Electric Current and Circuits – Learn


An electric current is the movement of an electric charge in a wire. Electric current is also described as the flow of electrons. Electrons move easily through conductors. Electrical energy can be converted into many other forms, such as heat, light and sound energy. This makes electricity a very useful form of energy.

Current will flow in a circuit only when the circuit forms a continuous loop from one terminal of a power supply to the other terminal. Different electrical components such as a light bulb or switch can be added.

Circuit diagrams are used to clearly show how the components of an electric circuit are connected. The diagram below is a cutaway diagram of a torch. The other diagram is a much simpler circuit diagram of the torch showing how a battery, light bulb and switch are physically connected by a conductive material in a torch.

Standard symbols are used for the components as this makes circuit diagrams easier to interpret and understand. Below are common symbols used in circuit diagrams:


When an electric current flows, electrons all around the circuit move towards the positive terminal at the same time. This is called electron flow. Conventionally, current in a circuit is said to flow from the positive terminal to the negative terminal.

Current (I) is defined as the amount of charge, q, that passes through a conducting wire per second. It has the unit amperes or amps (A), which are equivalent to coulombs per second. If the number of electrons (ne) that flow through a particular point in the circuit is known, then this can be multiplied by the charge of one electron (qe = −1.6 × 10-19 C) and divided by the time to find the current.

The following equations are used when dealing with charge and current:

I=\cfrac { q }{ t } =\cfrac { { n }_{ e }{ q }_{ e } }{ t }

Where:

I = current (in amps)

t = time (in sec)

{ n }_{ e } = number of electrons

{ q }_{ e } = charge on an electron (in C) (qe = −1.6 × 10-19 C)


Example 1:

What is the current in a wire if 0.5 C pass a point in the circuit every 0.1 seconds?

Answer:

Using: I=\cfrac { q }{ t }

I=\cfrac { 0.5 }{ 0.1 }

I=5 amps


Example 2:

What is the current in a wire if 4.6875 × 1019 electrons pass a point in the circuit every 5 seconds?

Answer:

I=\cfrac { { n }_{ e }{ q }_{ e } }{ t }

 I=\cfrac { { (4.6875\times 10 }^{ 19 })({ 1.6\times 10 }^{ -19 }) }{ 5 }

I=1.5 amps


Example 3:

How many electrons flow pass a point each second if the current is 0.6A?

Answer:

I=\cfrac { { n }_{ e }{ q }_{ e } }{ t }

 0.6=\cfrac { { n }_{ e }({ 1.6\times 10 }^{ -19 }) }{ 1 }

 { n }_{ e }={ 3.75\times 10 }^{ 18 } amps

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