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Why are the chemicals able to produce a unique potential difference? Quantum mechanical descriptions of molecules, which take into account the types of atoms and numbers of electrons in them, are able to predict the energy states they can have and the energies of reactions between them.

In the case of a lead-acid battery, an energy of 2 eV is given to each electron sent to the anode. Voltage is defined as the electrical potential energy divided by charge: V = P E q size 12{V= { {P rSub { size 8{E} } } over {q} } } {} . An electron volt is the energy given to a single electron by a voltage of 1 V. So the voltage here is 2 V, since 2 eV is given to each electron. It is the energy produced in each molecular reaction that produces the voltage. A different reaction produces a different energy and, hence, a different voltage.

Terminal voltage

The voltage output of a device is measured across its terminals and, thus, is called its terminal voltage     V size 12{V} {} . Terminal voltage is given by

V = emf Ir , size 12{V="emf" - ital "Ir"} {}

where r size 12{r} {} is the internal resistance and I size 12{I} {} is the current flowing at the time of the measurement.

I size 12{I} {} is positive if current flows away from the positive terminal, as shown in [link] . You can see that the larger the current, the smaller the terminal voltage. And it is likewise true that the larger the internal resistance, the smaller the terminal voltage.

Suppose a load resistance R load size 12{R rSub { size 8{"load"} } } {} is connected to a voltage source, as in [link] . Since the resistances are in series, the total resistance in the circuit is R load + r size 12{R rSub { size 8{"load"} } +r} {} . Thus the current is given by Ohm’s law to be

I = emf R load + r . size 12{I= { {"emf"} over {R rSub { size 8{"load"} } +r} } } {}
This schematic drawing of an electrical circuit shows an e m f, labeled as script E, driving a current through a resistive load R sub load and through the internal resistance r of the voltage source. The current is shown flowing in a clockwise direction from the positive end of the source.
Schematic of a voltage source and its load R load size 12{R rSub { size 8{"load"} } } {} . Since the internal resistance r size 12{r} {} is in series with the load, it can significantly affect the terminal voltage and current delivered to the load. (Note that the script E stands for emf.)

We see from this expression that the smaller the internal resistance r size 12{r} {} , the greater the current the voltage source supplies to its load R load size 12{R rSub { size 8{"load"} } } {} . As batteries are depleted, r size 12{r} {} increases. If r size 12{r} {} becomes a significant fraction of the load resistance, then the current is significantly reduced, as the following example illustrates.

Calculating terminal voltage, power dissipation, current, and resistance: terminal voltage and load

A certain battery has a 12.0-V emf and an internal resistance of 0 . 100 Ω size 12{0 "." "100" %OMEGA } {} . (a) Calculate its terminal voltage when connected to a 10.0- Ω size 12{"10" "." 0- %OMEGA } {} load. (b) What is the terminal voltage when connected to a 0 . 500- Ω size 12{0 "." "500-" %OMEGA } {} load? (c) What power does the 0 . 500- Ω size 12{0 "." "500-" %OMEGA } {} load dissipate? (d) If the internal resistance grows to 0 . 500 Ω size 12{0 "." "500 " %OMEGA } {} , find the current, terminal voltage, and power dissipated by a 0 . 500- Ω size 12{0 "." "500-" %OMEGA } {} load.

Strategy

The analysis above gave an expression for current when internal resistance is taken into account. Once the current is found, the terminal voltage can be calculated using the equation V = emf Ir size 12{V="emf" - ital "Ir"} {} . Once current is found, the power dissipated by a resistor can also be found.

Solution for (a)

Entering the given values for the emf, load resistance, and internal resistance into the expression above yields

I = emf R load + r = 12 . 0 V 10 . 1 Ω = 1 . 188 A . size 12{I= { {"emf"} over {R rSub { size 8{"load"} } +r} } = { {"12" "." 0" V"} over {"10" "." "1 " %OMEGA } } =1 "." "188"" A"} {}

Enter the known values into the equation V = emf Ir size 12{V="emf" - ital "Ir"} {} to get the terminal voltage:

Questions & Answers

what is thermodynamics
wana Reply
thermodynamic is a branch of physics that teaches on the relationship about heat and anyother form of energy
Emmanuel
if l cary box and stop is ther any work
Tamirat Reply
no that because u have moved no distance. for work to be performed a force needs to be applied and a distance needs to be moved
Emmanuel
Different between fundamental unit and derived unit
Alimi Reply
fundamental unit are independent quantities that do not depend on any other unit while derived unit are quantities that depend on two or more units for it definition
Emmanuel
what is nuclear fission
Sadik Reply
hello
Shawty
are you there
Shawty
miss your absence here...
Shawty
what is a vector
Temitayo Reply
vectors are quantities that have numerical value or magnitude and direction.
Muhammad
what is regelation
Oladele
vector is any quantity that has magnitude and direction
Emmanuel
Physics is a physical science that deals with the study of matter in relation to energy
Divine Reply
Hi
Jimoh
hello
Salaudeen
hello
Sadik
Yes
Maxamuud
hi everyone
Muhammad
what is physics
Rhema Reply
physics is a physical science that deals with the study of matter in relation to energy
Osayuwa
a15kg powerexerted by the foresafter 3second
Firdos Reply
what is displacement
Xolani Reply
movement in a direction
Jason
hello
Hosea
Hey
Smart
haider
Explain why magnetic damping might not be effective on an object made of several thin conducting layers separated by insulation? can someone please explain this i need it for my final exam
anas Reply
Hi
saeid
hi
Yimam
Hi
Jimoh
An object made of several thin conducting layers separated by insulation may not be affected by magnetic damping because the eddy current produced in each layer due to induction will be very small and the opposing magnetic flux produced by the eddy currents will be very small
Muhammad
What is thê principle behind movement of thê taps control
Oluwakayode Reply
while
Hosea
what is atomic mass
thomas Reply
this is the mass of an atom of an element in ratio with the mass of carbon-atom
Chukwuka
show me how to get the accuracies of the values of the resistors for the two circuits i.e for series and parallel sides
Jesuovie Reply
Explain why it is difficult to have an ideal machine in real life situations.
Isaac Reply
tell me
Promise
what's the s . i unit for couple?
Promise
its s.i unit is Nm
Covenant
Force×perpendicular distance N×m=Nm
Oluwakayode
İt iş diffucult to have idêal machine because of FRİCTİON definitely reduce thê efficiency
Oluwakayode
It is difficult to have an ideal machine in real life situation because in ideal machines all the input energy should be converted to output energy . But , some part of energy is always lost in overcoming friction and input energy is always greater than output energy . Hence , no machine is ideal.
Muhammad
if the classica theory of specific heat is valid,what would be the thermal energy of one kmol of copper at the debye temperature (for copper is 340k)
Zaharadeen Reply
Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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