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Electric current

Examples and exercises using U=R I and P=U I

It consists of electric charges in movement. Usually electrons inside a wire (but not always).

Electric charges move because of a electric potential (or voltage). It is similar to objects moving down because of the gravitational potential.

A collection of electric charges moving is an electric current.

The higher the voltage, the larger the current. The intensity of the current also depends on 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 against this movement. 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, and vice-versa.

Also, the more electrons in the wire, the higher the current.

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).

Some typical values of electric currents:

Human nervous system 20 µA
Train motor 600 A
Lightning 10 to 100 kA

One ampere (A) equals one coulomb per second (C/s).


Using 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

Electric current and power

An Electric current can transport energy, and as a result, power.

Electrical power is the amount of electrical energy transferred per second.

The power of an electric system depends on the voltage and also on the current. If a system has high voltage and high current (example:lightning), the power developed will be high.

The relationship between electric power (measured in watts), voltage and current is very simple:




-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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difference between alternate and direct current What is voltage?