Topic 2
Magnetostatics
Magnetic Field, Magnetic Fields
Due to Currents, Induction and
Inductance
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Section 2.1
MAGNETIC FIELD
(Halliday/Resnick/Walker Ch.28)
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Contents
•
What Produces a Magnetic Field?
•
Definition of Magnetic Field
•
Crossed Fields: Discovery of the Electron; The Hall Effect
•
A Circulating Charged Particle
•
Magnetic Force on a Current-Carrying Wire
•
Torque on a Current Loop
•
The Magnetic Dipole Moment
•
Magnetic Dipole in a Magnetic Field
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Permanent Magnets
A magnet has two poles, a north pole and a south pole. It is said to be a magnetic
dipole.


No Magnetic monopole (individual magnetic charge) available in nature.
- A fundamental difference between electrostatics & magnetostatics
Note: The electric dipole is made of individual negative and positive charges
("monopoles") and can be separated from each other.
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Magnetic Force and Magnetic Field
• Magnetic Force: When a permanent magnet is brought close to iron nails, it attracts
them (exert force on the iron nails).
Fig. 28-1
**Geo-north pole is the south pole of a magnet
 A magnetic field exists in the region around a magnet. The magnetic field is a
vector that has both magnitude and direction.

The direction of the magnetic field at any point in space is the direction indicated
by the north pole of a small compass needle placed at that point.**
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What Produces a Magnetic Field?
Two ways to produce magnetic fields:
(a). Use moving electrically charged particles to make an electromagnet. The
current produces a magnetic field that is utilizable, e.g., to sort scrap metal.
(b). By means of elementary particles such as electrons, because these particles have
an intrinsic magnetic field around them.
Q: What generates earth’s magnetic field?
Hint: All celestial bodies with solid core have no magnetic field.
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Definition of Magnetic Field
 𝐵 is defined to be a vector quantity that is directed along the zero-force axis.

To define a magnetic field 𝐵 , in terms of the magnetic force 𝐹𝐵 exerted on a
moving electrically charged test particle:
 Fire a charged particle through the point at which 𝐵 is to be defined, using
various directions and speeds for the particle and determining the force 𝐹𝐵
that acts on the particle at that point.
 The magnetic force on the charged particle is defined as
where q is the charge of the particle.
 The magnitude of this force:
 The direction of 𝐹𝐵 is perpendicular to the direction of
.
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Finding the Magnetic Force on a Particle
•
When a charge is placed in a magnetic field, it experiences a magnetic force if two
conditions are met:
 The charge must be moving. No magnetic force acts on a stationary charge.
 The velocity of the moving charge must have a component that is perpendicular to
the direction of the field.
•
Right-Hand Rule
Fig. 28-2
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Units of Magnetic Field
•
The SI unit for 𝐵 is newton per coulomb-meter per second, i.e., tesla (T):
•
An earlier (non-SI) unit for 𝐵 is the gauss (G), and
Earth’s magnetic field is about 0.3 to 0.6 gauss (depends on location on earth)
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Magnetic Field Lines

One can represent magnetic fields with field lines, as one did for electric fields.
The lines originate from the north pole and end
on the south pole; they do not start or stop in
mid-space.
 Similar rules apply:
 The direction of the tangent to a magnetic
field line at any point gives the direction of 𝐵
at that point.
 The spacing of the lines represents the magnitude
of 𝐵 .
Fig. 28-4(a)
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Earth’s B from Dynamo Effect: a conducting fluid in motion generates B
Bearth generated by molten iron in outer-core, reverses every few 100,000 years
Geo-north pole is south pole of a magnet
Slide 5 =>
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The outer core (2400 km < r < 4700 km) is liquid iron/nickel – a conducting fluid.
Motion of this fluid produces the earth’s magnetic field via the dynamo effect.
Tic ~ 5700 oK ~ Tsun, Pic ~ (3.3 – 3.6) E6 atm
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Example 1: Magnetic force on a moving
charged particle
A uniform magnetic field 𝐵 , with magnitude 1.2 mT, is directed vertically upward
throughout the volume of a laboratory chamber. A proton with kinetic energy 5.3 MeV
enters the chamber, moving horizontally from south to north. What magnetic deflecting
force acts on the proton as it enters the chamber? The proton mass is 1.67×10-27 kg and
neglect Earth’s magnetic field.
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Calculations:
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Crossed Fields: Discovery of the Electron

Both an electric field 𝐸 and a magnetic field 𝐵 can produce a force on a charged
particle.

