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I have a doubt that is: What does this statement mean: "a 6 V battery". Does this mean that the EMF of the battery is 6 V or the terminal voltage of the battery is 6 V?

If the battery has internal resistance then what will be considered emf or terminal voltage if the same statement is given ??

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EMF or Terminal voltage will be considered same if battery has no internal resistance. If battery has some internal resistance then terminal voltage will be different(less) from the EMF or potential difference from the battery.

If a battery has internal resistance then what will be considered if the same statement is given.

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In your questions all the cases are assumed to be ideal unless mentioned. Therefore the electromotive force and the terminal voltage are equal in that case as internal resistance of the battery is considered negligible(if not given) .

If the battery has internal resistance then the emf remains constant,but the terminal voltage decrease by a value which is equal to the potential drop across the internal resistorr

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  • $\begingroup$ If you placed an ideal voltmeter across a battery, namely one that draws no current, then the voltmeter would give the emf of the battery since no current is drawn and no pd drop across the internal resistance. In practice real voltmeters draw (ideally very small) current to work and so won't measure the emf of the battery. $\endgroup$ – jim May 8 '16 at 10:00
  • $\begingroup$ @jim the potential drop across the internal resistance itself depends on the rest of the circuits resistance and net current flowing. My statement is correct $\endgroup$ – Shubham May 8 '16 at 10:01
  • $\begingroup$ not trying to take away from your answer, just adding a little bit more. $\endgroup$ – jim May 8 '16 at 10:03
  • $\begingroup$ I dont think your first comment is correct. The ideal voltmeter in your case will show the terminal potential not emf. Voltmeter is actually a modified galvanometer. You have to have some current pass through it. For calculation's purpose the current through voltmeter is taken0@jim $\endgroup$ – Shubham May 8 '16 at 10:07
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The 6V in 6V battery is a label which gives an indication of the sort of voltage which might be obtained from such a battery.
For example if your ^ V battery was an lead-acid battery and it was fairly new and fully charged its voltage would be 6.3 volt and if older or partially discharges then it is more likely to be 6 V.

A 1.5 V alkaline battery at the start of its life would give 1.55 V. and a 1.2 V NiMH battery freshly charged would give 1.4 V but very soon this would drop to 1.2 V and then stay there for some time.

You will note that I have only used the term voltage without any qualification. The problem with real batteries is that even if they are of the same type and made by the same manufacturer they are not all they same.
If you measure the terminal potential difference with no current drain from a series of batteries of the same type you will not get the same value for all of them. Their age, how they have been used, their state of charge, the number of times they have been recharged, the temperature etc will affect the value.
Then if you start drawing current from a battery its characteristics will change and very much so as the current increases. For a start the terminal potential difference will drop due to a catch all parameter called internal resistance which for a battery is not really a constant.

Battery manufacturers produce copious data sheets which show extremely well how the electrical parameters of a battery change. They also give good advice as to what sort of battery to use in a particular application.

To illustrate the variability of the terminal voltage of a battery here are a couple of graphs for the Energizer NH15-2300 1.2 V NiMH rechargeable battery.

enter image description here

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