Bredhurst Receiving and Transmitting Society


Syllabus Sections:-

3. Technical Aspects

Potential Difference and Electromotive Force.

3a.1 Understand the difference between potential difference (p.d.) and electromotive force (e.m.f.).

Potential Difference and Electromotive Force

Potential Difference and Electromotive Force is a complex idea to grasp so let us first introduce you to a battery.

Up to now in both the Foundation and Intermediate licence courses you have only considered a battery, in its simplest form, as a source of electrons. The battery as a source of electrons does not change but the concept of the battery does.

Take a look at the diagram above. There is the usual circuit diagram for a battery but attached to is is a resistor and then the terminals have been shown out side these two circuit diagrams items. A battery is therefore in reality a combination of the source of electrons and a resistor.

Note on the battery diagram above the letter E. This stands for electro motive force. No don't panic at this new concept just follow it through.

In the Intermediate licence course you were told that you can measure the potential difference of a battery by connecting a voltmeter across its terminals. This measured potential difference is the same as the emf of the battery BUT only when it is not connected to a circuit.

So what is the significance of all of this.

Note carefully the words above "only when it is not connected to a circuit" as when the battery is connected to a circuit the potential difference measured by the voltmeter will be lower than the emf because of the internal resistance of the battery.

Understand the concept of source resistance (impedance) and voltage drop due to current flow.

So now you can see why we have had to introduce you to this new concept and some of the energy used in the circuit will be used in this internal resistance (source resistance or impedance) -the resistance within the battery itself.

So looking at the circuit above you will observe that we have a single resistor connected across a battery.

"r" is shown as the internal resistance of the battery with emf E, connected to a circuit with resistance R. The voltmeter whilst it is connected to the battery terminals is measuring the potential difference across R and not the emf of the battery which is equivalent to the potential difference across (R + r).

So there will be a voltage drop on the voltmeter when in circuit due to the current flor through the internal resistance.

So to sum up :-

  1. the electromotive force (EMF) is therefore the maximum potential difference between the terminals of a power source that is a battery.

  2. that there will be a current drop due to the current flow through the circuit and through the battery

  3. the internal resistance is also know as the source resistance (or source impedance)

Still mystified well look at this another way :-

A car battery is a good source of emf. In an ideal source of emf it will maintain a constant potential difference between its terminals, independent of the current, I, through it or the resistance, R, across it.

The formula for an ideal source of emf is V= I x R, which is known as Ohm's Law which you were introduced to in the Foundation licence as a magic triangle.

The potential difference across a real source in a circuit, however, is not equal to the ideal emf. The reason is that the flow of electrons passing through the materials of the battery encounters an internal resistance r and experiences a drop in potential difference equal to I x r .

Thus the equation for a source with internal resistance is V = (emf - I x r) and this potential is called the terminal voltage.

Therefore the voltage in a car battery can be less than 12 V (depending on its state of charge or internal resistance) while it is producing a current.


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