Electric current
It consists of electrons in movement, usually inside a wire (but not always).
The electrons move because of the electric potential (or voltage). It is similar to objects moving down because of the gravitational potential.
A collection of electrons moving is an electric current.
The higher the voltage, the larger the current.
But the current also depends on another parameter: the resistance to movement. The higher the resistance, the more difficult for the electrons to move, the lower the current.
Although a wire is a medium where the electrons can move, it exerts a resistance. The resistance will depend on the temperature and also of the material which the wire is made. Copper, for instance, offers lower resistance than most metals. If the temperature gets lower, the resistance also goes down.
The relationship between voltage (U), resistance(R) and current (I) is easily determined using Ohm's law: U=RI.
It can be rearranged, so that it becomes: I = U / R
Voltage is measured in volts (V), current is measured in amperes (A) and resistance is measured in ohms(Ω).
EXAMPLES
Ohm's law:
-Calculate the voltage needed to create an electric current of 10A in a circuit where the resistance is 10 Ω
U=RI -> U=10*10 = 100 V
-Calculate the current in a toaster that has a heating
element of 10 Ω when connected to a 220 V mains supply.
I = U / R -> I= 220/10= 22 A
- A light bulb in a car works with a voltage of 12V. If the current on it is 1 A, what is its resistance?
U=RI -> R= U / I -> R = 12/1 = 12 Ω
P = U I :
-What is the power developed by a car light bulb that works with 12 V and a current of 1.2 A ?
P = UI = 12*1.2 = 14.4 W
-Calculate the power of a hair dryer that operates on
220 volts and draws a current of 5 amperes.
P = UI = 220*5 = 1100 W
-If an electric shower has a power of 2000 W, and it is plugged in a 220 V mains, what is the current in its wires?
P = UI -> I = P/U -> I = 2000/220 = 9.1 A
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