NDA (II) 2016 General Ability Test · Question 130 of 150

Correct — C, the heating effect of current. A fuse is a short length of thin wire of low melting point, wired in series with the circuit so that it carries the whole current. Joule's law of heating says the heat produced in a resistance R by a current I flowing for time t is I²Rt, so the heat rises with the square of the current. Double the current and the fuse wire receives four times the heat. When the current exceeds the rated value, that heat raises the wire past its melting point in a fraction of a second, the wire melts, the circuit breaks, and the rest of the wiring is saved.

• (a)chemical effect of current — The chemical effect is what happens when current is passed through an electrolyte — electroplating, electrolysis, the charging of a cell. A fuse is a solid metal wire in a dry circuit and nothing is decomposed or deposited in it.
• (b)magnetic effect of current — The plausible near-miss, because the magnetic effect really does power a protective device — the miniature circuit breaker, whose electromagnet trips a latch on a surge. But a fuse has no coil and no moving part; it simply melts. Keep the two apart: the breaker uses magnetism, the fuse uses heat.
• (d)optical property of current — There is no such standard effect of an electric current. Current does produce light in a filament lamp, but only by heating the filament until it glows, which is again the heating effect in disguise.

An electric current has three classical effects: it heats a conductor, it produces a magnetic field around itself, and it drives chemical change when it passes through an electrolyte. The heating effect is quantified by Joule's law, in which the heat developed equals the square of the current multiplied by the resistance and the time. A fuse is engineered around that law — its wire is deliberately made thinner and of higher resistivity and lower melting point than the circuit it protects, usually a tin-lead or similar alloy, so it is always the first thing in the circuit to give way.

Two design points make the fuse work, and both are worth knowing because they are asked separately. First, the fuse must be in series, so that the full circuit current passes through it; a fuse in parallel would carry almost nothing and would never blow. Second, the fuse wire needs a low melting point but must not have a large resistance, or it would waste power and drop voltage in normal operation. The square in Joule's law is what gives the device its sharpness — a modest overload produces a disproportionately large jump in heating, so the fuse acts quickly rather than degrading slowly.

• Joule's law of heating: the heat produced is I²Rt, so heating rises with the square of the current.
• A fuse is connected in series with the circuit and is made of an alloy of low melting point, such as a tin-lead alloy.
• A fuse is rated by the current it can carry safely; it melts when the current exceeds that rating, breaking the circuit.
• A miniature circuit breaker performs the same protective job but uses the magnetic effect of current, plus a thermal element, to trip a switch instead of melting a wire.

• Marking the magnetic effect because circuit breakers use it; the fuse is purely thermal.
• Believing a fuse must have a very high resistance — it needs a low melting point and only a modest resistance, otherwise it would waste power all the time.
• Wiring a fuse in parallel; it must be in series or it never sees the fault current.

NDA asks the three effects of current in rotation — usually by naming a device and asking which effect it exploits — so build the mapping from device to effect and the item takes seconds.

• practice — not a real PYQ

  If the current through a fuse wire is doubled, the rate at which heat is produced in it becomes

  • (a)half
  • (b)double
  • (c)four times
  • (d)unchanged

  Answer(c) four times — heating goes as the square of the current in Joule's law, which is what makes a fuse act sharply on an overload.

• practice — not a real PYQ

  An electric bell works on which effect of electric current?

  • (a)Heating effect
  • (b)Magnetic effect
  • (c)Chemical effect
  • (d)Photoelectric effect

  Answer(b) Magnetic effect — the electromagnet pulls the armature, which strikes the gong and simultaneously breaks the circuit.