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A primary cell is any kind of battery in which the electrochemicalreaction is not reversible. A common example of a primary cell is the disposable battery. Unlike a secondary cell, the reaction cannot be reversed by running a current into the cell; the chemical reactants cannot be restored to their initial position and capacity. Primary batteries use up the materials in one or both of their electrodes.
Comparison with rechargeables
Rechargeable batteries are economical to use when their initially higher cost and cost of a charging system can be spread out over many use cycles; for example, in hand-held power tools, it would be very costly to replace a high-capacity primary battery pack every few hours of use.
Primary batteries are useful where long periods of storage are required; a primary battery can be constructed to have a lower self-discharge rate than a rechargeable battery, so all its capacity is available for useful purposes. Applications that require a small current for a long time, for example a smoke detector, also use primary batteries since the self-discharge current of a rechargeable battery would exceed the load current and limit service time to a few days or weeks. For example, a flashlight used for emergencies must work when needed, even if it has sat on a shelf for an extended period of time. Primary cells are also more cost-efficient in this case, as rechargeable batteries would use only a small fraction of available recharge cycles. Reserve batteries achieve very long storage time (on the order of 10 years or more) without loss of capacity, by physically separating the components of the battery and only assembling them at the time of use. Such constructions are expensive but are found in applications like munitions, which may be stored for years before use.
A primary cell becomes polarized when in use. This means that hydrogen accumulates at the cathode and reduces the effectiveness of the cell. To remove the hydrogen, a depolarizer is used and this may be mechanical, chemical or electrochemical.
Attempts have been made to make simple cells self-depolarizing by roughening the surface of the copper plate to facilitate the detachment of hydrogen bubbles. These attempts have had little success. Chemical depolarization utilizes an oxidizing agent, such as manganese dioxide (e.g. LeclanchÃ© cell and Zinc-carbon cell) or nitric acid (e.g. Bunsen cell and Grove cell), to oxidize the hydrogen to water. Electrochemical depolarization exchanges the hydrogen for a metal, such as copper (e.g. Daniell cell), or silver (e.g. Silver-oxide cell).
Anode and cathode
The plate which carries the positive terminal (usually carbon) is termed the cathode and the plate which carries the negative terminal (usually zinc) is termed the anode. This is the reverse of the terminology used in an electrolytic cell. The reason is that the terms are related to the passage of electric current through the electrolyte, not the external circuit
Inside the cell the anode is the electrode where chemical oxidation occurs, as it accepts electrons from the electrolyte. The cathode is defined as the electrode where chemical reduction occurs, as it donates electrons to the electrolyte.
Outside the cell, different terminology is used. Since the anode accepts electrons from the electrolyte, it becomes negatively charged and is therefore connected to the terminal marked "−" on the outside of the cell. The cathode, meanwhile, donates electrons to the electrolyte, so it becomes positively charged and is therefore connected to the terminal marked "+" on the outside of the cell.
Old textbooks sometimes contain different terminology that can cause confusion to modern readers. For example, a 1911 textbook by Ayrton and Mather describes the electrodes as the "positive plate" and "negative plate" in a way that contradicts modern usage.
In scholarship, a secondary source is a document or recording that relates or discusses information originally presented elsewhere. A secondary source contrasts with a primary source, which is an original source of the information being discussed; a primary source can be a person with direct knowledge of a situation, or a document created by such a person. Secondary sources involve generalization, analysis, synthesis, interpretation, or evaluation of the original information. Primary and secondary are relative terms, and some sources may be classified as primary or secondary, depending on how it is used. An even higher level, the tertiary source, resembles a secondary source in that it contains analysis, but attempts to provide a broad overview of a topic that is accessible to newcomers.
Many sources can be considered either primary or secondary, depending on the context in which they are used. Moreover, the distinction between primary and secondary sources is subjective and contextual, so that precise definitions are difficult to make. For example, if a historical text discusses old documents to derive a new historical conclusion, it is considered to be a primary source for the new conclusion, but a secondary source of information found in the old documents. Other examples in which a source can be both primary and secondary include an obituary or a survey of several volumes of a journal counting the frequency of articles on a certain topic.
