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In a circuit we can have four elements:
(i) source of emf,
 This is a source of emf and usually can be a battery.
(ii) resistor,
This is the symbol of resistor and the resistance is measured in ohm (Ω).
(iii) capacitor
 This is the symbol of a capacitor with capacitance C, which is measured in farads
(iv) inductor.
This is the symbol of an inductor with inductance L, measured in units of
henrys (H).
The above described circuit elements can be part of a circuit and in addition to these we can have voltmeter (V) and ammeter (A) in a circuit as well. Consider a circuit as follows:
The above circuit shows a linear relationship between voltage and resistance for an ohmic conductor
Now let us consider another circuit in which a battery is connected to a resister. We assume that the connecting wires have no resistance. The positive terminal of battery is at higher potential than the negative terminal. If the internal resistance of the battery is neglected then the potential difference across the battery is equal to its emf. Since, a real battery is always having an internal resistance r, the potential decreases by an amount Ir. If the emf of the battery is ε then the voltage across it is:
If the resistance of the resistor is R then 
Thus, the current across the circuit will be,
Thus, the electric current in this simple circuit depends upon both the load resistance R external to the battery and the internal resistance of the battery. In real world circuits R >> r and we can neglect r. This is another example of understanding and analyzing a circuit.
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Electromagnetism