Which of the following is a secondary cell?

The correct answer is D, 20 A. The working rule for charging a storage battery is that the charging current should be about one-tenth of the ampere-hour capacity of the battery, which is called the C by 10 rate. For a battery rated 200 ampere-hours, one-tenth of 200 gives 20 amperes, and at that rate a fully discharged battery takes roughly ten to twelve hours to charge. A rate much higher than this heats the plates, boils the electrolyte and shortens the life of the cell, while a much lower rate leaves the battery undercharged and allows sulphation. A, 12 A, and B, 15 A, are wrong because they are below the one-tenth value and are simply arbitrary figures for a 200 ampere-hour cell. C, 10 A, is the trap: it is one-twentieth of the rating, and candidates who remember the words ten per cent but apply them to the wrong number mark it.

The correct answer is C, 12 V. When cells are joined in parallel all their positive terminals are connected together and all their negative terminals together, so each cell is across the same two points and the voltage of the combination is the voltage of one cell, that is 12 volts. What the parallel arrangement does add is the current capacity: six identical cells in parallel can supply six times the current, or the same current for six times as long, and the internal resistance of the group falls to one-sixth of that of a single cell. B, 72 V, is wrong because adding the voltages is what happens in a series connection, where the positive of one cell is joined to the negative of the next. A, 6 V, and D, 2 V, are wrong because no method of grouping identical cells reduces the terminal voltage below that of one cell; 2 V happens to be the voltage of a single lead-acid cell, of which six in series make such a 12 volt battery.

The correct answer is B, the anode. The anode is the electrode joined to the positive terminal of the supply, so negatively charged ions, the anions, travel to it and give up electrons there; loss of electrons is oxidation. A is wrong because the cathode is joined to the negative terminal, attracts the positively charged cations and gives them electrons, so reduction, not oxidation, occurs there. The rule holds in a galvanic cell as well, where the anode is the negative terminal but is still the seat of oxidation, which is the point examiners test. C is wrong because the electrolyte is only the medium in which the ions move; the reactions happen where the ions meet the electrodes. D is wrong because a salt bridge belongs to a galvanic cell with two half-cells and merely completes the circuit while keeping the solutions electrically neutral; it is not a site of oxidation.

The correct answer is B, 96,500 coulombs. One faraday is the charge carried by one mole of electrons, obtained by multiplying the charge on a single electron by the Avogadro number, and it comes to about 96,500 coulombs; passing one faraday through an electrolyte liberates one gram equivalent of the substance at an electrode. A is wrong because 9,650 coulombs is one-tenth of the correct value and is a careless-reading trap. C is wrong because the figure 1.6 belongs to the charge on one electron, which is 1.6 into ten raised to the power minus nineteen coulombs, not to the faraday. D is wrong because 6.023 into ten raised to the power twenty-three is the Avogadro number, a count of particles and not a quantity of charge, though the two are linked in the very calculation that gives the faraday.

The correct answer is B, the cathode. The metal ions in the plating bath carry a positive charge, so they move towards the negative electrode and are reduced to metal there; the article must therefore be the cathode for the coating to build up on it. A is wrong because the anode is a plate of the pure plating metal, which dissolves into the solution as the plating proceeds and so keeps the concentration of the bath steady, which is why the bath does not have to be renewed often. C is wrong because the electrolyte is a solution of a salt of the plating metal, such as a copper salt for copper plating, and is not an electrode at all. D is wrong because the polarity cannot be chosen freely: reversing it would strip metal from the article instead of depositing it, which is in fact how electro-polishing and anodising work.