# 21.2 Electromotive force: terminal voltage  (Page 7/12)

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Individual solar cells are connected electrically in modules to meet electrical-energy needs. They can be wired together in series or in parallel—connected like the batteries discussed earlier. A solar-cell array or module usually consists of between 36 and 72 cells, with a power output of 50 W to 140 W.

The output of the solar cells is direct current. For most uses in a home, AC is required, so a device called an inverter must be used to convert the DC to AC. Any extra output can then be passed on to the outside electrical grid for sale to the utility.

## Take-home experiment: virtual solar cells

One can assemble a “virtual” solar cell array by using playing cards, or business or index cards, to represent a solar cell. Combinations of these cards in series and/or parallel can model the required array output. Assume each card has an output of 0.5 V and a current (under bright light) of 2 A. Using your cards, how would you arrange them to produce an output of 6 A at 3 V (18 W)?

Suppose you were told that you needed only 18 W (but no required voltage). Would you need more cards to make this arrangement?

## Section summary

• All voltage sources have two fundamental parts—a source of electrical energy that has a characteristic electromotive force (emf), and an internal resistance $r$ .
• The emf is the potential difference of a source when no current is flowing.
• The numerical value of the emf depends on the source of potential difference.
• The internal resistance $r$ of a voltage source affects the output voltage when a current flows.
• The voltage output of a device is called its terminal voltage $V$ and is given by $V=\text{emf}-\text{Ir}$ , where $I$ is the electric current and is positive when flowing away from the positive terminal of the voltage source.
• When multiple voltage sources are in series, their internal resistances add and their emfs add algebraically.
• Solar cells can be wired in series or parallel to provide increased voltage or current, respectively.

## Conceptual questions

Is every emf a potential difference? Is every potential difference an emf? Explain.

Explain which battery is doing the charging and which is being charged in [link] .

Given a battery, an assortment of resistors, and a variety of voltage and current measuring devices, describe how you would determine the internal resistance of the battery.

Two different 12-V automobile batteries on a store shelf are rated at 600 and 850 “cold cranking amps.” Which has the smallest internal resistance?

What are the advantages and disadvantages of connecting batteries in series? In parallel?

Semitractor trucks use four large 12-V batteries. The starter system requires 24 V, while normal operation of the truck’s other electrical components utilizes 12 V. How could the four batteries be connected to produce 24 V? To produce 12 V? Why is 24 V better than 12 V for starting the truck’s engine (a very heavy load)?

## Problem exercises

Standard automobile batteries have six lead-acid cells in series, creating a total emf of 12.0 V. What is the emf of an individual lead-acid cell?

2.00 V

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Anderson
Sure
What is Boyce law
how to convert meter per second to kilometers per hour
Divide with 3.6
Mateo
multiply by (km/1000m) x (3600 s/h) -> 3.6
2 how heat loss is prevented in a vacuum flask
what is science
Helen
logical reasoning for a particular phenomenon.
Ajay
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due to non in contact mean no conduction and no convection bec of non conducting base and walls and also their is a grape between the layer like to take the example of thermo flask
Abdul
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what if time is not given in finding the average velocity?
the magnetic circuit of a certain of the flux paths in each of the long and short sides being 25cm and 20cm reprectielectrove. there is an air gap of 2mm long in one the long sides if a flux density of 0.8weber/m is to produce in the magnet of 1500 turns..
How do you calculate precision
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Fillemon
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Sacky
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There are very large numbers of charged particles in most objects. Why, then, don’t most objects exhibit static electricity?
Because there's an equal number of negative and positive charges... objects are neutral in nature
NELSON
when a ball rolls on a smooth level ground,the motion of its centre is?
what is electro magnetic field?
Mary
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NELSON
Electromagnetic field is caused by moving electric charge
when a ball rolls on a smooth level ground,the motion of its centre is?
Mumeh
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displacement is the change of position 8======✊=D 💦💦
Anderson
what is the meaning of elasticity
is the ability of a material to or any object to expand to a limit point
king
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Emi
what does emf/R mean
What is work
work is the product of force and perpendicular distance
DAVID