Electrical current is the movement of electrical charge from one location to another; this flow is measured as amperes (A) (defined as 1 coulomb per second). Current flows when there is a potential difference between two locations; this potential difference is measured as volts (V). The intervening material resists the flow of electrical current; this resistance is measured as ohms. Ohm's law describes the relation between potential difference (V), current (I), and resistance (R) for simple materials: V = I * R.
Materials that allow electrical current to flow easily with low resistance are called conductors. Conductors include the metals copper and aluminum and electrolyte solutions such as normal saline. Materials that do not allow electrical current flow are called insulators. Most biologic materials conduct electricity to some extent and can be classified as having low, moderate, or high resistance (Table 196-1). Tissues with a high fluid and electrolyte content conduct electricity better than tissues with less fluid and electrolyte content. Bone is the biologic tissue most resistant to the passage of electricity. Skin resistance varies widely ( Table 196-2). Moist skin has reduced resistance that, according to Ohm's law, enables more current to flow such that voltages that ordinarily would not cause much damage can now become fatal.
TABLE 196-1 Body Tissue Resistance
As current flows through a resistor, energy is deposited into the material in the form of heat. The amount energy can be calculated according to Joule's law: energy = I2 * R * time.
High voltage is usually defined in the medical literature as greater than 1000 V. However, there is evidence that the risk for serious and fatal electrical injury increases significantly with voltages above 600 V; therefore, for medical decision making, a value of 600 V is a more appropriate threshold to separate low from high voltage. 3 Interestingly, the National Electric Code defines high voltage as greater than 600 V. Power lines in US residential areas are typically 7620 V that is stepped down by transformers to 220 V as they enter a home (Table 196:3). The voltage in US household circuits is 110 V. Clothes dryers, electric ranges, heavy-duty appliances, and many air conditioners operate on special circuits that carry 220 V. Although high voltage is more dangerous than low voltage, because low voltage is more accessible to the population, 40 to 60 percent of electrical injuries and 50 percent of the deaths are due to low-voltage electricity.
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