Voltage is related to potential-energy difference. The voltage drop across any circuit element is directly proportional to the change in energy of a charge as it traverses the circuit element. Specifically, 1 volt = 1 joule/coulomb. The potential energy (with respect to some reference point) is equal to the voltage multiplied by the charge.
Current refers to the motion of charges. The current through a given surface (e.g. the cross-section of a wire) is defined as the net charge passing through that surface per unit time. The unit for current is the ampere: 1 ampere = 1 coulomb/second.
The product of voltage and current has units of joules/second, otherwise known as watts.
If the voltage drop across a circuit element equals the change in potential energy per unit charge, and the current equals the amount of charge moving through the element per unit time, then their product equals the power released within the device!
The power dissipated within any device is given by
P = IV (2.11)
For resistive elements (or when an effective resistance can be defined), Eq. 2.11 can be combined with Ohm’s law to give:
P = IV = I 2 R = V 2 /R (2.12)
Resistors, diodes, transistors, relays, integrated-circuit chips, etc., are rated (in part) by their maximum allowed power. Exceeding these ratings can have disastrous effects on your circuit, and may even cause a fire! To illustrate this point, our first exercise will deliberately lead to the destruction of a carbon-film resistor.
0 comments:
Post a Comment