20.6 Electric Hazards and the Human Body There are two known hazards of electricity-thermal and shock. A thermal hazard is one where excessive electric power causes undesired thermal effects, such as starting a fire in the wall of a house. A shock hazard occurs when electric current passes through a person. Shocks range in severity from painful, but otherwise harmless, to heart-stopping lethality. This section considers these hazards and the various factors affecting them in a quantitative manner. Electrical Safety: Systems and Devices will consider systems and devices for preventing electrical hazards. Thermal Hazards Electric power causes undesired heating effects whenever electric energy is converted to themal energy at a rate faster than it can be safely dissipated. A classic example of this is the short circuit, a low-resistance path between terminals of a voltage source. An example of a short circuit is shown in Figure 20.21. Insulation on wires leading to an appliance has wom through, 788 Chapter 20 | Electric Current, Resistance, and Ohm's Law allowing the two wires to come into contact Such an undesired contact with a high voltage is called a short. Since the resistance of the short, r. is very small, the power dissipated in the short, P = v2/r.is very large. For example, if V is 120 V and r is 0.100 2. then the power is 144 kW, much greater than that used by a typical household appliance. Thermal energy delivered at this rate will very quickly raise the temperature of surrounding materials, melting or perhaps igniting them. (a) Figure 20.21 A short circuit is an undesired low-resistance path across a voltage source. (a) Worn insulation on the wires of a toaster allow them to come into contact with a low resistance r. Since P = V/r. thermal power is created so rapidly that the cord melts or burns. (b) A schematic of the short circuit.
20.6 Electric Hazards and the Human Body There are two known hazards of electricity-thermal and shock. A thermal hazard is one where excessive electric power causes undesired thermal effects, such as starting a fire in the wall of a house. A shock hazard occurs when electric current passes through a person. Shocks range in severity from painful, but otherwise harmless, to heart-stopping lethality. This section considers these hazards and the various factors affecting them in a quantitative manner. Electrical Safety: Systems and Devices will consider systems and devices for preventing electrical hazards. Thermal Hazards Electric power causes undesired heating effects whenever electric energy is converted to themal energy at a rate faster than it can be safely dissipated. A classic example of this is the short circuit, a low-resistance path between terminals of a voltage source. An example of a short circuit is shown in Figure 20.21. Insulation on wires leading to an appliance has wom through, 788 Chapter 20 | Electric Current, Resistance, and Ohm's Law allowing the two wires to come into contact Such an undesired contact with a high voltage is called a short. Since the resistance of the short, r. is very small, the power dissipated in the short, P = v2/r.is very large. For example, if V is 120 V and r is 0.100 2. then the power is 144 kW, much greater than that used by a typical household appliance. Thermal energy delivered at this rate will very quickly raise the temperature of surrounding materials, melting or perhaps igniting them. (a) Figure 20.21 A short circuit is an undesired low-resistance path across a voltage source. (a) Worn insulation on the wires of a toaster allow them to come into contact with a low resistance r. Since P = V/r. thermal power is created so rapidly that the cord melts or burns. (b) A schematic of the short circuit.
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Electric Hazards and the Human Body
• Define thermal hazard, shock hazard, and short circuit.
• Explain what effects various levels of current have on the human body.
Transcribed Image Text:20.6 Electric Hazards and the Human Body
There are two known hazards of electricity-thermal and shock. A thermal hazard is one where excessive electric power causes
undesired thermal effects, such as starting a fire in the wall of a house. A shock hazard occurs when electric current passes
through a person. Shocks range in severity from painful, but otherwise harmless, to heart-stopping lethality. This section
considers these hazards and the various factors affecting them in a quantitative manner. Electrical Safety: Systems and
Devices will consider systems and devices for preventing electrical hazards.
Thermal Hazards
Electric power causes undesired heating effects whenever electric energy is converted to themal energy at a rate faster than it
can be safely dissipated. A classic example of this is the short circuit, a low-resistance path between terminals of a voltage
source. An example of a short circuit is shown in Figure 20.21. Insulation on wires leading to an appliance has wom through,
788
Chapter 20 | Electric Current, Resistance, and Ohm's Law
allowing the two wires to come into contact Such an undesired contact with a high voltage is called a short. Since the resistance
of the short, r. is very small, the power dissipated in the short, P = v2/r.is very large. For example, if V is 120 V and r is
0.100 2. then the power is 144 kW, much greater than that used by a typical household appliance. Thermal energy delivered
at this rate will very quickly raise the temperature of surrounding materials, melting or perhaps igniting them.
(a)
Figure 20.21 A short circuit is an undesired low-resistance path across a voltage source. (a) Worn insulation on the wires of a toaster allow them to
come into contact with a low resistance r. Since P = V/r. thermal power is created so rapidly that the cord melts or burns. (b) A schematic of
the short circuit.
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