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10.6 Household wiring and electrical safety  (Page 4/11)

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Summary

  • The two types of electric hazards are thermal (excessive power) and shock (current through a person). Electrical safety systems and devices are employed to prevent thermal and shock hazards.
  • Shock severity is determined by current, path, duration, and ac frequency.
  • Circuit breakers and fuses interrupt excessive currents to prevent thermal hazards.
  • The three-wire system guards against thermal and shock hazards, utilizing live/hot, neutral, and ground wires, and grounding the neutral wire and case of the appliance.
  • A ground fault circuit interrupter (GFCI) prevents shock by detecting the loss of current to unintentional paths.

Key equations

Terminal voltage of a single voltage source V terminal = ε − I r eq
Equivalent resistance of a series circuit R eq = R 1 + R 2 + R 3 + ⋯ + R N − 1 + R N = ∑ i = 1 N R i
Equivalent resistance of a parallel circuit R eq = ( 1 R 1 + 1 R 2 + ⋯ + 1 R N ) −1 = ( ∑ i = 1 N 1 R i ) −1
Junction rule ∑ I in = ∑ I out
Loop rule ∑ V = 0
Terminal voltage of N voltage sources in series V terminal = ∑ i = 1 N ε i − I ∑ i = 1 N r i = ∑ i = 1 N ε i − I r eq
Terminal voltage of N voltage sources in parallel V terminal = ε − I ∑ i = 1 N ( 1 r i ) −1 = ε − I r eq
Charge on a charging capacitor q ( t ) = C ε ( 1 − e − t R C ) = Q ( 1 − e − t τ )
Time constant τ = R C
Current during charging of a capacitor I = ε R e − t R C = I o e − t R C
Charge on a discharging capacitor q ( t ) = Q e − t τ
Current during discharging of a capacitor I ( t ) = − Q R C e − t τ

Conceptual questions

Why isn’t a short circuit necessarily a shock hazard?

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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?

Not only might water drip into the switch and cause a shock, but also the resistance of your body is lower when you are wet.

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Problems

(a) How much power is dissipated in a short circuit of 240-V ac through a resistance of 0.250 Ω ? (b) What current flows?

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What voltage is involved in a 1.44-kW short circuit through a 0.100 - Ω resistance?

12.0 V

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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 ; (b) if she is standing barefoot on wet grass and has a resistance of only 4000 k Ω .

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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 . What is the smallest voltage on the case of the radio that could cause ventricular fibrillation?

400 V

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A man foolishly tries to fish a burning piece of bread from a toaster with a metal butter knife and 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?

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

a. 6.00 mV; b. It would not be necessary to take extra precautions regarding the power coming from the wall. However, it is possible to generate voltages of approximately this value from static charge built up on gloves, for instance, so some precautions are necessary.

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Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
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John Reply
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Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
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emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
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Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
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Mohammed
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Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
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Reofrir Reply
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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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