When the two fields are perpendicular to each other, they are said to be crossed
fields.

Need to examine what happens to electrons as they move through crossed fields.
J.J. Thompson: discovery of the electron by observation of charged particles –
namely, electrons – as they move through crossed fields.
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Crossed Fields: J.J. Thompson Experiment

Principle
 Charged particles are emitted by a hot filament at the rear of the evacuated tube.
 They are accelerated by an applied potential difference V.
 After they pass through a slit in screen C, they form a narrow beam.
 They then pass through a region of crossed fields, headed toward a fluorescent screen S,
where they produce a spot of light.
 The forces on the charged particles in the crossed-fields region can deflect them from
the center of the screen.
Fig. 28-7
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
Experimental Test
 Set E=0 and B=0 and note the position of the spot on screen S due to the
undeflected beam.
 Turn on
and measure the resulting beam deflection y.
 Maintaining , now turn on 𝐵 and adjust its value until the beam returns to the
undeflected position (with the forces in opposition, they can be made to cancel).
Total force on a charged particle:
F=q(E+vxB)
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
Deflection Formula
 The deflection of the particle at the far end of the plates is
where: v -- Particle’s speed; m -- Mass; q -- Charge; L -- Length of the plates.
 When the two fields in Fig. 28-7 are adjusted so that the two deflecting forces acting
on the charged particle cancel, we have
 Thus, the speed of the charged particles passing through the crossed fields can be
measured by
 Substituting v into the deflection equation and rearranging yield
Q: What about gravitational force?
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Crossed Fields: The Hall Effect

A beam of electrons in a vacuum can be deflected by a magnetic field.
Q. Can the drifting conduction electrons in a copper wire also be deflected by a
magnetic field?
 The Hall effect allows to find out whether the charge carriers in a conductor are
positively or negatively charged.
 The number of such carriers per unit volume of the conductor can also be
measured.
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
Consider a copper strip of width d, carrying a current i, whose conventional direction
is from the top of the figure to the bottom.
Fig. 28-8
(a)
The situation immediately after the
magnetic field is turned on. The
curved path that will then be taken
by an electron is shown.
(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, they would pile up on
the right side, and the right side
would be at the higher potential.
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•
Apply an external magnetic field
, pointing into the plane of the figure.
•
Each drifting electron in the strip will experience a magnetic force
right hand side (Fig.(a)).
•
The magnetic force will be given as
towards
vd -- drift speed
• Presence of magnetic deflection causes accumulation of electrons on the right hand
edge of the strip.
• This accumulation will develop a potential difference V between the right and left
edge (Fig.(b)).
•
V is called Hall potential difference, and will generate an electric field
that
•
This electric field will try to deflect the electrons toward left hand side.
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•
When the electric and magnetic forces are in balance
E  vd B
But,
where J (= i/A) is the current density, A (= ld) the cross-sectional area, e the
electronic charge, and n the number density of charge carries (their number per
unit volume).
Hall probe: infer B from E
•
Therefore,
where, l -- the thickness of the strip.
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Example 2: Potential difference set up across
a moving conductor
Fig.28-9 shows a solid metal cube, of edge length d=1.5 cm, moving in the positive y
direction at a constant velocity
of magnitude 4.0 m/s. The cube moves through a
uniform magnetic field o𝐵 of magnitude 0.050 T in the positive z direction.
(a) Which cube face is at a lower electric potential and which is at a higher electric
potential because of the motion through the field?
(b) What is the potential difference between the faces of higher and lower electric
potential?
Fig. 28-9
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Calculations:
(a). Some of those conduction electrons are deflected by
to the left cube face, making
that face negatively charged and leaving the right face positively charged.
Thus, the left face is at a lower electric potential, and the right face is at a higher
electric potential.
(b).
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A circulating Charged Particle

Consider a particle of charge magnitude |q|
and mass m moving perpendicular to a
uniform magnetic field 𝐵 at speed v.

