CDS (I) 2021 General Knowledge · Question 107 of 120

Correct — C, James P. Joule. James Prescott Joule established in the 1840s that a current passing through a resistance produces heat in a fixed and measurable amount, a result now written as H = I²Rt — the heat produced goes as the square of the current, directly as the resistance, and directly as the time. That relation is Joule's law of heating, and it explains every device that turns electricity into warmth: the electric iron, the room heater, the geyser, the filament lamp and the fuse. Joule also measured the mechanical equivalent of heat, showing that a given amount of work always yields the same quantity of heat, and the SI unit of energy carries his name for that body of work.

• (a)Isaac Newton — Newton belongs to the century before electricity was studied as a science — the laws of motion, universal gravitation and the splitting of white light by a prism. He wrote nothing on current electricity.
• (b)Galileo Galilei — Galileo worked on falling bodies, the pendulum and the telescope in the early seventeenth century, two hundred years before a steady electric current could even be produced.
• (d)J.J. Thomson — Thomson discovered the electron in 1897 and proposed the plum-pudding model of the atom. His work explains what carries the current, not the law governing the heat it produces.

An electric current does three things that can be observed and measured: it heats the conductor, it produces a magnetic field around it, and it can drive chemical change in a solution. The heating effect is quantified by Joule's law, H = I²Rt. Because the current is squared, doubling the current makes four times the heat, which is why a small rise in current can burn out a component.

The question is a straight attribution item, and the fastest way through it is chronology. Galileo and Newton belong to the seventeenth century, and the steady electric current only became available after Volta's pile in 1800, so neither can be the answer. That leaves two nineteenth-century figures, and Thomson's contribution is the electron rather than any law of heating. The corroborating clue is the unit: the joule is the SI unit of energy, and units are named after the people who worked on the quantity. Devices built to exploit the heating effect use high-resistance alloy elements, and devices built to avoid it use low-resistance copper.

• Joule's law of heating states that H = I²Rt — heat produced varies as the square of the current, as the resistance and as the time.
• Heating appliances use nichrome, an alloy of high resistivity with a high melting point that does not oxidise readily when hot.
• The electric fuse is a deliberate application of the heating effect — a thin wire that melts before the circuit is damaged.
• Joule also determined the mechanical equivalent of heat, linking work and heat as forms of the same quantity.
• The joule, the SI unit of energy and work, is named after him.

• Attributing electrical laws to Newton or Galileo, who worked long before steady currents existed.
• Confusing Joule's law of heating with Ohm's law — one gives the heat produced, the other the relation between voltage and current.
• Forgetting that the current is squared in the formula, so that halving the current cuts the heat to a quarter.

As a scientist-to-discovery match, or as a formula-based numerical on the heat produced in a resistor.

• practice — not a real PYQ

  If the current through a resistor is doubled while the resistance and the time are unchanged, the heat produced becomes

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

  Answer(c) four times — the heat varies as the square of the current in H = I²Rt, so doubling the current multiplies the heat by four.

• practice — not a real PYQ

  The element of an electric iron is made of nichrome chiefly because nichrome

  • (a)has very low resistivity
  • (b)has high resistivity and a high melting point
  • (c)is a good conductor of heat only
  • (d)is magnetic

  Answer(b) has high resistivity and a high melting point — it produces plenty of heat for a given current and survives being run red-hot without oxidising away.