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ELECTROMOTIVE FORCE (emf) & INTERNAL RESISTANCE

Electromotive Force (emf)

What is emf?

It is defined as the energy provided by the source (battery/cell) to each unit charge that flows in a circuit. It means that, if we connect a battery in circuit, the battery will give energy to the charge, enable it to flow.

emf = Energy (U)/ charge (q)

We can see here that emf has the same unit as that of potential difference that is VOLT. So, please bear in our mind, electromotive force (emf) is not actually a force, it is a potential difference of unit of VOLT.


 Internal Resistance (r)

 Consider this ⇩ case to comprehend more:

We start with Diagram 2,

Diagram 2

A battery of 1.5 V is connected to a circuit which consist of on external resistor, R. At the same time, we connect a voltmeter and the voltmeter reading shows the reading of 1.3 V !!! That is less than the emf value.

The concept here is that the battery gives 1.5 joules of electrical energy to each coulomb (charge) that passes through it. But the energy that is transferred in the external resistor R is only 1.3 joules per coulomb. The circuit seems to be loosing energy!!But to WHERE?

Some of the energy is 'lost' as the charges moves against a resistance of the battery itself.

See!!, the battery itself has some resistance, and it is called internal resistance, r.

Knowing that , the internal resistance can not be seen with our naked eye. Meaning, if we open (by peeling) the battery body, the internal resistor in not there physically. The truth is, it exists because of the chemical content of the battery. However, we usually draw the battery like this ⇩, as if the internal resistor is there.

The equation that fit the concept is:

emf = V+ Ir

The whole energy carried per unit charge (voltage) is dissipated in internal resistance r (Ir) and external resistor R (V=IR). Let say a battery supplies 1.5 V to the charge to flow in the circuit, after the passing r and R, the charge has NO MORE ENERGY!!


Diagram 1

A 1.5 V battery is connected to a voltmeter and the voltmeter reading shows the reading of 1.5 V. It's THE SAME with the emf of the battery.

What happens here is, the voltmeter only measure the energy per unit charge (emf) of the battery. The current is not flowing. The current is also not flowing through the internal resistor, r . Hence, the is no voltage drop (energy lost) in the internal resistor.

Thank you,
Mdm Ummi Atiah KMPk
~ May it Benefits You ➽💕

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