Physics and Astronomy Outreach Program at the University of British Columbia

1. Human Body Electric Shock Lecture Notes Physics and Astronomy Outreach Program at the University of British Columbia

2. Goals Electric Shock • Physics: To show the fundamental laws applied in a ground fault circuit interrupter (GFCI). • Biology: To understand the effect of electrical current on the human body. Physics and Astronomy Outreach Program at the University of British Columbia

3. Big Ideas Electric Shock • Physics: Ohms law determines current flow through a resistor. Electromagnetic induction can be used to prevent electrical shocks. • Biology: Human body acts like a resistor. Electrical currents interfere with nerve impulses. This affects muscle contraction, heart functioning and breathing. Burns can also occur. Physics and Astronomy Outreach Program at the University of British Columbia

4. Human body Electric Shock • Human body acts like a resistor. V =IR • Dry skin1 : R = 10-100 kΩ • Wet skin1 : R ~ 1 kΩ • Inside body1: Water and electrolytes. Each limb has R = ~ 500Ω • Rubber shoes2: R = 20MΩ • Leather shoes, dry2, R = 100-500 kΩ • Leather shoes, wet2, R = 5 – 20 kΩ • Total resistance depends on path of current. Physics and Astronomy Outreach Program at the University of British Columbia

5. Electric shock Electric Shock • Effect of current on body (60 Hz AC)1: • 0-1 mA Imperceptible • 1-3 mA Perceptible, mild • 3-5 mA Annoyance • 6-9 mA Muscles contract and cannot let-go • 30 mA Asphyxiation • 80 mA Ventricular fibrillation • 5-10 A Cardiac arrest, burns • Duration of exposure important as well. Physics and Astronomy Outreach Program at the University of British Columbia

6. GFCIs Electric Shock • Ground fault circuit interrupters (GFCIs) are • installed in power outlets in wet environments • such as bathrooms, kitchens, outside, pools etc. • They sense the development of an imbalance • in the current going out of the outlet and the • current returning to the outlet. • They will shut off the current flow if an • imbalance is sensed. Physics and Astronomy Outreach Program at the University of British Columbia

7. GFCIs cont’d Electric Shock • The development of an • imbalance in current • will cause a change in the • B- field inside the sense coil. • A change in magnetic flux induces a voltage • activates electronics • which stops the current flow.

8. GFCIs cont’d Electric Shock • Designed to shut off in 7.5 s for a 5 mA imbalance and within 25 ms for greater than a 264 mA imbalance3. • The duration of shock is limited, not the amount of current. • GFCIs do not protect against touching the hot and neutral wires. Physics and Astronomy Outreach Program at the University of British Columbia

9. References Electric Shock [1] Magison, E.C., Electrical instruments in hazardous locations 3rd ed., Instrument Society of America (1972) [2]http://www.allaboutcircuits.com/vol_1/chpt_3/3.html [3] Quick, R. C., IEEE Transactions on Industry Applications, Vol IA-11, No. 1 January/February 1975 Physics and Astronomy Outreach Program at the University of British Columbia