The magnetic force continuously deflects the
particle, and since 𝐵 and are always
perpendicular to each other, this deflection
causes the particle to follow a circular path.

The magnetic force acting on the particle
has a magnitude of |q|vB. For uniform
circular motion
Fig. 28-10
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 Solving for r, the radius of the circular path is
 K.E. = mv2/2 = (qBr)2/2m ~ r2
See “cyclotron” on Text p. 748
-
T, f, w independent of v
until relativistic effect
sets in => Synchrotron
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Example 3: Uniform circular motion of a
charged particle in a magnetic field
Figure below shows the essentials of a mass spectrometer, which can be used to measure
the mass of an ion; an ion of mass m and charge q is produced in source S. The initially
stationary ion is accelerated by the electric field due to a potential difference V. Suppose
that in a certain trail B = 80.0mT and V = 1000V, and ions of charge q=+1.610-19C strike
the plate at x = 1.6254 m. What is the mass m of the individual ions?
Fig. 28-12
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Calculations:
Make sure to use a consistent set of units, SI units here
q - Coulomb, m – kg, v – m/s, B – Tesla, E – Volts/m
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420 m diameter Shanghai Synchrotron
Magnetron generates μwave power
Μicrowave oven
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Magnetic Force on a Current-Carrying Wire

A magnetic field exerts a sideways force on electrons moving in a wire.

This force must be transmitted to the wire itself. Charge of current carrier does not matter.
Fig. 28-14
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 Consider a wire segment of length L, carrying current i, placed in a uniform magnetic
field coming out of the page. All the conduction electrons in this segment will drift
past plane xx in a time t =L/vd .
In that time a charge will pass through that
plane and is given by
or
Fig. 28-15: shows what happens
inside the wire of Fig.28-14(b).
f = F / vol. = F / AL = i LB / AL = i B / A = j B
Where j = i / A = current density & f = force density
In general, f = j x B
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•
If the magnetic field is not perpendicular to the wire, the magnetic force is given by
Here
is a length vector that has magnitude
L and is directed along the wire segment in the
direction of the (conventional) current.
• If a wire is not straight or the field is not uniform
 Imagine the wire broken up into small straight segments.
 The force on the wire as a whole is the vector sum of all the forces on the
segments that make it up.
• In the differential limit,
Find the resultant force on any given arrangement of
currents by integrating equation over that arrangement.
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Example 4: Magnetic force on a wire
carrying current
A straight, horizontal length of copper wire has a current i =28A through it. What are
the magnitude and direction of the minimum magnetic field B needed to suspend the
wire, i.e., to balance the gravitational force on it? The linear density (mass per unit
length) of the wire is 46.6 g/m.
Calculations:
Fig. 28-17
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Torque on a Current Loop

With the knowledge that a force is exerted on a current-carrying conductor when the
conductor is placed in an external magnetic field, this force can produce a torque on a
current loop placed in the magnetic field.

A rectangular loop of wire, carrying a current and free to rotate about a fixed axis, is
placed in a magnetic field. Magnetic forces on the wire produce a torque that rotates it.
n vertically upward
B horizontal  θ = 90o – see #33
The two magnetic forces F and
–F produce a torque on the
loop, tending to rotate it about
its central axis.
Fig. 28-18
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•
Let us look at the net torque on the loop due to these forces. Consider a rectangular
wire loop of side lengths a and b carrying current i in a uniform magnetic field.
Fig. 28-19

The normal vector
magnetic field.

In (c),
(in (b)), is used to define the orientation of the loop in the
is shown at an arbitrary angle θ to the direction of the magnetic field 𝐵 .
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
For side 2, the magnitude of the force acting on this side is
F2 = ibB sin(90°-q) = ibB cosq = F4
F2 and F4 cancel out exactly.


