SOURCES OF MAGNETIC FIELDS
The magnetic field lines inside the bar magnet point from the south pole toward the
north pole, whereas between the two charges of the electric dipole the electric field
lines are in the opposite directions.
• Magnetic Fields are produced by moving electric
charges.
• The magnetic field lines form closed “loops”. While
electric field lines start on positive charges and end on
negative charges.
• The unit of magnetic field is a derived unit of measure
called the “tesla” (T) in the SI System. The earth also
acts approximately as a bar magnet, with a magnetic
field of approximately 50𝜇𝑇 near the earth’s surface.
MAGNETIC FIELDS PRODUCED BY AN ELECTRIC CURRENT
Magnetic field lines circulate around a current-carrying wire, but how do we
determine in which direction they circulate?
PLOTTING MAGNETIC FIELDS AND FIELD LINES
MAGNETIC FIELDS AND SUPERPOSITION
• The qualitative pattern of the magnetic field lines in most other situations can be
understood using these results along with the principle of superposition:
The total magnetic field produced by two or more different sources is equal to the
sum of the fields produced by each source individually.
We can apply the principle of superposition to find the direction of the magnetic
field produced by a current loop.
Use the principle of superposition to sketch qualitatively the magnetic field lines near
two long, straight wires, each parallel to the x-axis and lying in the x-y plane. Assume
each wire carries a current I, but with the currents in opposite directions.
MAGNETIC FORCE ON A MOVING CHARGE
• Magnetic forces acts on electric charges, such as an individual electrons,
protons, or ions. The magnetic force depends on the velocity of the charge,
and it is nonzero only if the charge is in motion.
Formative quiz
1. ILLUSTRATE and DIFFERENTIATE the magnetic field lines
produced by a bar magnet and an electric field lines produced by
two opposite charges.
2. Use the principle of superposition to sketch qualitatively the
magnetic field lines near two long, straight wires, each parallel to
the x-axis and lying in the x-y plane. Assume each wire carries a
current I, but with the currents in opposite directions. (Step-byStep)
•Give the three key-points for the motion of a charged particle in
a magnetic field.
•Assume B = 2 T, v = 55 m/s, 𝜃 = 30°, compute the electrons 𝑭𝑩
and 𝑭𝒈𝒓𝒂𝒗 . Determine which is stronger between the force by
computing their ratio.
•Find the radius of the motion of the electron with v = 50m/s
perpendicular to the magnetic field of 1.5 T.
•State what is Ampere Law?
•Consider a wire with 𝐼 = 0.5𝐴 enclosed to a circular tube path with
diameter of 2𝑐𝑚. Find the magnitude of the magnetic field.
•A current loop has an area of 4𝜋𝑐𝑚2. If 𝐼 = 1.0𝐴, find the
magnitude of its magnetic field.
•What is the magnitude of the magnetic field of a solenoid with a
length of 20cm long, current of 1.0 Ampere and 20 windings?