What is the relationship between the directions of the field and the velocity vector when the force equals zero? (Note: There are actually two
directions in which you can set the velocity vector to achieve this, one 180° opposed to the other.)
For your next challenge, set the parameters in the simulation to maximize the amount of force on the particle. As with your first task, there are
two directions that result in a maximum force on the charge. You will find yourself clicking the up and down arrows on parameters like “Charge”
until you reach their maximum value in the simulation. The maximum amount of force you can achieve in this simulation is 1.68×1 0 í^8 newtons.
For your final task, change the charge from positive to negative while keeping the initial velocity the same. What effect does this have on the
magnitude and direction of the force?
If you are surprised by the results of any of these configurations, refer back to the section that discussed the effect of a magnetic field on a
moving charged particle.
28.10 - Determining the strength of a magnetic field
A magnetic field exerts a force on a moving charged particle. When the charge, velocity
and field are known, the direction and magnitude of that force can be calculated. This
process can also be reversed: Just as an electric field can be measured using a test
charge, the strength of a magnetic field can also be determined using a test charge.
The process is similar, but with a couple of twists.
In an electric field, a stationary positive test charge is used to determine the
electrostatic force at a given point. The amount of force is then divided by the test
charge to determine the electric field strength. The direction of the force on the test
charge indicates the direction of the field.
In a magnetic field, the force must be determined using a moving charge. Physicists
fire a charged particle through a magnetic field and measure how the force exerted on it
by the field alters its velocity. They then use Newton’s second law, together with the
observed acceleration and the mass of the particle, to calculate the force the field exerts
on it.
The angle between the velocity and the magnetic field vectors must also be known to
determine the field. (A compass can be used to determine the direction of the field.)
Once the force and angle are known, they can be used in the equation shown on the
right to find the strength of the magnetic field. This is the same equation shown earlier
to determine the amount of force exerted by a given magnetic field, but here it is solved
for the field strength.
Measuring a magnetic field
Use a moving test charge
Measure acceleration of charge
Newton’s 2nd relates acceleration, force
With force known, field can be found