MOvInG ChArGeS
& MaGnEtIsM
Classmate
-
BEST NOTES
-
All Previous
Year Q 's
ApNIKAkSHA_
• o
ignoring
✓
@
, tf MOVING CHARGES AND MAGNETISM s
s
Oersted’s experiments
Hans Christian Oersted noticed that a current carrying wire caused a noticeable deflection in a nearby magnetic
compass needle. On conducting a few experiments, he found that-
- The compass needle aligns tangentially to an imaginary circle at the center of which is the current carrying wire
and whose plane is perpendicular to the wire.
- Reversing the direction of current, reverses the direction of orientation of the compass needle.
- The deflection increases on bring the wire closer to the needle or increasing the current.
- Iron filings sprinkled around the wire, arrange themselves in concentric circles with the wire as the center.
Conclusion- Moving charges or currents produce a magnetic field in the surrounding space.
Note: Important convention-
Current emerging out of the plane is depicted by a dot ••
| going into the plane is depicted by a cross
Magnetic field (B)
:
Like static charges produce electrostatic field, moving charges produce both electrostatic and magnetic field.
It is a vector quantity| SI Unit- Tesla (T) or Webber meter-2 (Wb m-2)|cgs unit- Gauss(G) 1G = 10-4 T
Principle of superposition of magnetic field
Total magnetic field at point due to multiple sources is the vector sum of all magnetic field of each individual source.
Magnetic force (PYQ 2020, 2019, 2018, 2017, 2016, 2014, 2013, 2011)
1. Magnetic Force on a moving, charged particle
The magnetic force Fb on charge q, moving with velocity v in a magnetic field B is given as-
- It depends on q, v, and B. Force on a negative charge is opposite to that on a positive charge
- The magnetic force q (v × B) includes a vector product of velocity and magnetic field. So, force will always be
perpendicular to v and B. Direction of force is given by cross product rule or Flemings’ left-hand rule
- Since Fb is perpendicular to v, the work done by magnetic force is always zero.
i.e. in a magnetic field, speed of particle remains constant, only the direction of velocity can change.
- Magnetic force becomes zero when v = 0 i.e. only moving charges can experience magnetic force.
- Magnetic force depends on the frame of reference.
- Magnetic force becomes zero when v is parallel to B i.e. when particle is moving along or opposite to the field, it
experiences no magnetic force.
Case 1: v is perpendicular to B
If a charged particle moving with velocity v enters a '
Magnetic field B perpendicularly, the force acts such
T q
That the particle moves in a circular path (see fig) with ••
I
Be
The same speed v.
Let radius of circle be r. The magnetic force that the
→
Particle experiences provides the centripetal force B
'
FI • B
Required to move in the circle. Therefore, we can write-
q
E
(Where p is momentum, KE is kinetic energy and V is potential difference)
, Time period of the particle is independent of speed and radius of the particle i.e. if a charged particle of mass m and
charge q enters a magnetic field with different velocities, in each case its time period remains same.
Note: Use the following table to quickly solve the questions asking you to compare Time period and radius of various
charged particles in magnetic fields!
Proton Electron Alpha particle Deuteron
Charge +e -e +2e +e
Mass m m/1840 4m 2m
Important PYQs
¥¥¥÷i7
.
¥ Ques: An alpha particle is accelerated through a potential difference of 10kV and moves along the x-axis. It enters in
.
a region of uniform magnetic field B= 2 × 10-3 T acting along the Y-axis. Find the radius of its path. (mass of alpha
particle = 6.4 × 10-27 kg) (PYQ 2020) [3M]
Ans:
¥ Ques: An alpha particle and a proton of the same kinetic energy are allowed are in turn allowed to pass through a
.
magnetic field B, acting normal to the direction of the motion of the particles. Calculate the ratio of radii of the
circular paths described by them. (PYQ 2019) [1M]
Ans:
P
I
¥ Ques: a) Write the expression for the magnetic force acting on a charged particle moving with velocity v in the
.
presence of magnetic field B
b) A neutron, an electron and an alpha particle moving with equal velocities, enter a uniform magnetic field going
into the plane of the paper as shown in the fig. Trace their trajectories and justify your answer. (PYQ 2016)
Ans: a) F = q (v × B)
b) The neutron will not be deflected as it is neutral in nature. The electron and the alpha particle will go around
circular trajectories (since they enter perpendicular to the field). However, they will be deflected in opposite
directions as they are oppositely charged, however they both wont be able to complete their circular
trajectories as they will exit the field.
