Transcript Chapter 22
Chapter 22 Magnetism (Lecture II) Dr. Jie Zou PHY 1161 1 Outline Motion of charged particles in a magnetic field Magnetic force on a current-carrying wire Electric current and magnetic fields Magnitude of the magnetic field of a current-carrying wire Solenoids Dr. Jie Zou PHY 1161 2 The Motion of Charged Particles in a Magnetic Field The electromagnetic flowmeter The operating principle of a mass spectrometer Dr. Jie Zou PHY 1161 3 The Magnetic Force on a Current-Carrying Wire Magnitude of the magnetic force on a current-carrying wire Direction of the magnetic force on a current-carrying wire Dr. Jie Zou F = ILBsin I: Current in the wire (A); L: Length of the wire (m); B: Magnetic field (T); : The angle between the direction of the magnetic field and the current. Given by the same Magnetic Force RHR for charges PHY 1161 4 Example 22-4: Magnetic Levity A copper rod 0.150 m long and with a mass of 0.0500 kg is suspended from two thin, flexible wires. At right angles to the rod is a uniform magnetic field of 0.550 T pointing into the page. Dr. Jie Zou Find the direction and magnitude of the electric current needed to levitate the copper rod. PHY 1161 5 Electric Current and Magnetic Fields The source of any magnetic field is the motion of electric charge. Magnetic field produced by a straight and infinitely long currentcarrying wire: Dr. Jie Zou PHY 1161 The magnetic field “circulates” around the wire. Find the direction of the magnetic field using the magnetic field RHR. 6 Conceptual Checkpoint 22-5 The magnetic field shown in the sketch is due to the horizontal, current-carrying wire. Dr. Jie Zou Does the current in the wire flow to the left or to the right? PHY 1161 7 Magnitude of the Magnetic Field of a Current-Carrying Wire Magnitude of the magnetic field, B, produced by a straight and infinitely long currentcarrying wire: Dr. Jie Zou B = (0I)/(2r) 0 = 4 x 10-7 Tm/A, the permeability of free space. The field doubles if the current I is doubled; the field halves if the distance from the wire, r, is doubled. PHY 1161 8 Example 22-6: An Attractive Wire A 52-C charged particle moves parallel to a long wire with a speed of 720 m/s. The separation between the particle and the wire is 13 cm, and the magnitude of the force exerted on the particle is 1.4 x 10-7 N. Dr. Jie Zou Find the magnitude of the magnetic field, B. Find the current in the wire, I. PHY 1161 9 Solenoids A solenoid carrying a current produces an intense, nearly uniform magnetic field inside the loops. Magnitude of the magnetic field inside a solenoid: Dr. Jie Zou A solenoid is also called an electromagnet. The magnetic field inside the solenoid is directed along its axis. The field outside the solenoid is almost zero. B = 0 (N/L) I = 0 nI n: the number of loops per unit length PHY 1161 10 Example 22-7: Through the Core of a Solenoid A solenoid is 20.0 cm long, has 200 loops, and carries a current of 3.25 A. Dr. Jie Zou Find the magnitude of the force exerted on a 15.0-C changed particle moving at 1050 m/s through the interior of the solenoid, at an angle of 11.5° relative to the solenoid’s axis. PHY 1161 11 Homework #7 Chapter 22, P. 793-795, Problems: #31, 33, 49, 57 (Physics, Walker, 4th edition). Dr. Jie Zou PHY 1161 12