PHY 112 (Section 001 )

PHY 112 (SECTION 001 )Lecture 12,13 Chapter 28 of R Haliday Book

Fall 2011 (PHY 112 Section 1) 2

Outline• Magnetic field (28.1-28.3)• Hall Effect (28-5), • Magnetic Force on a Current Carrying Wire (28-8).• Torque on a Current Loop (28-9). • Biot-Savart Law (29-2), • Force Between Parallel Currents (29-3).• Ampere's Law (29-4), • Solenoids and Toroids (29-5)


What Produces a Magnetic Field?• One way is to use moving electrically charged particles, such as a current in a wire, to make an electromagnet. The current produces a magnetic field that can be used. (computer hard drive).

• The other way to produce a magnetic field is by means of elementary particles (such as electrons) because these particles have an intrinsic magnetic field around them. That is, the magnetic field is a basic characteristic of each particle just as mass and electric charge (or lack of charge) are basic characteristics.


Definition of • We determined the electric field at a point by putting a test particle of charge q at rest at that point and measuring the electric force acting on the particle.

• If a magnetic monopole were available, we could define in a similar way. Because such particles have not been found, we must define in another way, in terms of the magnetic force exerted on a moving electrically charged test particle.


• A charged particle is fired through the point at which B is to be defined, using various directions and speeds for the particle and determining the force that acts on the particle at that point. After many such trials we would find that when the particle's velocity is along a particular axis through the point, force is zero.

• For all other directions of ,the magnitude of where is the angle between the zero-force axis and the direction of .


• We can then define a B as


Direction of F

The right-hand rule



Lines of Mag Field




HALL EFFECT • A beam of electrons in a vacuum can be deflected by a

magnetic field. Can the drifting conduction electrons in a copper wire also be deflected by a magnetic field? In 1879, Edwin H. Hall, then a 24-year-old graduate student at the Johns Hopkins University, showed that they can.

• This Hall effect allows us to find out whether the charge carriers in a conductor are positively or negatively charged. Beyond that, we can measure the number of such carriers per unit volume of the conductor.


• Figure 28-8a shows a copper strip of width d, carrying a current i whose conventional direction is from the top of the figure to the bottom. The charge carriers are electrons and, as we know they drift (with drift speed vd) in the opposite directior, from bottom to top.


A strip of copper carrying a current i is immersed in. (a) The situation immediately after the magnetic field is turned on. The curved path that will then be taken by an electron. (b) The situation at equilibrium, which quickly follows. Note that negative charges pile up on the right side of the strip, leaving uncompensated positive charges on the left. Thus, the left side is at a higher potential than the right side. (c) For the same current direction., if the charge carriers were positively charged



A flexible wire passes between the pole faces of a magnet. (a) Without current in the wire, the wire is straight. (b) With upward current, the wire is deflected rightward. (c) With downward current, the deflection is leftward. The connections for

getting the current into the wire at one end and out of it at the other end are not shown.

Magnetic Force on a current

carrying conductor




A curved wire carries a current

A curved wire carrying a current I in a uniform magnetic field. The total magnetic force acting on the wire is equivalent to the force on a straight wire of length L’ running between the ends of the curved wire.


A current-carrying loop of arbitrary shape

A current-carrying loop of arbitrary shape in a uniform magnetic field. The net magnetic force on the loop is zero.

Calculating the Magnetic FieldDue to a Current

A wire of arbitrary shape carrying a current i. We want to find the magnetic field B at a nearby point p.

Biot–Savart law:

The right-hand rule for determining the direction of the magnetic field surrounding a long, straight wire carrying a current. Note that the magnetic field lines form circles around the wire.

Magnetic Field of a Solenoid

It concerns the magnetic field produced by the current in a long, tightly wound helical coil of wire. Such a coil is called a solenoid (Fig. 29-17). We assume that the length of the solenoid is much greater than the diameter.

Magnetic Field of a Toroid• Figure a shows a toroid, which we may describe as a (hollow) solenoid that has been curved until its two ends meet, forming a sort of hollow bracelet.

Magnetic Field Due to a straight Wire Segment

Magnetic Field Due to a straight

Magnetic Field Due to a curved Wire Segment

The magnetic field at O due to the current in the curved segment AC is into the page. The contribution to the field at O due to the current in the two straight segments is zero.

The magnetic field at O due to the current inthe straight segments AA’ and CC’ is zero because ds is parallel to along these paths;

Each length element ds along path AC is at the same distance R from O, and the current in each contributes a field element dB directed into the page at O. Furthermore, at every pointon AC, ds is perpendicular to hence,

The direction of B is into the page at O because ds×r is into the page for every length element.

Magnetic Field on the axis of a Circular Current Loop

Magnetic field lines surrounding a current loop.

THE MAGNETIC FORCE BETWEEN TWOPARALLEL CONDUCTORS

Magnetic Field Outside a Long Straight Wire with Current

PHY 112 (Section 001 )

Documents

current loop

current i

magnetic field b

intrinsic magnetic field

curved wire

copper wire

magnetic fielddue

righthand rule fall