Chapters 16 & 17

Physics 101

Combined Chapters on Electricity and Magnetism

Introduction to Hyperphysics

• Do a “Google Search” for hyperphysics• http://hyperphysics.phyastr.gsu.edu/hbase/HFrame.html

Hyperphysics Introducory Page

Electricity in the Atmosphere

Note that the earth (ground) is normally negatively charged. The person to the right in the diagram distorts the electric field and in reality the person’s head is at ground potential.

Electric Currents in the Atmosphere

• A small current passes from the sky to the earth. It is on the order of 10 A

• Where do these charges come from?• Ans: Ionizations produced by cosmic rays• The balloon experiment proved it did not come

from the earth’s natural radioactivity.• Atmospheric physics became an interesting

branch of physics.• How does the atmospheric electric field continue

to exist

Typical Electric Current on a Clear Day

The daily variation of the atmospheric electric potential with time (time is shown as GMT or “Zulu Time”).

The potential has its peak value at 7:00 PM no matter where you measure it: Atlantic Ocean, Pacific Ocean, Arctic Ocean, etc.)

What Maintains the Negative Charge on the Earth?

• There must be a source of large negative currents which must be flowing from the top of the atmosphere to the surface of the earth to keep it charged negatively!

• Where are the “batteries” to do this?• Can you think of any mechanisms?

The answer is LIGHTNING!

Lightning storms carry negative charges to the earth. There are about 300 thunderstorms per day all over the earth!

The total amount of thunderstorm activity is highest on the whole earth at 7:00 PM Zulu Time (GMT).

With voltages of hundreds of millions and currents in the thousands amperes these lightning strikes maintain the earth’s negative charge.

Atmospheric Physics is exciting to study. We still do not know everything about the atmosphere!

And now you know “The rest of the story”.

A comparison of Electrical and Gravitational Forces

2

21

r

mmGF

2

21

r

qqkF

G = 6.6 x 10-11 m3 / kg s2

k = 9 x 109 N m2/C2

Gravity or Electricity?

Gravity 1 x 10-47 N Electrical 2.3 x 10-8 N

Electricity Wins Hands Down!