Question Video: Recognizing That a Cell with Constant emf Has a Constant Charge Separation | Nagwa Question Video: Recognizing That a Cell with Constant emf Has a Constant Charge Separation | Nagwa

Question Video: Recognizing That a Cell with Constant emf Has a Constant Charge Separation Science • Third Year of Preparatory School

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If the emf of a cell does not change when it is connected to a circuit, which of the following must be true of the charges on the terminals of the cell? [A] The amount of positive charge on the positive cell terminal does not change, but the amount of negative charge on the negative terminal does change. [B] The amount of positive charge on the positive cell terminal changes, but the amount of negative charge on the negative terminal does not change. [C] The amount of positive charge on the positive cell terminal does not change, and the amount of negative charge on the negative terminal does not change.

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Video Transcript

If the emf of a cell does not change when it is connected to a circuit, which of the following must be true of the charges on the terminals of the cell? (A) The amount of positive charge on the positive cell terminal does not change, but the amount of negative charge on the negative terminal does change. (B) The amount of positive charge on the positive cell terminal changes, but the amount of negative charge on the negative terminal does not change. Or (C) the amount of positive charge on the positive cell terminal does not change, and the amount of negative charge on the negative terminal does not change.

In circuit diagrams, we usually represent a cell like this, where each line represents a terminal of the cell. There is one positively charged terminal and one negatively charged terminal. To answer this question, we’re going to represent our cell slightly differently, like this. Here, we’ve drawn some of the charges on each of the terminals. The positive charges are shown in red, and the negative charges are shown in blue. In this cell, the opposite charges have been separated. There is an overall positive charge here, an overall negative charge here, and these regions of charge have a distance between them.

Recall that opposite charges attract. There is an attractive force between positive charges and negative charges that pulls them towards each other. In order to keep opposite charges separate and on different terminals of a cell, we need to do work on the charges. When we do work on the charges, we can give them enough energy to overcome the attractive force and remain separate. This creates an electric potential difference between the two terminals of the cell. This potential difference can also be called an emf. In short, because there’s a separation of charge across the terminals of the cell, there is also an emf across the terminals of the cell. The magnitude of this emf depends on the amount of charge that has been separated.

If the amount of charge on each terminal increases, so the positive terminal becomes more positively charged and the negative terminal becomes more negatively charged, it would take more work to keep all of the extra positive and negative charges separate. This corresponds to a greater emf across the terminals of the cell. Similarly, if the amount of charge on each terminal decreased, less work would need to be done to separate the charges. This corresponds to a smaller emf across the terminals.

In this question, we’re told that the emf of the cell does not change when it is connected to the circuit. This means that the amount of charge on each terminal does not change. If we look through the options we’ve been given, we see that this corresponds to option (C). The amount of positive charge on the positive cell terminal does not change, and the amount of negative charge on the negative terminal does not change. So, the correct answer to this question must be option (C).

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