Theory:

Battery:
We can call cells as ‘batteries' only when two or more cells are combined. A cell is a single unit that converts chemical energy into electrical energy, whereas a battery is a collection of cells.
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Combination of cells
  
A simple electric circuit
An electric circuit is the continuous closed path along which current flows from the positive terminal to the negative terminal of the battery.
Construction of a circuit:
  
A cell, connecting wires, a bulb, and a switch make up an electric circuit.
 
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Simple circuit
  
A cell or a battery is the main source of electric current.
 
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Open and closed circuit
 
A simple electric circuit can be formed by connecting the two terminals of the electric cell to the bulb. This creates a complete path for the current flow between the electric cell's two terminals.
An electric switch or a key is connected to the circuit to stop or allow the current flow.
If a key is in open condition (switched off), the current will not flow across the circuit. Thus, it is an open circuit. The bulb does not glow in this type.
If the key is in closed (switched on) condition, the current will flow, making it a closed circuit. The bulb glows in this type.
 
A simple electric circuit can be formed by connecting the two terminals of the electric cell to the bulb. This creates a complete path for the current to flow between the two terminals of electric cell.
 
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Flow of current
 
The current always flows from the positive terminal of the battery to the negative terminal.