AP Physics 2
Chapter 19 Summary
Magnetic Fields
Conventionally Magnetic Field lines point outward from the north pole and terminate inward at the south pole.
A stationary charged particle is not affected by a stationary magnetic field, however a charged particle moving through
a magnetic field experiences a force. The magnitude of this force is defined as:
Magnetic Force is given by the equation: F  qvB sin  where q is the charge on the particle, v is its velocity, B is the
magnetic field strength, and θ is the angle between the magnetic field and the motion of the particle.
Direction of the force is determined by the right hand rule.
Magnetic force on a Current Carrying Conductor
The force on a wire follows the same right hand rule and is given by the equation: F  BIl sin  where B is the strength
of the magnetic field, I is the current in the wire, L is the length of the wire, and θ is the angle between the field and the
wire.
Torque in a Current Carrying Loop
In a Current Carrying Loop, the two opposing forces will produce a torque and the force will be the greatest when B is
parallel to the plane of the loop.
This torque can be calculated using:   NBIA sin  where τ is the torque, N is the number of turns or loops in the
wire, B is the magnetic field strength, A is the area of the loop, and θ is the angle between the Magnetic field and the
line perpendicular to the loop.
μ is called the magnetic moment of the loop and is determined using:   IAN hence torque can also be calculated
using the equation:   B sin  .
Magnetic Field of a Long Straight Wire and Ampere’s Law
Current carrying wires actually produce their own magnetic field and the magnetic field surrounds the wire.
The magnitude of this field can be determined using: B 
0 I
, where B stands for the magnetic field strength, r is the
2r
distance from the wire of the point you are measuring, μ0=4πx10-7 Tm/A which is a constant known as the permeability
of free space.
Finding the Magnetic Field for More than One Wire
 If there are two wires you need to find the magnetic field for each wire
 Find the direction of the magnetic field for each wire
 If the magnetic fields are in the same direction you may add
 If they are in opposite directions subtract
Magnetic Force Between Two Parallel Conductors
The magnetic force between two parallel conductors is measure in N/m or force per unit length. This is determined by
the equation:
F  0 I1 I 2

where µ0 is the permeability of free space constant, d is the distance separating the 2 wires,

2d
and I1 and I2 are the currents in each wire.
The force is attractive if the currents are in the same direction and repulsive if in opposite directions.