Forces F1 and F3 have the common magnitude iaB. As Fig.(c) shows, these two forces
do not share the same line of action; so they produce a net torque.
For
N loops (or turns), when A=ab, the area of the loop, the total torque is
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The Magnetic Dipole Moment

Definition:
 The magnitude of
is given by
where: N -- Number of turns in the coil; i -- Current through the coil; A -- Area
enclosed by each turn of the coil.
 The direction of
plane of the coil.
is that of the normal vector to the
Use right-hand rule. That is, grasp the coil with the fingers
of your right hand in the direction of current i; the outstretched thumb of that hand gives the direction of .
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Magnetic Dipole in a Magnetic Field

The torque on the coil due to a magnetic field
In vector form:

Just as in the electric case, the magnetic dipole in an external magnetic field has an
energy that depends on the dipole’s orientation in the field:
U at minimum when θ = 0 (or μ||B)

From the above equations, the unit of
square meter.
can be the joule per tesla (J/T), or the ampere-
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Fig. 28-20
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Some Magnetic Dipole Moments
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Section 2.2
MAGNETIC FIELDS DUE TO
CURRENTS
(Halliday/Resnick/Walker Ch.29)
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Contents
•
Calculating the Magnetic Field due to a Current
•
Force Between Two Parallel Currents
•
Ampere’s Law
•
Solenoids and Toroids
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Calculating the Magnetic Field due
to a Current

A moving charge produce magnetic field and its magnitude and direction are given by
"Biot-Savart law“.

Figure 29-1 shows a wire of arbitrary shape carrying a current i. We want to find the
magnetic field 𝐵 at a nearby point P.
Fig. 29-1
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 Therefore, in vector form
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Magnetic Field due to a Current in a
Long Straight Wire
•
Consider a very long (infinite long) straight wire.
•
The magnitude of the magnetic field at a perpendicular distance R from the wire
carrying a current i is given by
(Note: the proof of the equation is attached in Appendix (the last slide))
•
The field lines of 𝐵 from concentric circles around
the wire. The lengths of the two vectors 𝐵 in
Fig.29-2 show the 1/R decrease.
Fig. 29-2
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
Right-hand rule: find the direction of the magnetic field set up by a current-length
element.
Fig. 29-4
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Magnetic Field due to a Current in a
Circular Arc of Wire
 Let us calculate magnetic field at the center of a circular arc of wire carrying current i.
 Magnitude of
, of each element at
the center of the arc is given as
Fig. 29-6
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•
The net field at the center can be computed by adding the sum of magnitude due to all
the segments
•
For a very small segment ds,
• The discrete summation can be written as an integral
• When arc is a complete circle ( = 2π), the field at the center of a circular wire will be
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Example 1: Magnetic field at the center of a
circular arc of a circle
The wire in Fig. 29-7a carries a current i and consists of a circular arc of radius R and
central angle π/2 rad, and two straight sections whose extensions intersect the center C
of the arc.
What magnetic field 𝐵 does the current produce at C?
Fig. 29-7
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Calculations:
ds || r  θ = 0
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Example 2: Magnetic field off to the side
of two long straight currents
Figure 29-8(a) shows two long parallel wires carrying currents i1 and i2 in opposite
directions. What are the magnitude and direction of the net magnetic field at point P?
Assume the following values: i1 =15 A, i2 = 32 A, and d = 5.3 cm.
Fig. 29-8
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Calculations:
In general, <P not 90o ,
split B1, B2 into x & y
components and add
them
i
  tan
 250
i2
1 1
The angle between the direction of 𝐵 and the x axis is
  450  700
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Force Between Two Parallel Currents
 Consider two such wires separated by a distance d and carrying currents ia and ib:
 Current ia produces the magnetic field 𝐵 , which causes a force 𝐹𝑏𝑎 on a length L
of wire b with current ib :
Where
Fig. 29-9
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 A general procedure for finding the force on a current-carrying wire:

