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NOTES 36B - Topic 5 - Electricity & Magnetism

NOTES 36B - Topic 5 - Electricity & Magnetism - --------------------------------------------------------------------------------------------- • Every current-carrying wire generates a circular magnetic field around itself. If 2 current-carrying wires are close together, they will attract or

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NOTES 36B - Topic 5 - Electricity & Magnetism

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  1. NOTES 36B - Topic 5 - Electricity & Magnetism ---------------------------------------------------------------------------------------------- • Every current-carrying wire generates a circular magnetic field around itself. If 2 current-carrying wires are close together, they will attract or repel each other according to the directions of the currents.

  2. 5.3.9 Problems w/B and FB from Two, Parallel I-carrying Wires • The magnetic force between two parallel, current-carrying wires: FB = ( μo / 2π) ( I1, I2l / L) ; ...where FB = magnetic force per meter of wire (N m-1), μo = constant = permeability of free space = 4π x 10-7 T m A-1; I1, I2= currents in wires (A); l = length of 2 parallel current-carrying wires; L = distance of separation of wires (m);

  3. 5.3.10 The Ampere (A) and the Coulomb (C) - Latest Definitions • 1.00 Ampere (A) is the amount of current flowing in each of two long parallel wires, separated by exactly 1.00 m, in a vacuum, that results in a magnetic force between the wires of exactly 2.00 x 10-7 N m-1 ; • 1.00 Coulomb (C) is exactly 1.00 ampere-second; 1.00 C = 1.00 A

  4. 5.3.11 The Galvanometer and the Electric Motor • The DC Motor - (see diagram below) - a coil of wire with an attached split ring (commutator); - the coil is suspended in a permanent magnetic field; - as I passes through coil, a B is produced around the coil which interacts (attracted to/repelled from) with the field of the permanent magnet; - the coil experiences a torque and rotates about a pivot in proportion to the amount and direction on the current; - when the contacts (brushes) cross the gap in the split ring, the current changes direction, the FB’s reverse, and the coil continues its rotation;

  5. • The Galvanometer - (see diagram below) - a coil of wire with an attached pointer; - the coil is suspended in a permanent magnetic field; -as I passes through coil, a B is produced around the coil which interacts (attracted to, repelled from) with the field of the permanent magnetic; -the coil experiences a torque a rotates about a pivot in proportion to the amount and direction on the current; - when the current stops, a spring returns the coil to its “zero” position;

  6. 5.3.12 Problems Involving B around a Current-carrying Wire B near a current-carrying wire is directly related to I and indirectly related to r (distance from wire - m); As an equation: B = uo I /2πr (Ampere’s Law) , ...where uo is a constant...the permeability of free space; uo = 4π x 10-7 T m A-1;

  7. • Sample Problem: A vertical electrical wire in the wall of a building carries a direct current of 25 A upward. What is B at a point that is 1.0 m horizontally away from the wire? Given: Unknown: Equation:

  8. 5.3.13 Problems Involving B around a Current-carrying Solenoid • A solenoid is made of many parallel loops of wire; usually has a hollow center; • The B of a current-carrying solenoid moves through the center of the loops and curves around outside the solenoid; it is nearly uniform with the tube of the solenoid; • Outside the solenoid, the B follows the same rule as for a single current-carrying wire: B = uo I /2πr ; • Inside the solenoid, the B is the sum of the B’s of all the current-carrying wires: B = uo I N / L , where N = # loops and L = length of solenoid; or B = uo I n , where n = # loops per meter;

  9. Sample Problem: Estimate the strength of the magnetic field inside a 20. cm long solenoid made of 800. loops of wire that carries a current of 3.0 A. Given: Unknown: Equation:

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