Lecture 28: MON 23 MARLecture 28: MON 23 MARMagnetic Fields Due to Currents: Biot-Savart LawMagnetic Fields Due to Currents: Biot-Savart Law

Physics 2113

Jonathan Dowling

Jean-Baptiste Biot (1774-1862)

Felix Savart (1791–1841)

What Are We Going to Learn?What Are We Going to Learn?A Road MapA 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. • Electromagnetic waves light waves• Geometrical Optics (light rays). • Physical optics (light waves)

Electric Current: Electric Current: A Source of Magnetic FieldA Source of Magnetic Field

I

B

Wire withcurrentINTO page

B

• Orested’s Observation : an

electric current creates a

magnetic field

• Simple experiment: hold a

current-carrying wire near

a compass needle!

B

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.

New Right Hand Rule!New 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.

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B

Direction of B! i

29.2: Magnetic Field due to a Long Straight Wire:

Fig. 29-4 A right-hand rule gives the direction of the magnetic field due to a current in a wire. (a) The magnetic field B at any point to the left of the wire is perpendicular to the dashed radial line and directed into the page, in the direction of the fingertips, as indicated by the x. (b) If the current is reversed, at any point to the left is still perpendicular to the dashed radial line but now is directed out of the page, as indicated by the dot.

Superposition: ICPPSuperposition: ICPP• 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?

B

I-OUT I-OUT

I-IN I-IN

When we computed the electric field due to charges we usedCoulomb’s law. If one had a large irregular object, one broke itinto infinitesimal pieces and computed dE,

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.

““Coulomb’s” Law For E-FieldsCoulomb’s” Law For E-Fields

The Biot-Savart LawThe Biot-Savart LawFor B-FieldsFor B-Fields

• Quantitative rule for computing the magnetic field from any electric current

• Choose a differential element of wire of length dL and 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

μ0 =4π×10–7 T•m/A(permeability constant of free space)

μ0 =4π×10–7 T•m/A(permeability constant of free space)

Jean-Baptiste Biot (1774-1862)

Felix Savart (1791-1841)

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Biot-Savart Law for B-Fields

Coulomb Law for E-Fields

Biot-Savart Requires A Right-Hand Rule

Both Are 1/r2 Laws!The has no units.

Biot-Savart LawBiot-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

Field of a Straight WireField of a Straight Wire

Is the B-Field From a Power Line Dangerous?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?

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 μT= 1 μT

Recall that the earth’s magnetic field is ~10–4T = 100 μTRecall that the earth’s magnetic field is ~10–4T = 100 μT

Probably not dangerous!

Biot-Savart LawBiot-Savart Law• 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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Example, Magnetic field at the center of a circular arc of a circle pg 768:

ICPP: What is the direction of the B field at point C?

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Example, Magnetic field at the center of a circular arc of a circle.:

Example, Magnetic field off to the side of two long straight currents: