Theory:

A simple electric circuit
An electric circuit is a continuous closed path along which current flows from the positive terminal to the negative terminal of the battery.
Construction of a circuit
 
An electric circuit consists of a cell, connecting wires, a bulb, and a switch.

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A cell or a battery is the main source of electric current.
A cell is a single unit, whereas a battery is a collection of cells.
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The connecting wires are used for carrying current across the circuit, and a bulb (or a lamp) is a small device that consumes the electricity.
An electric switch or a key is connected to the circuit to stop or allow the flow of current.
If a key is in open condition (switched off state), the current will not flow across the circuit; thus, it is an open circuit. A bulb does not glow in this type.
If the key is closed (switched on), then the current will flow, making it a closed circuit. A bulb glows in this type.
The two terminals of the electric cell connected to the bulb can form a simple electric circuit. This provides a complete path for current to flow between the two terminals of the electric cell.
 
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The direction of the current is always from the positive terminal to the negative terminal of the battery.
 
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In this arrangement, one end of a wire is connected to the base of a bulb, and the other end is given to the negative terminal of a cell. When the tip of the base of the bulb is in contact with the positive terminal of the cell, the bulb glows.
 
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When the bulb is taken away from the terminal of the electric cell, the bulb does not glow. A bulb becomes fused if there is a break in the filament. Due to the break in the path of the current, the fused bulb does not glow.