Thus far, we have discussed static charges. Static charges alone are useful, but not nearly as much as charges in motion. As you recall, electrons are most easily moved. However, for sake of ease in sign convention (keeping things positive, where positive), we define the following:
current (I) - the rate at which positive charge "flows"
The unit is the coulomb per second, defined as an ampere (A). One ampere (or amp) is a tremendous amount of current - more than enough to kill a person. In fact, you can feel as little as 0.01 A. Typical currents in a circuit are on the order of mA (milliamperes).
We also define other new quantities in electricity: voltage, resistance, power
voltage (V) - the amount of available energy per coulomb of charge
resistance (R) - the amount by which the voltage is "dropped" per ampere of current
You can also think of resistance of that which "resists" current. Typically, resistors are made of things that are semi-conductors (they conduct current, but less well than conductors, and better than insulators). Resistors are often made of carbon, but can also be made of silicon and other materials. The unit is a volt per ampere, defined as an ohm (Greek symbol, omega).
A convenient way to relate all the variables is embodied by Ohm's Law:
As in, "Twinkle, twinkle little star, V is equal to I R."
Well, it works for me
current (I) - the rate at which positive charge "flows"
I = Q/t
The unit is the coulomb per second, defined as an ampere (A). One ampere (or amp) is a tremendous amount of current - more than enough to kill a person. In fact, you can feel as little as 0.01 A. Typical currents in a circuit are on the order of mA (milliamperes).
We also define other new quantities in electricity: voltage, resistance, power
voltage (V) - the amount of available energy per coulomb of charge
V = E/Q
resistance (R) - the amount by which the voltage is "dropped" per ampere of current
R = V/I
You can also think of resistance of that which "resists" current. Typically, resistors are made of things that are semi-conductors (they conduct current, but less well than conductors, and better than insulators). Resistors are often made of carbon, but can also be made of silicon and other materials. The unit is a volt per ampere, defined as an ohm (Greek symbol, omega).
A convenient way to relate all the variables is embodied by Ohm's Law:
V = I R
As in, "Twinkle, twinkle little star, V is equal to I R."
Well, it works for me
What exactly *IS* a circuit?
An electrical circuit can be thought of as a complete "loop" through which charge can travel. Therefore, it actually has to be physically complete - there can be no openings. That is, the current actually has to have a full path to take.
But there is an exception:
If the supplied voltage is high enough, charge can "jump" an "open circuit." This is clearly a dangerous situation, and one way in which a person can get shocked. Think of the unfortunate situation of sticking your finger (or a paper clip, etc.) into an electrical outlet (or something like a toaster, for that matter). You would "bridge" the circuit, becoming in effect, a resistor.
That's bad.
But there is an exception:
If the supplied voltage is high enough, charge can "jump" an "open circuit." This is clearly a dangerous situation, and one way in which a person can get shocked. Think of the unfortunate situation of sticking your finger (or a paper clip, etc.) into an electrical outlet (or something like a toaster, for that matter). You would "bridge" the circuit, becoming in effect, a resistor.
That's bad.
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