Magnetic field and force IB DP Physics Study Notes - 2025 Syllabus
Magnetic field and force IB DP Physics Study Notes
Magnetic field and force IB DP Physics Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Physics syllabus with Students should understand
magnetic field lines.
Standard level and higher level: 8 hours
Additional higher level: 6 hours
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Magnetic force
∙The magnetic force can be demonstrated using two bar magnets, which are metallic bars that have north and south poles:
∙From a) and b) we see that like poles repel.
∙From c) and d) we see that unlike poles attract.
∙Both statements together are called the pole law.
∙Note the similarity with the charge law.
∙Because of their historical use for navigation, magnetic poles of detection devices are defined like this:
∙The pole labeled “North” is really seeking the North Pole.
∙The pole labeled “South” is really seeking the South Pole.
From the pole law we see that the north geographic pole is actually a south magnetic pole!
∙When we say “north-seeking” we mean that the north pole of a hanging, balanced magnet will tend to point toward the north geographic pole of the earth.
∙We call the lines along which the magnets align themselves the magnetic field lines.
∙The symbol B is used to represent the magnetic flux density and is measured in Tesla (T).
∙Note that B is a vector since it has direction.
∙By convention, the direction of the magnetic field lines is the direction a north-seeking pole would point if placed within the field:
∙Just as in any field, the strength of the B-field is proportional to the density of the field lines.
∙At either pole of the earth the B-field is thus the greatest.
Sketching and interpreting magnetic field patterns
EXAMPLE:
A bar magnet is a piece of ferrous metal which has a north and a south pole. Looking at the B-field about such a magnet, determine the north and the south poles.
SOLUTION:
∙By convention, the direction of the magnetic field lines is the direction a north-seeking pole would point if placed within the field.
∙The poles are as shown. Why?
∙By the pole law (S) is attracted to (N), and (N) is attracted to (S).
Sketching and interpreting magnetic field patterns
∙A bar magnet is a magnetic dipole because it has two poles, N and S.
∙Compare the field lines of the magnetic dipole with the electric dipole, which also has two poles, (+) and (-).
∙Externally, they are identical. How do they differ internally?
∙We can take an electric dipole and split it into its constituent monopoles:
FYI
∙An electric monopole is a charge.
∙Now we ask, can we do the same thing to a magnetic dipole?
Can we split a magnet and isolate the poles?
FYI
∙The answer is: No.
∙To date no one has succeeded in isolating a magnetic monopole.
∙Become rich and famous: Discover or create one!
Magnetic field caused by a current
∙Consider a current-carrying wire as shown.
∙If we place compasses around the wire we discover that a magnetic field is produced which is tangent to a circle surrounding the wire.
∙This is a strange phenomenon: Namely, the magnetic field lines do not originate on the wire. They encircle it. They have no beginning, and no end.
∙Furthermore, if we reverse the direction of the current, the magnetic field lines will also reverse their directions.
Determining magnetic field direction – straight wire
∙There is a “right hand rule” for a current carrying wire which helps us remember the direction of the B-field.
∙Imagine grasping the wire with your right hand in such a way that your extended thumb points in the direction of the current.
∙Then your fingers will wrap around the wire in the same direction as the B-field lines.
Determining magnetic field direction – straight wire
∙There are sketching conventions for drawing B-fields. They are as follows
EXAMPLE:
Using the drawing conventions just shown, sketch in the B-field for the current-carrying wire shown here.
SOLUTION:
∙Use the right hand rule to determine the direction, then sketch in the field symbols.
∙Note that on right the side of the wire the B-field enters the slide.
∙On the left side the B-field exits the slide.
FYI
∙The field gets weaker the farther you are from the wire. How can you tell from the picture?
Observe what happens if we bend a straight current-carrying wire into a loop:
FYI
∙The B-field inside a loop is stronger than outside.
Determining magnetic field direction – wire loop
∙There is a “right hand rule” for a current carrying loop which helps us remember the direction of the B-field.
∙Imagine placing the heel of your right hand on the loop in such a way that your fingers curl in the direction of the current.
∙Then your extended thumb points in the direction of the B-field.
∙Of course, you could just use the straight-wire RHR and grasp the loop itself, if you like.
Determining magnetic field direction – solenoid
A solenoid is just a series of loops stretched out as shown.
∙There is a RHR for solenoids.
∙With your right hand, grasp the solenoid in such a way that your fingers curl around it in the direction of the current-carrying loops.
∙Then your extended thumb points in the direction of the B-field.
∙Of course, you could just use the loop RHR and grasp the end loop itself, if you like.
Sketching and interpreting magnetic field patterns
∙The B-field looks like this around a solenoid:
∙Note the concentration of the B-field lines inside the solenoid, and the micro-loops close to the wires.
∙If we place an iron core inside the solenoid we have what is called an electromagnet.
∙The ferrous core enhances the strength of the B-field.
IB Physics Magnetic field and force Exam Style Worked Out Questions
Question
Electrons in a conductor are moving down the page. A proton outside the wire is moving to the right.
What is the direction of the magnetic force acting on the proton?
A. Down the page
B. Up the page
C. Out of the page
D. Into the page
▶️Answer/Explanation
Ans B
Question
Electrons are moving in a long wire that is normal to the plane of the paper. The electrons move into the paper.
What is the direction of the magnetic field at point P?
▶️Answer/Explanation
Ans C