Again, use this procedure to compute the force on
wire a due to the current in wire b
the force is directly toward wire b
Note:
The force depends only on I
NOT on the charge on current
carrier – electron or ion
the two wires with parallel currents
attract each other.
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Ampere’s Law
•
Gauss law is used to compute electric field in certain symmetric charge distribution,
similarly if the current distribution is symmetric.
Ampere's law can be used to find the magnetic field with considerably less effort.
• Ampere’s law:
• The loop on the integral sign means that the dot product is to be integrated around a
closed loop, called an Amperian loop. The current ienc is the net current encircled by
that closed loop.
• In the Figure, only the currents encircled by the loop, i.e., i1 and i2, are used in
Ampere’s law.
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
How to compute integral
?
Calculate
 For calculating ienc , choose any arbitrary direction as direction of Amperian loop.
 Fig. 29-12 shows how to assign a sign to a
current used in Ampere’s law:
Curl your right hand around the Amperian loop,
with the fingers pointing in the direction of
integration. A current through the loop in the
general direction of your outstretched thumb is
assigned a plus sign, and a current generally in
the opposite direction is assigned a minus sign.
For the above example
Fig. 29-12
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Magnetic Field Outside a Long Straight Wire
Carrying Current
•
Fig.29-13 shows a long straight wire that carries current i directly out of the page.
•
All of the current is encircled and thus all is used in Ampere’s law.
Fig. 29-13
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Magnetic Field Inside a Long Straight Wire
Carrying Current
•
Fig.29-14 shows the cross section of a long straight wire of radius R that carries a
uniformly distributed current i directly out of the page.
Fig. 29-14
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Example 3: Ampere’s Law to find the field
inside a long cylinder of current
Figure 29-15(a) shows the cross section of a long conducting cylinder with inner radius
a = 2 cm and outer radius b = 4 cm. The cylinder carries a current out of the page, and
the magnitude of the current density in the cross section is given by J = cr2 , with c =
3×106 A/m4 and r in meters. What is the magnetic field at a point that is 3.0 cm from
the central axis of the cylinder?
Fig. 29-15(a)
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Calculations:
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Solenoids and Toroids
(1): Magnetic Field of a Solenoid
•
•
Solenoid: a long, tightly wound helical coil of wire.
Assumption: the length of the solenoid is much greater than the diameter.
Fig. 29-16

