Studio Session 5

Magnetic fields

Moving charged particles produce magnetic fields and are acted on by magnetic fields.  Currents are moving charges and therefore they produce and are acted on by magnetic fields.  Permanent magnets are the result of "magnetization currents" flowing inside the material.  During this session you will use a compass needle to measure the magnetic field produced by a current-carrying wire and you will observe the deflection of electrons in a magnetic field and use this deflection to determine the electron's charge to mass ratio.

Equipment Needed:

Open a Microsoft Word document to keep a log of your experimental procedures, results and discussions.  This log will become your lab report.  Address the points highlighted in blue.  Answer all questions.


Currents produce magnetic fields and current-carrying wires are deflected in magnetic field.

power suppliesExperiment 1

Use the "Low Voltage" power supply.  Before you turn it on, turn the voltage knob all the way up and the current knob all the way down.  After you have connected a load, you can then control the current passing through the load by slowly turning the current knob clockwise.

(a)  Let the wire cross the needle at right angle as shown.
the wire crosses the needle at right angl     the wire crosses the needle at right angl
Slowly turn up the current to ~ 3 A.  Do you see any deflection of the needle? 
Turn the current back down before the wire gets hot.

Record your observation.  Is the needle deflected?  Do you expect a deflection?  Why or why not?
Switch the direction of the current flow by switching the red and black lead at the power supply.
Record your observation.  Is the needle deflected?  Do you expect a deflection?  Why or why not?

(b)  Now align the wire with the needle. 
he wire aligns with the needle     he wire aligns with the needle
Let the current pass over the needle from North to South.
Slowly turn up the current to ~ 3 A.  Do you see any deflection of the needle?  T
urn the current back down before the wire gets hot.

Record your observation. Is the needle deflected?  Do you expect a deflection?  Why or why not?
Let the current pass over the needle from South to North and repeat.
What do you see?  Can you explain your observation?


vector addition of magnetic fieldsNear Knoxville, TN, the strength of the Earth magnetic field is ~ 53 microT.  It has a declination D (deviation from North) of about 3o and an inclination I (downward tilt) of ~ 65o, so its horizontal component has a magnitude of approximately 53 microT*cos(65o) = 22 microT.
Earth magnetic field

The magnetic field of the wire and the Earth magnetic field are vectors and add vectorially.  If the wire is aligned with the compass needle when no current flows through the wire, and the magnitude of average field due to the wire at the compass needle is approximately equal to magnitude of the horizontal component of the Earth field, you should see a deflection of ~45o.

Can you deflect the needle by ~ 45o with a current of less than 4 A?  Would you expect to be able to? 
What current do you need to produce a magnetic field of magnitude 22 microT at a distance of the needle from the wire?

(Review:  The magnetic field produced by a steady current flowing in a  long straight wire)


Activity 1

Magnetic fields exert forces on other moving charge.  The force a magnetic field exerts on a charge q, moving with velocity v, is called the Lorentz force.
It is given by F = qv B

Assume a charged particle is moving with velocity v through a region with magnetic field B.  Predict the direction of the magnetic force for each situation below.   Assume that the particle is positively charged.  Record your predictions in your log.

(a)B and v(b)B and v

(c)B and v(d)B and v

Experiment 2

Mass spectrometry has become an important measurement tool in clinical chemistry, microbiology, toxicology and in the pharmaceutical world.  A mass spectrometer deflects ionized and accelerated molecular fragments using a magnetic field and sorts them according to their charge to mass ratio.  You will use the PASCO e/m apparatus like a mass spectrometer to determine the electron's charge to mass ratio, e/m, by measuring the radius of curvature of an electron's path in a uniform magnetic field of known strength.

We have two different models of the Pasco e/m apparatus.  Identify the model provided to your group and then choose the appropriate write-up for this model.

PASCO Model SE-9638   PASCO Model SE-9629 
PASCO Model SE-9638
Got to the write-up for this apparatus!
Complete the experiment and answer
all questions in blue font.
PASCO Model SE-9629
Got to the write-up for this apparatus!
Complete the experiment and answer
all questions in blue font.

Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.