The resistance of a uniform metallic wire is

The correct answer is D, it gets halved. By Ohm's law, at a constant temperature the current through a conductor is directly proportional to the potential difference across its ends, since the current equals the potential difference divided by the resistance. The resistance of the conductor does not change when the voltage is changed, so halving the potential difference halves the current exactly.

Option A is wrong because a smaller driving voltage cannot push a larger current through the same resistance. Option B is wrong for the same reason, and it describes what would happen if the voltage were doubled rather than halved. Option C, that the current gets decreased, is true as far as it goes but it is not the best answer, because the question can be answered exactly: the current does not merely fall, it falls to one half. In a question of this kind always give the precise proportional change rather than the vague direction of change.

The correct answer is B, potential difference. The potential difference between two points in a circuit is defined as the work done in moving a unit positive charge from one point to the other. From this definition comes the unit: one volt is one joule of work per coulomb of charge, and the quantity is measured with a voltmeter connected in parallel across the two points.

Option A, pressure distance, is not a physical quantity at all, and pressure belongs to mechanics rather than to circuits. Option C, potential distance, is likewise not a defined term; the word potential appears only to make the option look plausible. Option D, pressure difference, is a real quantity but belongs to fluids, where it drives the flow of a liquid or a gas; it is sometimes used only as an analogy for voltage while teaching, never as the name of the electrical quantity. The definition asked for is that of potential difference.

The correct answer is D, microwave. A microwave oven does not cook by passing a current through a resistance. It generates microwaves in a magnetron, and these waves make the water molecules in the food rotate rapidly, so the food heats from within by dielectric heating. The heating effect of current plays no part in the cooking itself.

Option A, the electric heater, works exactly on Joule heating: current through a nichrome element produces heat proportional to the square of the current, the resistance and the time. Option B, the filament bulb, passes current through a thin tungsten wire that becomes white hot and glows, which is the same effect taken to a very high temperature. Option C, the electric iron, again uses a coiled resistance element inside the sole plate. Since three of the four are ordinary resistance heating devices, the microwave oven is the odd one out.

The correct answer is C, 6000 ohm. By Ohm's law the resistance equals the potential difference divided by the current. Here the current must first be converted into the base unit: two milliampere is 0.002 ampere. Dividing twelve volt by 0.002 ampere gives six thousand ohm, that is six kilo-ohm.

Option A, 600 ohm, is what you get if the milliampere is mistaken for a hundredth of an ampere instead of a thousandth, a common slip under time pressure. Option B, 6 ohm, is the answer to a different question, namely twelve volt divided by two ampere, and it ignores the prefix milli altogether. Option D, 60 ohm, does not follow from the numbers at all and is there only to fill the choice. The lesson is to convert every prefix, milli, micro, kilo or mega, into the base unit before dividing.

The correct answer is A, tungsten. Tungsten has the highest melting point of all metals, well above three thousand degrees Celsius, so a thin tungsten wire can be heated until it glows white without melting. It also has a fairly high resistivity and a low rate of evaporation at working temperature, which is why the filament lasts for hundreds of hours inside a bulb filled with an inert gas.

Option B, silver, is the best conductor of electricity of all metals, but for that very reason it would produce hardly any heat, and it melts at a temperature far too low for a filament. Option C, nichrome, is the right choice for the element of a heater or a toaster, where a red heat is wanted, but it cannot reach the white heat needed to give useful light. Option D, copper, is used for connecting wires precisely because it has a low resistance and stays cool while carrying current.