Fig. 29-17
Fig.29-17: A vertical cross section through the central axis of a “stretched-out”
solenoid. The solenoid’s magnetic field is the vector sum of the fields produced by
the individual turns (windings) that make up the solenoid.
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
Fig. 29-19: Application of Ampere’s law to a section of a long ideal solenoid carrying a
current i. The Amperian loop is the rectangle abcda.
Fig. 29-19
n -- Number of turns per unit length of the solenoid.
Note: Magnetic field inside a solenoid depends only on the number of turns per unit length and
current, it is independent of the area (radius
64 of solenoid) of the loops.
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(2): Magnetic Field of a Toroid
•
Toroid: a (hollow) solenoid that has been curved until its two ends meet, forming a
sort of hollow bracelet.
Fig. 29-20
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•
What is the magnetic field inside a toroid?
 Consider a circular Amperian loop of radius r inside the toroid.
 If N is the total number of turns, according to Ampere’s law:
where i is the current in the toroid windings (and is positive for those windings
enclosed by the Amperian loop).
 The magnetic field inside the toroid at a distance r from the center of the toroid
ring
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Example 4: The field inside a solenoid
A solenoid has length L = 1.23 m and an inner diameter d = 3.55 cm, and it carries a
current i = 5.57 A. It consists of 5 close-packed layers, each with 850 turns along
length L. What is B at its center?
Calculations:
N
n  # turns per length 
L
B  o ni
 (1.26 10 6 )  5 
850
(5.57)
1.23
 0.024T
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Section 2.3
INDUCTION AND INDUCTANCE
(Halliday/Resnick/Walker Ch.30)
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Contents
• Faraday’s Law of Induction
• Lenz’s Law
• Induction and Energy Transfer
• Induced Electric Fields
• Inductors and Inductance
• RL Circuits
• Energy Stored in a Magnetic Field
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Two Experiments
Exp.(1): An ammeter registers a current in the wire loop when the magnet is moving
with respect to the loop (Fig.30-1).
The current produced
in the loop is called
induced current.
Fig. 30-1
Fig. 30-2
Exp.(2): An ammeter registers a current in the left-hand wire loop just as switch S is
closed or opened. No motion of the coils is involved (Fig.30-2).
•
Faraday summarized the results of these experiments in what is known as Faraday's
law of Induction.
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Faraday’s Law of Induction
• Faraday’s Law of Induction
•
Suppose a loop enclosing an area A is placed in a magnetic field
flux through the loop is
. Then the magnetic
•
If the loop lies in a plane, and the magnetic field is uniform and perpendicular to the
plane of the loop, then
•
The SI unit for magnetic flux is the tesla–square meter, which is called the weber
(abbreviated Wb):
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•
•
Faraday's law can be restated in quantitative form as
Induced emf in a coil:
 If there are N turns in a coil, the change in magnetic flux through the coil will
induce emf in each turn.
 The total emf induced in the coil will be the sum of these individual induced emfs.
 Assume that the coil is tightly wound (closely packed), so that the same magnetic
flux passes through all the turns, the total emf induced in the coil is
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Example 1: Induced emf in a coil due to a
solenoid
The long solenoid S shown (in cross section) in Figure has 220 turns/cm and carries a
current i = 1.5 A; its diameter D is 3.2 cm. At its center we place a 130-turn closely packed
coil C of diameter d = 2.1 cm. The current in the solenoid is reduced to zero at a steady
rate in 25 ms. What is the magnitude of the emf that is induced in coil C while the current in
the solenoid is changing?
Calculations:
Calculate the flux
Use Faraday's law
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Lenz’s Law
•
A rule for determining the direction of an induced current in a loop:
• Opposition to Pole Movement
 The approach of the magnet’s north pole increases the magnetic
flux through the loop, inducing a current in the loop.
 To oppose the magnetic flux increase being caused by the
approaching magnet, the loop’s north pole must face toward
the approaching north pole so as to repel it.
 The current induced in the loop must be counterclockwise.
Fig. 30-4
 If the magnet is moving away from the loop, the induced
magnet will try to attract it, and the induced current will be
clockwise.
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• Opposition to Flux Change: always resist the change in magnetic flux.
 If flux is increasing, the induced magnet will try
to decrease it.
 If flux is decreasing, the induced magnet will try
to increase it.
•
Once we know the direction of induced
magnetic field, we can figure out the
direction of induced current by using the
right hand rule.
Fig. 30-5
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Example 2: Induced emf and current due to
a changing uniform magnetic field
Fig.30-6 shows a conducting loop consisting of a half-circle of radius r = 0.20 m and
three straight sections. The half-circle lies in a uniform magnetic field that is directed
out of the page; the field magnitude is given by B = 4.0t2+2.0t+3.0, with B in teslas and
t in seconds. An ideal battery with emf εbat=2.0 V is connected to the loop. The
resistance of the loop is 2.0Ω.
(a) What are the magnitude and direction
of the emf induced around the loop
by field at t = 10 s?
(b) What is the current in the loop at
t = 10 s?
Fig. 30-6
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Calculations:
(a)
(b)
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Example 3: Induced emf and current due to
a changing non-uniform magnetic field
Fig. 30-7 shows a rectangular loop of wire immersed in a non-uniform and varying
magnetic field
that is perpendicular to and directed into the page. The field’s
magnitude is given by B = 4 t2x2, with B in teslas, t in seconds, and x in meters. The
