|
|
30.3 Ampère’s Law Oersted’s 1819 discovery about deflected compass needles demonstrates that a SECTION 3 0.3 • Ampère’s Law 933 Quick Quiz 30.2 For I 1 " 2 A and I 2 " 6 A in Figure 30.8, which is true: (a) F 1 " 3F 2 , (b) F 1 " F 2 /3, (c) F 1 " F 2 ? Quick Quiz 30.3 A loose spiral spring carrying no current is hung from the ceiling. When a switch is thrown so that a current exists in the spring, do the coils move When a conductor carries a steady current of 1 A, the quantity of charge that flows The value 2 & 10 ' 7 N/m is obtained from Equation 30.12 with I 1 " I 2 " 1 A and a " 1 m. Because this definition is based on a force, a mechanical measurement can be The SI unit of charge, the coulomb, is defined in terms of the ampere: Andre-Marie Ampère French Physicist (1775–1836) Ampère is credited with the discovery of electromagnetism— the relationship between electric currents and magnetic fields. Ampère’s genius, particularly in mathematics, became evident by the time he was 12 years old; his personal life, however, was filled with tragedy. His father, a wealthy city official, was guillotined during the French Revolution, and his wife died young, in 1803. Ampère died at the age of 61 of pneumonia. His judgment of his life is clear from the epitaph he chose for his gravestone: Tandem Felix (Happy at Last). (Leonard de In deriving Equations 30.11 and 30.12, we assumed that both wires are long compared with their separation distance. In fact, only one wire needs to be long. The Active Figure 30.9 (a) When no current is present in the wire, all compass needles point in the same direction (toward the Earth’s north pole). (b) When the wire carries a strong current, the compass needles deflect in a direction tangent to the circle, which is the direction of the magnetic field created by the current. (c) Circular magnetic field lines surrounding a current-carrying conductor, displayed with iron filings. © Richard Megna, Fundamental Photographs (a) (b) I = 0 I ds B At the Active Figures link at http://www.pse6.com, you can change the value of the current to see the effect on the compasses. (c) |