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Photograph of an electric arc produced between two multi stranded wires close to each other but not in contact.
Is this electric arc dangerous? The answer depends on the AC frequency and the power involved. (credit: Khimich Alex, Wikimedia Commons)

Section summary

  • The two types of electric hazards are thermal (excessive power) and shock (current through a person).
  • Shock severity is determined by current, path, duration, and AC frequency.
  • [link] lists shock hazards as a function of current.
  • [link] graphs the threshold current for two hazards as a function of frequency.

Conceptual questions

Using an ohmmeter, a student measures the resistance between various points on his body. He finds that the resistance between two points on the same finger is about the same as the resistance between two points on opposite hands—both are several hundred thousand ohms. Furthermore, the resistance decreases when more skin is brought into contact with the probes of the ohmmeter. Finally, there is a dramatic drop in resistance (to a few thousand ohms) when the skin is wet. Explain these observations and their implications regarding skin and internal resistance of the human body.

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What are the two major hazards of electricity?

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Why isn’t a short circuit a shock hazard?

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What determines the severity of a shock? Can you say that a certain voltage is hazardous without further information?

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An electrified needle is used to burn off warts, with the circuit being completed by having the patient sit on a large butt plate. Why is this plate large?

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Some surgery is performed with high-voltage electricity passing from a metal scalpel through the tissue being cut. Considering the nature of electric fields at the surface of conductors, why would you expect most of the current to flow from the sharp edge of the scalpel? Do you think high- or low-frequency AC is used?

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Some devices often used in bathrooms, such as hairdryers, often have safety messages saying “Do not use when the bathtub or basin is full of water.” Why is this so?

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We are often advised to not flick electric switches with wet hands, dry your hand first. We are also advised to never throw water on an electric fire. Why is this so?

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Before working on a power transmission line, linemen will touch the line with the back of the hand as a final check that the voltage is zero. Why the back of the hand?

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Why is the resistance of wet skin so much smaller than dry, and why do blood and other bodily fluids have low resistances?

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Could a person on intravenous infusion (an IV) be microshock sensitive?

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In view of the small currents that cause shock hazards and the larger currents that circuit breakers and fuses interrupt, how do they play a role in preventing shock hazards?

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Problem exercises

(a) How much power is dissipated in a short circuit of 240-V AC through a resistance of 0 . 250 Ω size 12{0 "." "250" %OMEGA } {} ? (b) What current flows?

(a) 230 kW

(b) 960 A

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What voltage is involved in a 1.44-kW short circuit through a 0 . 100 - Ω size 12{0 "." "100""-" %OMEGA } {} resistance?

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Find the current through a person and identify the likely effect on her if she touches a 120-V AC source: (a) if she is standing on a rubber mat and offers a total resistance of 300 k Ω size 12{"300"" k" %OMEGA } {} ; (b) if she is standing barefoot on wet grass and has a resistance of only 4000 k Ω size 12{"4000"" k" %OMEGA } {} .

(a) 0.400 mA, no effect

(b) 26.7 mA, muscular contraction for duration of the shock (can't let go)

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While taking a bath, a person touches the metal case of a radio. The path through the person to the drainpipe and ground has a resistance of 4000 Ω size 12{"4000" %OMEGA } {} . What is the smallest voltage on the case of the radio that could cause ventricular fibrillation?

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Foolishly trying to fish a burning piece of bread from a toaster with a metal butter knife, a man comes into contact with 120-V AC. He does not even feel it since, luckily, he is wearing rubber-soled shoes. What is the minimum resistance of the path the current follows through the person?

1 . 20 × 10 5 Ω size 12{1 "." "20"´"10" rSup { size 8{5} } %OMEGA } {}

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(a) During surgery, a current as small as 20.0 μA size 12{"20" "." 0 μ"A"} {} applied directly to the heart may cause ventricular fibrillation. If the resistance of the exposed heart is 300 Ω size 12{"300" %OMEGA } {} , what is the smallest voltage that poses this danger? (b) Does your answer imply that special electrical safety precautions are needed?

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(a) What is the resistance of a 220-V AC short circuit that generates a peak power of 96.8 kW? (b) What would the average power be if the voltage was 120 V AC?