& MaGnEtIsM
Classmate
-
BEST NOTES
-
All Previous
Year Q 's
ApNIKAkSHA_
• o
ignoring
✓
@
, tf MOVING CHARGES AND MAGNETISM s
s
Oersted’s experiments
Hans Christian Oersted noticed that a current carrying wire caused a noticeable deflection in a nearby magnetic
compass needle. On conducting a few experiments, he found that-
- The compass needle aligns tangentially to an imaginary circle at the center of which is the current carrying wire
and whose plane is perpendicular to the wire.
- Reversing the direction of current, reverses the direction of orientation of the compass needle.
- The deflection increases on bring the wire closer to the needle or increasing the current.
- Iron filings sprinkled around the wire, arrange themselves in concentric circles with the wire as the center.
Conclusion- Moving charges or currents produce a magnetic field in the surrounding space.
Note: Important convention-
Current emerging out of the plane is depicted by a dot ••
| going into the plane is depicted by a cross
Magnetic field (B)
:
Like static charges produce electrostatic field, moving charges produce both electrostatic and magnetic field.
It is a vector quantity| SI Unit- Tesla (T) or Webber meter-2 (Wb m-2)|cgs unit- Gauss(G) 1G = 10-4 T
Principle of superposition of magnetic field
Total magnetic field at point due to multiple sources is the vector sum of all magnetic field of each individual source.
Magnetic force (PYQ 2020, 2019, 2018, 2017, 2016, 2014, 2013, 2011)
1. Magnetic Force on a moving, charged particle
The magnetic force Fb on charge q, moving with velocity v in a magnetic field B is given as-
- It depends on q, v, and B. Force on a negative charge is opposite to that on a positive charge
- The magnetic force q (v × B) includes a vector product of velocity and magnetic field. So, force will always be
perpendicular to v and B. Direction of force is given by cross product rule or Flemings’ left-hand rule
- Since Fb is perpendicular to v, the work done by magnetic force is always zero.
i.e. in a magnetic field, speed of particle remains constant, only the direction of velocity can change.
- Magnetic force becomes zero when v = 0 i.e. only moving charges can experience magnetic force.
- Magnetic force depends on the frame of reference.
- Magnetic force becomes zero when v is parallel to B i.e. when particle is moving along or opposite to the field, it
experiences no magnetic force.
Case 1: v is perpendicular to B
If a charged particle moving with velocity v enters a '
Magnetic field B perpendicularly, the force acts such
T q
That the particle moves in a circular path (see fig) with ••
I
Be
The same speed v.
Let radius of circle be r. The magnetic force that the
→
Particle experiences provides the centripetal force B
'
FI • B
Required to move in the circle. Therefore, we can write-
q
E
(Where p is momentum, KE is kinetic energy and V is potential difference)
, Time period of the particle is independent of speed and radius of the particle i.e. if a charged particle of mass m and
charge q enters a magnetic field with different velocities, in each case its time period remains same.
Note: Use the following table to quickly solve the questions asking you to compare Time period and radius of various
charged particles in magnetic fields!
Proton Electron Alpha particle Deuteron
Charge +e -e +2e +e
Mass m m/1840 4m 2m
Important PYQs
¥¥¥÷i7
.
¥ Ques: An alpha particle is accelerated through a potential difference of 10kV and moves along the x-axis. It enters in
.
a region of uniform magnetic field B= 2 × 10-3 T acting along the Y-axis. Find the radius of its path. (mass of alpha
particle = 6.4 × 10-27 kg) (PYQ 2020) [3M]
Ans:
¥ Ques: An alpha particle and a proton of the same kinetic energy are allowed are in turn allowed to pass through a
.
magnetic field B, acting normal to the direction of the motion of the particles. Calculate the ratio of radii of the
circular paths described by them. (PYQ 2019) [1M]
Ans:
P
I
¥ Ques: a) Write the expression for the magnetic force acting on a charged particle moving with velocity v in the
.
presence of magnetic field B
b) A neutron, an electron and an alpha particle moving with equal velocities, enter a uniform magnetic field going
into the plane of the paper as shown in the fig. Trace their trajectories and justify your answer. (PYQ 2016)
Ans: a) F = q (v × B)
b) The neutron will not be deflected as it is neutral in nature. The electron and the alpha particle will go around
circular trajectories (since they enter perpendicular to the field). However, they will be deflected in opposite
directions as they are oppositely charged, however they both wont be able to complete their circular
trajectories as they will exit the field.
