EECS
2015+1-1,2,3+all.pdf

3 compare your result with the scenario described

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3. Compare your result with the scenario described above (in the text accompanying Figure 4). Explain which component of the total electric resistance made the greatest difference between the two scenarios. Reasoning: 1. The current enters the worker’s body through the palm of the hand holding the drill. The current exits the worker’s body through the feet and the soles of the shoes. Between the entrance and exit, the current flows from the hand to the arm, the chest, most likely involving the heart and lungs; then to the legs and the feet, 2. The total electric resistance for the path of the current determined above is the sum of three resistances (for each of them, we use the worst-case scenario): (a) The resistance of the skin of the hand that holds the drill handle; we can approximate this as the resistance of the hand holding an 1 ½ -inch pipe, that is 1 k Ω for dry skin (Figure 1) (b) The resistance of internal organs, which is 200 Ω (Figure 1) (c) The resistance of the sole of dry shoes, which is given as 100 k Ω . Thus the total resistance equals 101.2 k Ω . Assuming the voltage of 120 V, apply Ohm’s law to obtain the current through the worker’s body: ࠵? = 120 ࠵? 101 . 2 ࠵?࠵? = 1 . 186 ࠵?࠵? This current causes pain (see Figure 1 in Unit 1-2-1) but does not exceed the Let-Go threshold. The worker should not experience muscle paralysis thus should be able to turn off the drill and/or drop it in order to get rid of the electric shock. 3. Compared with the potentially lethal outcome of the scenario described in the text accompanying Figure 4, the situation suggested in this problem is nearly harmless. The main difference between the two scenarios is due to the difference between electric resistances of the dry shoes and the immersed feet. Book Page 110
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EE for 21 st century 1-2 Avoid electrocution 1-2-2 Resistance of the human body © 2015 Alexander Ganago Page 12 of 13 Last printed 2015-07-24 1:06 PM File: 2015 1-2-2 Human R.docx T YPICAL QUESTIONS FOR SELF - ASSESSMENT 1 Electric resistance of internal organs in the human body is much higher than the electric resistance of healthy, dry skin. True / False 2 Any person who works with electricity barefoot and with wet hands is in greater danger of electric shock than someone doing the same work with dry hands and wearing dry shoes. True / False 3 Electric resistance of the human skin does not depend on the area of contact: it is the same for a finger touch and for a hand grasp. True / False 4 If you only hold a power electric tool with one hand, your risk of electric shock is minimal and does not depend on whether your boots are dry or soggy. True / False 5 If you are caught in the middle of a thunderstorm, the choices are: 1. Don’t panic; walk as usual. 2. Walk with long strides. 3. Run. 4. Make shorter steps What is the correct course of action?
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  • Fall '07
  • Ganago
  • Electric charge, Alexander Ganago

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