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Lecture 22: MON 09 MAR Ch.29.1–2: Magnetic Fields Due to Currents

Physics 2102 Jonathan Dowling. Lecture 22: MON 09 MAR Ch.29.1–2: Magnetic Fields Due to Currents. Biot-Savart Law. Felix Savart (1791–1841). Jean-Baptiste Biot (1774-1862). What Are We Going to Learn? A Road Map.

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Lecture 22: MON 09 MAR Ch.29.1–2: Magnetic Fields Due to Currents

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  1. Physics 2102 Jonathan Dowling Lecture 22: MON 09 MARCh.29.1–2: Magnetic Fields Due to Currents Biot-Savart Law Felix Savart (1791–1841) Jean-BaptisteBiot (1774-1862)

  2. What Are We Going to Learn?A Road Map • Electric charge Electric force on other electric charges Electric field, and electric potential • Moving electric charges : current • Electronic circuit components: batteries, resistors, capacitors • Electric currentsMagnetic field  Magnetic force on moving charges • Time-varying magnetic field  Electric Field • More circuit components: inductors. • Electromagneticwaveslight waves • Geometrical Optics (light rays). • Physical optics (light waves)

  3. I Electric Current: A Source of Magnetic Field • Observation: an electric current creates a magnetic field • Simple experiment: hold a current-carrying wire near a compass needle! B Wire with current INTO page B B

  4. Hans Christian Oersted was a professor of science at Copenhagen University. In 1820 he arranged in his home a science demonstration to friends and students. He planned to demonstrate the heating of a wire by an electric current, and also to carry out demonstrations of magnetism, for which he provided a compass needle mounted on a wooden stand. While performing his electric demonstration, Oersted noted to his surprise that every time the electric current was switched on, the compass needle moved. He kept quiet and finished the demonstrations, but in the months that followed worked hard trying to make sense out of the new phenomenon.

  5. B i i Direction of B! Casey’s Right Hand Rule! • Point your thumb along the direction of the current in a straight wire • The magnetic field created by the current consists of circular loops directed along your curled fingers. • The magnetic field gets weaker with distance: For long wire it’s a 1/R Law! • You can apply this to ANY straight wire (even a small differential element!) • What if you have a curved wire? Break into small elements.

  6. B Superposition I-OUT I-OUT • Magnetic fields (like electric fields) can be “superimposed” -- just do a vector sum of B from different sources • The figure shows four wires located at the 4 corners of a square. They carry equal currents in directions indicated • What is the direction of B at the center of the square? I-IN I-IN

  7. Biot-Savart Law When we computed the electric field due to charges we used Coulomb’s law. If one had a large irregular object, one broke it into infinitesimal pieces and computed, Which we write as, If you wish to compute the magnetic field due to a current in a wire, you use the law of Biot and Savart.

  8. i dB The Biot-Savart Law Jean-Baptiste Biot (1774-1862) Felix Savart (1791-1841) • Quantitative rule for computing the magnetic field from any electric current • Choose a differential element of wire of length dLand carrying a current i • The field dB from this element at a point located by the vector r is given by the Biot-Savart Law m0 =4px10–7 T•m/A (permeability constant)

  9. i dB Both Are 1/r2 Laws! The has no units. Biot-Savart Law for B-Fields Coulomb Law for E-Fields Biot-Savart Requires A Right-Hand Rule

  10. Biot-Savart Law • An infinitely long straight wire carries a current i. • Determine the magnetic field generated at a point located at a perpendicular distance R from the wire. • Choose an element ds as shown • Biot-Savart Law: dB points INTO the page • Integrate over all such elements

  11. Field of a Straight Wire

  12. Is the B-Field From a Power Line Dangerous? A power line carries a current of 500 A. What is the magnetic field in a house located 100 m away from the power line? = 1 mT Recall that the earth’s magnetic field is ~10–4T = 100 mT Probably not dangerous!

  13. Biot-Savart Law i • A circular arc of wire of radius R carries a current i. • What is the magnetic field at the center of the loop? Direction of B?? Not another right hand rule?! TWO right hand rules!: If your thumb points along the CURRENT, your fingers will point in the same direction as the FIELD. If you curl our fingers around direction of CURRENT, your thumb points along FIELD!

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