# 21.2 Electromotive force: terminal voltage  (Page 6/12)

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Here, $I=\frac{\text{emf}}{\left({r}_{\text{tot}}\phantom{\rule{0.25em}{0ex}}+\phantom{\rule{0.25em}{0ex}}{R}_{\text{load}}\right)}$ flows through the load, and ${r}_{\text{tot}}$ 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.

## 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 $\frac{\text{mV}}{m}$ , while sharks have been found to be able to sense a field in their snouts as small as 100 $\frac{\text{mV}}{m}$ ( [link] ). Electric eels use their own electric fields produced by the electroplaques to stun their prey or enemies.

## 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 $\text{100}\phantom{\rule{0.25em}{0ex}}{\text{mA/cm}}^{2}$ of cell surface area is produced by typical single-crystal cells.

Calculate the work done by an 85.0-kg man who pushes a crate 4.00 m up along a ramp that makes an angle of 20.0º20.0º with the horizontal. (See [link] .) He exerts a force of 500 N on the crate parallel to the ramp and moves at a constant speed. Be certain to include the work he does on the crate an
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the information provided is not enough
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Hmmmm maybe the question is logical
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where are the parameters for calculation
HENRY
there is enough information to calculate an AVERAGE acceleration
Kwok
mistake, there is enough information to calculate an average velocity
Kwok
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Abel
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what are the types of radioactivity
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what is static friction
It is the opposite of kinetic friction
Mark
static fiction is friction between two surfaces in contact an none of sliding over on another, while Kinetic friction is friction between sliding surfaces in contact.
MINDERIUM
I don't get it,if it's static then there will be no friction.
author
It means that static friction is that friction that most be overcome before a body can move
kingsley
static friction is a force that keeps an object from moving, and it's the opposite of kinetic friction.
author
It is a force a body must overcome in order for the body to move.
Eboh
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Eboh
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more technically it is the product of mass of an object and Acceleration produced in it
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wave is disturbance in any medium
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on both a string and peldulum?
Anya
spring*
Anya
Yea
yusuf
Do you have a chit-chat contact
yusuf
I dont have social media but i do have an email?
Anya
Which is
yusuf
Where are you chatting from
yusuf
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Jimmy
teach him SHM init
Anya
Simple harmonic motion
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how many subject is in physics
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effiom
how many topic are in physics
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Yeah basics of physics prin8
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Heat nd Co for a level
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yh I need someone to explain something im tryna solve . I'll send the question if u down for it
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hallow
Boniface
Boniface
the range of objects and phenomena studied in physics is
Boniface