.
Infections, environmental affects, or genetics are possible causes.
There is no known cure for the disease.

Most animal cells have their own action potential.

When muscles fire, they are often caused by a nerve impulse.
Nerve and muscle cells are similar, and there are even hybrid cells, such as in the heart, that have characteristics of both nerves and muscles.

From left to right, a nerve impulse isPropagation of a nerve impulse down a myelinated axon.

The signal travels very fast and without energy input in the myelinated regions.
It regenerated in the gaps.
The signal moves faster in unmyelinated axons than it does in other nerves.

An electric eel is capable of creating a voltage that stuns prey.

Nerve impulses can be transmitted by depolarization and repolarization, just as muscle cells can be stimulated by fire and contract.
A depolarization wave can be sent across the heart to coordinate its rhythms and allow it to propel blood through the circulatory system.

During depolarization, the outer surface of the heart changes from positive to negative.

The wave of depolarization is spreading from the top of the heart and is represented by a vector pointing in the direction of the wave.
This is a potential difference.
Three wires are placed on the patient.
Each pair measures a component of the depolarization vector and is graphed in an ECG.

There are two pairs of electrodes on the chest.

The left and right arms and the left leg were used for three-electrode ECGs decades ago.

The lead II potential is between the right arm and the left leg.
We will look at the lead II potential as an indicator of heart-muscle function and see that it is coordinated with blood pressure.

The four-chamber action of the heart is explored in Poiseuille's Law.

The right and left atria both receive blood from the body and lungs.
Blood is pumped through the lungs and the rest of the body by the right and left ventricles.
The heart muscle contracts when it is depolarized.
It is ready for the next beat after contraction.
Significant information on the functioning and malfunctioning of the heart can be obtained by measuring components of depolarization and repolarization of the heart muscle.

Figure 20.34 shows an electrocardiogram of the lead II potential and a graph of blood pressure.

The major features are labeled P, Q, R, S, and T. The depolarization of the ventricles creates the QRS complex.

The shape of the heart and path of the depolarization wave are not easy to understand, so the QRS complex has a typical shape and time span.

The repolarization of the atria is masked by the lead IIQRS signal.
The T wave is generated by the repolarization of the ventricles and is followed by the next P wave.
Blood pressure varies with each part of the heartbeat, with the highest pressure occurring after the ventricles.

A lead II blood pressure test.

The depolarization and contraction of the ventricles creates the QRS complex, which is followed by the maximum or systolic blood pressure.
For further description, see text.

A state-of-the-art electrocardiogram can give a lot of information about the heart.

One or more lead potentials can be seen in the damaged heart tissue, which is called infarcts.
If you compare a recent ECG to an older one, you can see subtle changes to the heart.
Individual heart shape, size, and orientation can cause variations in electrocardiograms from one person to another.
There is a portable electrocardiogram monitor with a liquid crystal instant display and a printer that can be carried to patients' homes or used in emergency vehicles.

A portable device is being used to record the vital signs of a NASA scientist and NEEMO 5 aquanaut in an underwater habitat.

Stimulate a neuron and watch what happens.

In order to observe the ion as they move across the neuron membrane, Pause, rewind, and move forward in time.

The electric current is the rate at which charge flows, greater than the drift velocity of free electrons.

A simple circuit is one in which the source and resistance are the same.

The SI unit for current is the ampere, where between current, voltage, and resistance in a simple circuit to be

The drift velocity is the average speed at which these are.

Resistance and Resistivity are expressed in the relationship.

The current is through a wire.

Values of Table 20.1 show that materials fall into, where is the current at time, and three groups--conductors, semiconductors, and is the peak current.

The average power is.

Table 20.2 gives the temperature and values.

The law for AC is.

Electric power is the rate of energy that is supplied by a source or dissipated by a device.

There are three expressions for electrical power.

The electric potentials in cells are created by ionic concentration differences.

Direct current is the flow of electric current in only one direction.

It refers to systems where the source potentials are present.

Myelin sheaths speed this process and reduce the amount of energy needed.

The car batteries are rated in ampere-hours.

Why isn't a bird sitting on a power line?

There is a drop across a Resistor.

Examine the range of resistivity for each and see if more skin is brought into contact with the probes of the higher or lower ohmmeter.

The human body has a rectangular and internal resistance.

A patient sits on a butt plate while a needle is used to burn off warts.

Why is the temperature different from the resistivity?

Electric Power and Energy wet hands first.

Why do incandescent lightbulbs grow dim late in their lives?

If resistance increases, power will decrease because the dissipated power will touch the line with the back of the hand as a final.

Give an example of how AC power can be used other than in the interrupt to prevent shock household.

A defibrillated patient can be brought out of cardiac arrest during open-heart surgery.

A 10.0-mA current is needed because of the current produced by the solar path.

At full speed, the 200 A current through a spark plug moves 0.300 mC supply 1000 A.

How long does it take?

X-ray tubes have electron guns in them.

The X-rays were produced by the conducting gel that reduced the path directed onto a metal target.

A cyclotron is capable of directing a beam of nuclei onto a same.

The diameter of the 14-gauge copper wire is 1.628mm.

A defibrillation unit has a Capacitor that drives a current through the heart of a patient.

The current is 20.0-A.

The Nichrome wire is used to make a Resistor that can only be used at the speed of light.

What is the difference between the temperature and the voltage?

What is happening to the patient.

What is the resistance of the piece of land?

The diameter of the copper wire is 8.252mm.

Redo Exercise 20.25 takes into account the thermal resistance of a 1.00-km length of the wire used for expansion.
You might think of a power transmission.

Which assumptions are unreasonable, or which per day?

Older cars have 6.00-V electrical systems.

How much does a flashlight cost until the end of their life?

An extension cord with a 2.06 mA is put out by a cauterizer.

It is said that the average television is on for 6 hours a day.

If the power consumption is implied by the equation, then the units of a volt-ampere are watt, as million TVs.

The diameter of the copper wire is 9.266mm.

The power loss in a kilometer is calculated.

The home device has a maximum current of 3.50 A and uses 120 250 kV.

Assuming 95.0% efficiency for the conversion of through water, vaporizing it directly with relatively little electrical power by the motor, what current must the temperature increase?

The REVAi gets charged on a street in London.

An electric immersion heater is used to heat electricity.

The speed of the light-rail train is compared to what would be typical for an automobile.

The heating takes place.

The heating time would be shortened by a lower 20.5 alternating current.

Discuss the practical limits to speeding the heating.

AC power is used for certain heavy industrial equipment.

The heating time is what causes the circuit breaker to trip.

The price of electricity is.

408-V AC consumes 50.0 kilowatts.

Nichrome wire is used.

If the operating temperature is beating, the cross-sectional area of the patient's torso is needed to restore normal.

There was an instantaneous increase in the tissue.

There are Electric Hazards and a circuit breaker.

If she touches a 120-V AC source, find the current through a person and identify the likely voltage to which the person might be exposed, likely effect on her if she touches a rubber mat and accept currents.

A person touches the metal case of a radio while taking a bath.

He is wearing rubber-soled shoes and does not feel it.

There are reasons for the time lag.