17.5. Measuring Current and Voltage http://www.ck12.orgElectrical SafetyWhat would happen if a wire were connected to the positive terminal and the negative terminals of a battery? The
resistance of a typical wire is very small and therefore a huge current would result. The battery would soon discharge
and become useless. However, there is also the chance that the Joule heating of the wire may cause an electrical fire.
The insulation covering the wire could melt and then perhaps burn. The battery may even explode!
A situation where the positive and negative terminals of a power source are directly connected or a component of a
circuit is bypassed, is called an electrical short orashort circuit
There are times in a household when too many electrical appliances are connected to a circuit and draw too much
current. This also can cause too much heating in the wires, which may lead to an electrical fire. To protect against
too much current (amperage) in the circuit, fuses or circuit breakers are placed in series within the circuit. Fuses
enclosed in glass, which were once used in home service panels, had a thin wire that melted if a certain amount of
current was going through (usually, 15 A or 20 A). Most automobiles fuses are still enclosed in glass.
All new buildings today are equipped with circuit breakers. They are much more convenient than the older glass-
enclosed fuses, which needed to be replaced once they burned out. Circuit breakers use an electromagnetic switch,
opening the circuit when there is too much current. Circuit breakers can be flipped back into the proper position and
the current will flow once more. Nothing needs to be replaced.
Another very important safety precaution is making sure that an electrical circuit is grounded. A circuit that is
grounded provides a path for current from the metallic casing of an electrical appliance to, literally, the ground (or
metal connected to the ground).
If you look at most electrical power cords, you’ll see three prongs. One prong is at an average potential of 120V
(the live wire), the other at 0V(the neutral wire). These two prongs provide for a potential difference of 120Vin
order for an appliance to draw current. The third prong (the ground wire) is not a part of the circuit. It is connected
to the metal casing of the appliance and to ground. Under normal conditions, the third prong carries no current.
An electrical short occurs when the intended path of the current bypasses the appliance and goes directly back to
the voltage source, or directly to ground. For example, if the wire connected to the 120Vprong loses insulation
and comes into contact with the casing of an appliance, anyone who touches the appliance provides a shortened path
for the current back to ground, unless the appliance is grounded by the third prong. If the appliance is properly
grounded, the short circuit will now be between the casing and the ground, and the person will be protected.- Electric current is defined as the rate at which charges flow within a conducting wire past any point in the wire
theI→I=∆∆Qt - An ampere(A)is defined as acoulombsecond.
- Georg Simon Ohm first proposed that a currentIwas directly proportional to a potential differenceVfor metal
conductors as long as the temperature of the wire did not increase substantially. Under the same conditions, the
current is inversely proportional to the resistance. Combining these results we have
V=IR, which is often referred to as Ohm’s Law
- Theresistivityis a measure of the resistance of a material. For wires, the resistivity is directly proportional
to the lengthLof the wire and inversely proportional to the cross-sectional areaAof the wire. The constant of
proportionalityρis known as the resistivity of the material.
R=ρLA - The conductivityσof a material is the reciprocal of the resistivity
σ=^1 ρ - The equivalent resistance of resistors in series is
Requivalent=R 1 +R 2 +...