26.1 Action of Electric and Magnetic Fields on Matter

Chapter 26

A Key Point

Remember that current that moves through a magnetic field has a force that acts on it.

Flipping this around: If a conductor is physically moved through a magnetic field, a current is induced in the conductor.

Something to think about

Do we need a conductor to allow electrons to move through space?

More on this later….

Mass of the Electron

How do you measure something you can’t see or touch, and is so small to hardly exist at all???

Indirect Measurements!

Robert Millikan – American Physicist

Nobel Prize for Physics (1923) for his experiments on the photoelectric effect and on the charge carried by an electron.

The measurement of the electron's charge was achieved by Millikan and his famous experiment in 1909

Millikan’s oil drop experiment

This experiment is called the oil-drop experiment and it was the first successful scientific attempt to detect and measure the effect of an individual subatomic particle.

The values of E, the applied electric field, m the mass of a drop, and g, the acceleration due to gravity, are all known values. So it is “very” easy to obtain the value of q, the charge on the drop.

The charge q on a drop was always a multiple of 1.59 x 10-19 Coulombs.

This is less than 1% lower than the value accepted today: 1.602 x 10-19 C!

The mass of the electron is found indirectly from its charge (q), and its charge-to-mass ratio:

q/m

Charge-to-mass ratio

First measured by J.J. Thomson

1856 – 1940CRT (named the

Thomson q/m tube)

Thomson Tube

Forces exerted on the beam by

1. an electric field2. Magnetic field

These crossed electric and magnetic fields exert forces in opposite directions on the electrons

The electric field deflects the electrons upward (electric field intensity ) The force is qE

The magnetic field deflects the electrons downward The force is Bqv

When the beam is perfectly straight (undeflected), Bqv = Eq

EE

EEEE

E

Rearranging this equation:

Eqv

Bq

E

B

If the electric field is turned off: The magnetic field causes the electrons to

be pushed in a circular path• The field lines push perpendicular to the motion

of the motion of the electrons, causing centripetal acceleration.

The electrons follow a circular path with radius r.

2vBqv m

r

Solving for q/m

q v

m Br

Thomson was clever!

Thomson was able to measure the magnitude of the electric and magnetic fields, and used these to solve for velocity.

He then measured the amount of deflection when only the magnetic field was deflecting the beam of electrons… This allowed him to calculate r

Using r, Thomson then calculated the q/m ratio.

After many trials, he found this ratio to be 1.759 x 1011 C/Kg.

Using q = 1.602 x 10-19 C

19

11

31

1.602 10

1.759 10 /

9.107 10

qm

m

C

C Kg

kg

Thomson was able to repeat this experiment with positively charged particles (ions)

He used Hydrogen, heated it up and then pulled the protons through the tube

Mass of the proton: 1.67 x 10-27Kg

The masses of heavier gases were found by pulling off single electrons from their atoms e.g. helium, neon, argon

Isotopes

Thomson Actually observed two and sometimes

more than two dots on the screen in the CRT

This meant that there were atoms that had different masses, but the same properties!

He showed experimentally that isotopes existed!

Mass Spectrometer

Adaptation of the Thomson tubeUsed extensively as an analytical toolA substance of unknown materials

(elements) is heated to form a gasThe positive ions in the gas are

accelerated through the tube, and deflected by a magnetic field.

Each different ion will be deflected by a different amount (depending on mass).

These deflections are then compared to a set of controls (known elements)

2 2

2q v

m B r

•The radius r is found by measuring the distance between the two points

•These appear as dark marks on photographic film

Mass Spectrometers Very sensitive Can detect one molecule in 10 billion! Able to separate molecules of one-ten

thousandth of one percent.

Combustion subsystem:

sample dropper (a) combustion column (b) reduction column (c) gas traps (water and

optionally CO2) (d)

•Mass separation subsystem: •ion beam source (e)•flight tube (f)•magnetic beam deflector (g)•signal detectors (h)

Gas Chromatograph