(a) 1 . 00 Ω size 12{1 "." "00" %OMEGA } {}

(b) 14.4 kW

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A heart defibrillator passes 10.0 A through a patient’s torso for 5.00 ms in an attempt to restore normal beating. (a) How much charge passed? (b) What voltage was applied if 500 J of energy was dissipated? (c) What was the path’s resistance? (d) Find the temperature increase caused in the 8.00 kg of affected tissue.

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Integrated Concepts

A short circuit in a 120-V appliance cord has a 0 . 500 - Ω size 12{0 "." "500""-" %OMEGA } {} resistance. Calculate the temperature rise of the 2.00 g of surrounding materials, assuming their specific heat capacity is 0.200 cal/g ⋅º C size 12{0 "." "200"" cal/g" cdot °C} {} and that it takes 0.0500 s for a circuit breaker to interrupt the current. Is this likely to be damaging?

Temperature increases 860º C size 12{"860"°C} {} . It is very likely to be damaging.

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Construct Your Own Problem

Consider a person working in an environment where electric currents might pass through her body. Construct a problem in which you calculate the resistance of insulation needed to protect the person from harm. Among the things to be considered are the voltage to which the person might be exposed, likely body resistance (dry, wet, …), and acceptable currents (safe but sensed, safe and unfelt, …).

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Questions & Answers

meaning of the term si units
Chali Reply
what what causes electric current
Chali
what what causes electric current
Chali
Charges(electron)
Caleb
Systeme international unitq@1qa@aaq
Caleb
Correct one international system of units.
Caleb
electric dynamo
albert
Electric dynamo is the sources of electric magnetic forces which utilize electromagnetic induction.
albert
A stone is dropped down a well, if it take 5 seconds to reach the water, how dip is the well
Mollamin Reply
an aircraft at as steady velocity of 70m/so eastwards at a height of 800me drops a package of supplies .a, how long will it take for the package to rich the ground? b, how fast will it be going as it lands?
Ng Reply
what is hypothesis theory law
Tamba
physics is the science of measurement
Jide Reply
What is physics
Victor Reply
what is physics
Obaro
Good question! Physics is the study of the nature world . Does this help?
Yonn
physics is the study of matter in relations to energy.
Enoch
physics is the science of measurements
Jide
physics is a science concern with nature and properties of matter and energy
Ugomma
what is a parallelogram law of motion?
Nancy
describe how you would find the area of an irregular shaped body?
Chali
Definition for physics
Adesola Reply
It deal with matter and relation to energy
Soughie
physics is the Study of matter in relation to energy.
albert
physics is a natural science that study matter its behaviour and relation to energy.
mohammed
physic tells us more about quantities and measurement also
Kelly
life as we know it that can be measured and calculated
Jesus
what is a reference frame
Chukwu Reply
what is anatomy in relation to physics
Mubarak Reply
how does half life exist
Humble Reply
 The amount of time it takes a radioactive isotope to decay into a stable isotope is different for each radioactive isotope, and is characterized by its “half-life”. An isotope's half-life is the amount of time it takes for half the number of atoms of that isotope to decay to another isotope.
Nardine
what is the difference between Mass and weight
Pjustin
mass is constant while weight varies. unit of mass is kg, unit of weight is newton
Faith
how can a coin float in water and what principle governs it
Mercy Reply
in my opinion that work of surface tension but restrictions on coin is that coin do not break surface energy of molecules but some days before scientists prove that's another types of force
Aman
which force hold floating coins together thats my confusion
Aman
how can a coin float in water and what principle governs it
Mercy Reply
why many of the coin floating in water
Aman Reply
what is hook's law
Momoh Reply
provided the elastic limit is not exceeded,the extension produce in an elastic material is equal to the applied force
Mahmud
What is Andromeda
Chinecherem Reply
👡
Mahmud
What are the international agreed system of unit for physical measurements
Aisher Reply
m, kg, s
Faith
kg , m, s
Faith
si
Aman
meter , kilogram , second
Kelly
international system of units is agreed system because more units are not in mks system so si exist with ampere unit
Aman
Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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