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Here, I = emf r tot + R load size 12{I= { {"emf"} over { left (r rSub { size 8{"tot"} } +R rSub { size 8{"load"} } right )} } } {} flows through the load, and r tot size 12{r rSub { size 8{"tot"} } } {} is less than those of the individual batteries. For example, some diesel-powered cars use two 12-V batteries in parallel; they produce a total emf of 12 V but can deliver the larger current needed to start a diesel engine.

Part a shows parallel combination of two cells of e m f script E and internal resistance r sub one and internal resistance r sub two connected to a load resistor R sub load. Part b shows the combination of e m f of part a. The circuit has a cell of e m f script E with an internal resistance r sub tot and a load resistor R sub load. The resistance r sub tot is less than either r sub one or r sub two.
Two voltage sources with identical emfs (each labeled by script E) connected in parallel produce the same emf but have a smaller total internal resistance than the individual sources. Parallel combinations are often used to deliver more current. Here I = emf r tot + R load size 12{I= { {"emf"} over { left (r rSub { size 8{"tot"} } +R rSub { size 8{"load"} } right )} } } {} flows through the load.

Animals as electrical detectors

A number of animals both produce and detect electrical signals. Fish, sharks, platypuses, and echidnas (spiny anteaters) all detect electric fields generated by nerve activity in prey. Electric eels produce their own emf through biological cells (electric organs) called electroplaques, which are arranged in both series and parallel as a set of batteries.

Electroplaques are flat, disk-like cells; those of the electric eel have a voltage of 0.15 V across each one. These cells are usually located toward the head or tail of the animal, although in the case of the electric eel, they are found along the entire body. The electroplaques in the South American eel are arranged in 140 rows, with each row stretching horizontally along the body and containing 5,000 electroplaques. This can yield an emf of approximately 600 V, and a current of 1 A—deadly.

The mechanism for detection of external electric fields is similar to that for producing nerve signals in the cell through depolarization and repolarization—the movement of ions across the cell membrane. Within the fish, weak electric fields in the water produce a current in a gel-filled canal that runs from the skin to sensing cells, producing a nerve signal. The Australian platypus, one of the very few mammals that lay eggs, can detect fields of 30 mV m size 12{ { {"mV"} over {m} } } {} , while sharks have been found to be able to sense a field in their snouts as small as 100 mV m size 12{ { {"mV"} over {m} } } {} ( [link] ). Electric eels use their own electric fields produced by the electroplaques to stun their prey or enemies.

A photograph of a large gray tiger shark that swims along the bottom of a saltwater tank full of smaller fish at the Minnesota Zoo.
Sand tiger sharks ( Carcharias taurus ), like this one at the Minnesota Zoo, use electroreceptors in their snouts to locate prey. (credit: Jim Winstead, Flickr)

Solar cell arrays

Another example dealing with multiple voltage sources is that of combinations of solar cells—wired in both series and parallel combinations to yield a desired voltage and current. Photovoltaic generation (PV), the conversion of sunlight directly into electricity, is based upon the photoelectric effect, in which photons hitting the surface of a solar cell create an electric current in the cell.

Most solar cells are made from pure silicon—either as single-crystal silicon, or as a thin film of silicon deposited upon a glass or metal backing. Most single cells have a voltage output of about 0.5 V, while the current output is a function of the amount of sunlight upon the cell (the incident solar radiation—the insolation). Under bright noon sunlight, a current of about 100 mA/cm 2 size 12{"100"" mA/cm" rSup { size 8{2} } } {} of cell surface area is produced by typical single-crystal cells.

Questions & Answers

Who can help me with dynamics?
ivan Reply
radiation amuses mee....
pau Reply
can someone enumerate the First and second law of thermodynamics
oladele Reply
Good
Abdul Reply
radiation of phones kept amazing me
Okugbesan Reply
f=m(v-u)/t
Adeleke Reply
I understand light is a range of wavelenghts from em spectrum, but Where do photons come from in particular, how it is emitted from the sun?
Ian Reply
F=ma
pierre Reply
please what is the formula for calculating Newton second law of motion?
Ogodo Reply
what is emotion
Lilian Reply
properties of transverse waves
Abiodun Reply
is visible light electromagnetic wave?
akash Reply
Visible light is a range of wavelengths within the electro magnetic spectrum.
Robert
It is electro magnetic radiation from the sun.
Robert
please what is the formula for coefficient of kinetic friction
Seyi Reply
What is work
Sunbomustaphar Reply
1315.711980073m
babar Reply
what is thermo electric thermometer
Undie 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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