# Force on electric charge moving in a magnetic field

##### Intros
###### Lessons
1. The magnitude of the electromagnetic force
2. The direction of the electromagnetic force
3. The radius of the motion of moving charged particle in a magnetic field.
##### Examples
###### Lessons
1. The path of a charged particle in a uniform magnetic field is circular when the initial velocity is perpendicular to the field.

Which of the following are a valid expression for the radius of this orbit in terms of the magnetic field strength, and the particle's momentum and charge?

1. $Bqp$
2. $\frac{Bp}{q}$
3. $\frac{Bq}{p}$
4. $\frac{p}{Bq}$
1. A beam made up of ions of various charges and masses enters a uniform magnetic field as shown.

One type of ion is observed to follow path 2. Which path describes the one taken by an oppositely charged ion with twice the mass and twice the charge? (Assume all ions have the same speed.)

1. Path 1
2. Path 3
3. Path 4
4. Path 5
1. One method for determining masses of heavy ions involves timing their orbital period in a known magnetic field.
What is the mass of a single ion that makes 7.0 revolutions in 1.3 × 10-3$s$ in a 4.5 × 10-2 $T$ filed?
1. A positively charged object ($q$ =1.6 ×10-19 $C$) is traveling at 1.9 × 104m/s perpendicular to a 1.0 × 10-3 $T$ magnetic field. If the radius of the resulting oath is 0.40m, what is the object's mass?

1. 3.4 × 10-27 kg
2. 3.1 × 10-19 kg
3. 2.1 × 10-9 kg
4. 0.77kg