Download Magnetic Forces

4.5) Magnetic Forces
1) For a current carrying wire you use F = L I x B. For a free charge you use F = Q v x B.
An electron is moving through a uniform magnetic field that is down. What will happen to it if it is
moving: west?
up?
up and west?
2) Draw the forces and path of an electron moving to the right in the following magnetic fields.
Field into the page.
Field to the right.
Field Down
3) A magnet is brought near a black and white TV screen. Describe and explain what happens.
4) Charged particles can move really fast. A velocity selector can be used to measure this.
a) There are two charged plates producing a uniform electric field that will exert a force up on a
positive particle moving to the right. There will be two circular coils producing a uniform magnetic
field that will exert a force down on the particle. Place the field lines on the sides of the box and the
current lines around the box. Make a 3-D model.
b) Draw a ¾ view and a side view below. Use coloured arrows to clearly show the current, magnetic
field, magnetic force, electric field and electric force.
c) The strengths of the fields are adjusted so that the magnetic force is balanced by the electric force.
Solve for v. Calculate the speed if the electric field is 11 kV/m and the magnetic field is 0.20 T.
5) The charge to mass ratio of an electron was measured by J.J. Thompson using magnetic and electric
fields. Watch http://www.youtube.com/watch?feature=endscreen&v=JB6YT7mm9JQ&NR=1
a) Draw a diagram of the apparatus and label the given values.
b) The magnetic field causes the electrons to turn when the electric field is off. Use F = ma, to form an
equation for q/m.
c) The speed can measured using the electric and magnetic fields as shown in the question above.
Substitute the equation for the speed into the equation for q/m.
d) How do you calculate the strength of the electric field? Substitute this into the formula for v.
e) How do you calculate the strength of the magnetic field? Substitute this into the formula for v.
f) The video gives you all the necessary data to calculate q/m except the turns per length of the
electromagnet. Calculate what N/L must be using the universal constants for e, me and othe
permeability of free space (4 x 10-7 Tm/A).
6) Mass spectrometers use electric and magnetic fields to measure the masses of individual atoms and
molecules. Ions with a charge of -1 are accelerated by a potential difference of 250 V into a uniform
magnetic field of 1.00 T where their path curves with a radius of 1.35 cm.
a) Compare the set up to what Thompson used.
b) Use conservation of energy in the electric field to find an equation for mass.
c) Use F = ma to find another equation for mass.
d) You have two equations with two unknowns – mass and speed. Solve for m.
e) Use the data above to calculate the mass of the particle. (p. 398)
Textbook: 8.2, 8.3
p. 396 # 2-6, p.402 # 1 - 4, p. 435 # 10, 11, 16, 17