Whether a source is regarded as primary or secondary in a given context may change, depending upon the present state of knowledge within the field. For example, if a document refers to the contents of a previous but undiscovered letter, that document may be considered "primary", since it is the closest known thing to an original source, but if the letter is later found, it may then be considered "secondary".
Attempts to map or model scientific and scholarly communication need the concepts of primary, secondary and further "levels". One such model is the UNISIST model of information dissemination. Within such a model these concepts are defined in relation to each other, and the acceptance of this way of defining the concepts are connected to the acceptance of the model.
Other languages, like German, call the secondary sources SekundÃ¤rliteratur, leaving SekundÃ¤rquelle to historiography. A SekundÃ¤rquelle is a source that can tell about a (lost) PrimÃ¤rquelle, e.g. a letter is quoting from minutes that no longer exist and can not be consulted by the historian.
In science and medicine
In the sciences, a review article, book review, or meta-analysis are both examples of a secondary source. Some academic journals only publish reviews. Unlike in the humanities, scientific and medical peer reviewed sources are not generally considered secondary unless they are a review or a meta-analysis. Some scientific document search engines, such as PubMed, allow users to limit their searches to reviews and meta-analyses. Secondary sources can help trace the history of scientific and mathematical ideas, including who is credited as the original source of the idea.
A survey of previous work in the field in a primary peer-reviewed source is secondary. This vastly increases the amount of secondary source information available when there are few reviews in a field.
Library and information science
In library and information sciences, secondary sources are generally regarded as those sources that summarize or add commentary to primary sources in the context of the particular information or idea under study.
An important use of secondary sources in the field of mathematics has been to make difficult mathematical ideas and proofs from primary sources more accessible to the public; in other sciences tertiary sources are expected to fulfill the introductory role.
In humanities and history
Primary sources are those closest to an event, such as diaries and first-hand newspaper and magazine accounts. In the humanities, unlike the sciences and medicine, secondary sources in history and humanities are usually newspaper, magazine, academic journal, or other written accounts from the perspective of a different person than the person who experienced the event. In the humanities, a peer reviewed article is always a secondary source, but isn't necessarily secondary in the sciences.
The delineation of sources as primary and secondary first arose in the field of historiography, as historians attempted to identify and classify the sources of historical writing. In scholarly writing, an important objective of classifying sources is to determine the independence and reliability of sources. In contexts such as historical writing, it is almost always advisable to use primary sources if possible, and that "if none are available, it is only with great caution that [the author] may proceed to make use of secondary sources." Many scholars have commented on the difficulty in producing secondary source narratives from the "raw data" which makes up the past. Historian/philosopher Hayden White has written extensively on the ways in which the rhetorical strategies by which historians construct narratives about the past, and what sorts of assumptions about time, history, and events are embedded in the very structure of the historical narrative. In any case, the question of the exact relation between "historical facts" and the content of "written history" has been a topic of discussion among historians since at least the nineteenth century, when much of t
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Answers:primary cell can be used once i.e the reaction in a primary cell is irreversible . example-dry cell. secondary cell is that in which the cell can be used again by recharging i.e the the reaction in a secondary cell is reversible. example-lead-acid battery.
Answers:Primary cell: It is an electrochemical cell which acts as a source of electrical energy without being previously charged up by an electric current from an external source. Secondary cell: The cell in which electrical energy from an external source is first converted to chemical energy and then made to operate in opposite direction by removing the external source. In this cell, the reaction can be reversed practically. It can be recharged after its discharging. More clear cut explanation from the electrochemistry chapter from the following resource.
Answers:The main difference is the energy source. In primary active transport, a substance is moved across a cell membrane as a direct result of the cell accessing energy from ATP. In secondary active transport, the energy for the transport is instead derived from a differential in concentrations of a second substance; this differential is usually achieved with primary active transport.
Answers:When a cell expends energy directly from ATP the process is called Primary active process. Using another energy source, such as the potential energy stored in the ion gradient is called secondary active transport.