Chapter 19. Magnetism

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Nigel Mathews
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1 Chapter 19 Magnetism

2 The figure shows the path of a negatively charged particle in a region of a uniform magnetic field. Answer the following questions about this situation (in each case, we revert back to the original situation). 1 What is the direction of the magnetic field in the boxed region? 2 If the speed of the particle is doubled, how does the path change? 3 If the magnetic field strength is doubled, how does the path change? 4 If the original particle is replaced with a negative particle of twice the charge and the same mass, how does the path change? 5 If the original particle is replaced with a positive particle of the same charge and mass, how does the path change? 6 If the original particle is replaced with a negative particle of twice the mass and the same charge, how does the path change?

Hans Christian Oersted 1777 1851 Best known for observing that a compass needle deflects when placed

3 Hans Christian Oersted Best known for observing that a compass needle deflects when placed near a wire carrying a current First evidence of a connection between electric and magnetic phenomena

4 Magnetic Fields Long Straight Wire A current-carrying wire produces a magnetic field The compass needle deflects in directions tangent to the circle The compass needle points in the direction of the magnetic field produced by the current

5 Direction of the Field of a Long Straight Wire Right Hand Rule #2 Grasp the wire in your right hand Point your thumb in the direction of the current Your fingers will curl in the direction of the field

6 Magnitude of the Field of a Long Straight Wire The magnitude of the field at a distance r from a wire carrying a current of I is µ o = 4 π x 10-7 T. m / A B = µ 0I 2πr µ o is called the permeability of free space

7 Six charged particles have been placed near identical current carrying wires. These particles have the same mass and have the same speed at the instant shown. Rank the magnitude of the acceleration of each charge, greatest first.

8 Magnetic Force Between Two Parallel Conductors The force on wire 1 is due to the current in wire 1 and the magnetic field produced by wire 2 The force per unit length is: F 1 l = µ 0I 1 I 2 2πd

9 Force Between Two Conductors, cont Parallel conductors carrying currents in the same direction attract each other Parallel conductors carrying currents in the opposite directions repel each other

10 In each case below, two or three very long straight wires are parallel to each other. Rank the strength of the total magnetic force on each of the labeled wires, from largest to smallest.

11 Defining Ampere and Coulomb The force between parallel conductors can be used to define the Ampere (A) If two long, parallel wires 1 m apart carry the same current, and the magnitude of the magnetic force per unit length is 2 x 10-7 N/m, then the current is defined to be 1 A The SI unit of charge, the Coulomb (C), can be defined in terms of the Ampere If a conductor carries a steady current of 1 A, then the quantity of charge that flows through any cross section in 1 second is 1 C

12 Magnetic Field of a Current Loop The strength of a magnetic field produced by a wire can be enhanced by forming the wire into a loop All the segments, Δx, contribute to the field, increasing its strength

13 Magnetic Field of a Current Loop Total Field

14 Magnetic Field of a Current Loop Equation The magnitude of the magnetic field at the center of a circular loop with a radius R and carrying current I is B = µ 0I 2R With N loops in the coil, this becomes B = N µ 0I 2R

15 Magnetic Field of a Solenoid If a long straight wire is bent into a coil of several closely spaced loops, the resulting device is called a solenoid It is also known as an electromagnet since it acts like a magnet only when it carries a current

16 Magnetic Field of a Solenoid, 2 The field lines inside the solenoid are nearly parallel, uniformly spaced, and close together This indicates that the field inside the solenoid is nearly uniform and strong The exterior field is nonuniform, much weaker, and in the opposite direction to the field inside the solenoid

17 Magnetic Field in a Solenoid, 3 The field lines of the solenoid resemble those of a bar magnet

18 Magnetic Field in a Solenoid, Magnitude The magnitude of the field inside a solenoid is constant at all points far from its ends B = µ o n I n is the number of turns per unit length n = N / l The same result can be obtained by applying Ampère s Law to the solenoid

19 Magnetic Effects of Electrons Orbits An individual atom should act like a magnet because of the motion of the electrons about the nucleus Each electron circles the atom once in about every seconds This would produce a current of 1.6 ma and a magnetic field of about 20 T at the center of the circular path However, the magnetic field produced by one electron in an atom is often canceled by an oppositely revolving electron in the same atom

20 Magnetic Effects of Electrons Orbits, cont The net result is that the magnetic effect produced by electrons orbiting the nucleus is either zero or very small for most materials

21 Magnetic Effects of Electrons Spins Electrons also have spin The classical model is to consider the electrons to spin like tops It is actually a quantum effect

22 Magnetic Effects of Electrons Spins, cont The field due to the spinning is generally stronger than the field due to the orbital motion Electrons usually pair up with their spins opposite each other, so their fields cancel each other That is why most materials are not naturally magnetic

23 Magnetic Effects of Electrons Domains In some materials, the spins do not naturally cancel Such materials are called ferromagnetic Large groups of atoms in which the spins are aligned are called domains When an external field is applied, the domains that are aligned with the field tend to grow at the expense of the others This causes the material to become magnetized

24 Domains, cont Random alignment, a, shows an unmagnetized material When an external field is applied, the domains aligned with B grow, b

25 Domains and Permanent Magnets In hard magnetic materials, the domains remain aligned after the external field is removed The result is a permanent magnet In soft magnetic materials, once the external field is removed, thermal agitation causes the materials to quickly return to an unmagnetized state With a core in a loop, the magnetic field is enhanced since the domains in the core material align, increasing the magnetic field

Torque on a Current Loop

Today Chapter 19 Magnetism Torque on a current loop, electrical motor Magnetic field around a current carrying wire. Ampere s law Solenoid Material magnetism Clicker 1 Which of the following is wrong?