loop has width W = 3.0 m and height H = 2.0 m. What are the magnitude and direction
of the induced emf around the loop at t = 0.10 s?
Fig. 30-7
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Calculations:
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Induction and Energy Transfers
•
Consider a rectangular conducting wire loop placed in a uniform magnetic field (into
the page).
• If the loop is pulled at a constant
velocity , one must apply a
constant force
to the loop.
The power is P=Fv.
•
As the loop is pulled, the portion of
its area within the magnetic field,
and therefore the magnetic flux,
decrease. According to Faraday’s law,
a current is produced in the loop. The
magnitude of the flux through the
loop is
Fig. 30-8
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•
The change in magnetic flux due to this motion will induce emf in the loop
• The induced current is
R = resistance of the loop
•
The net deflecting force is
•
The power dissipated in the resistance is given by
•
The power delivered by applied force is
•
The work that you do in pulling the loop through the magnetic field appears as
thermal energy in the loop.
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Induced Electric Fields
•
Presence of induced current in a conducting ring due to changing flux of magnetic field
implies that an induced electric field is present.
•
Thus, one can reformulate Faraday's law as follow:
•
This induced field is present even in the absence of the conducting ring.
Fig. 30-11
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A Reformulation of Faraday’s Law
•
•
Consider a particle of charge q0 moving around the circular path. The work W done on it
in one revolution by the induced electric field is W = q0 , where is the induced emf.
The work:
where q0E is the magnitude of the force acting on the test
charge and 2πr is the distance over which that force acts.
•
Rewrite Eq. in a more general expression for the work done on a particle of charge q0
moving along any closed path:
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Example 4: Induced electric field due to
changing magnetic field
Fig. 30-11
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Calculations:
(a)
(b)
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Inductors and Inductance
Inductor
• Any conductor capable of producing magnetic field is called an inductor (symbol:
•
).
Simple example of an inductor is a solenoid.
Inductance
•
•
•
If we establish a current i in the windings (turns) of the solenoid (treated as an inductor),
the current produces a magnetic flux ΦB through the central region of the inductor.
The inductance of the inductor:
The SI unit of inductance is the tesla–square meter per ampere (Tm2/A), i.e., henry (H).
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Inductance of a Solenoid
• Consider a long solenoid of cross-sectional area A, with number of turns N, and of
length l. The flux is
(n is the number of turns per unit length.)
• The magnitude of B is given by:
• The inductance per unit length near the center of a long solenoid is
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Self-Induction
•
Change in current in one loop induces an emf in other loop.
•
Similarly, if the two loops are part of the same coil, the induction still occurs
—
a changing current in one loop of a coil induces a back emf in another loop of the
same coil.
•
A changing current in a single loop induces a back emf in itself. This is called selfinduction, and the emf that appears is called a self-induced emf.
•
It obeys Faraday’s law of induction just as other induced emfs do.
Fig. 30-13
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RL Circuits
•
Consider an RL circuit (Fig.30-15). Suppose at time t=0 the switch is closed on a. The
current will start increasing in the circuit and this increase will induce an emf
across the inductor L. Thus, the equivalent circuit will be as shown in Fig.30-16.
Fig. 30-15
•
•
Fig. 30-16
The loop gives
Solution of the above equation gives the current through the circuit at any time t
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That is,
•
As time goes on,
•
Once a steady current is flowing through the inductor, and now if we throw the switch
from a to b, the current will not go to zero instantaneously.
• According to Kirchhoff's rule
Since
•
Solution of the above equation leads to
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Example 5: RL circuit, immediately after
switching and after a long time
The Figure shows a circuit that contains three identical resistors with resistance R = 9Ω,
two identical inductors with inductance L = 2 mH, and an ideal battery with emf ε = 18V.
a) What is the current through the battery just after the switch is closed?
b) What is the current through the battery long after the switch has been closed?
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Calculations:
(a)
(b)
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Energy Stored in a Magnetic Field
• According to Kirchhoff's rule
×i
 The left side of equation represents the rate at which the emf device delivers
energy to the rest of the circuit.
 The rightmost term represents the rate at which energy appears as thermal
energy in the resistor.
 Energy that is delivered to the circuit but does not appear as thermal energy
must be stored in the magnetic field of the inductor.
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= i Ldi/dt = i εL = electric power input
Note: This is the rate at which magnetic potential energy UB is stored in the magnetic
field.
This represents the total energy stored by an inductor L carrying
a current i.
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Example 6: Energy stored in a magnetic field
A coil has an inductance of 53 mH and a resistance of 0.35Ω.
(a). If a 12V emf is applied across the coil, how much energy is stored in the magnetic
field after the current has built up to its equilibrium value?
(b). After how many time constants will half this equilibrium energy be stored in the
magnetic field?
Calculations:
(a)
(b)
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