Coulomb’s Law. Methods to electrically charge an object –Conduction: Direct contact: will transfer electrons, such as touching your car door in the winter.

Coulombrsquos Law

bull Methods to electrically charge an object

ndash Conduction bull Direct contact will transfer

electrons such as touching your car door in the winter

bull Friction rubbing your feet against carpet hair against a balloon

ndash Induction no direct contactbull Start with a neutral object Then bring an electrically charged object

near but not in contact with a neutral object

bull The charges in the neutral object will be ldquoinducedrdquo to separate to get closer or farther from the charged object

bull If provided a pathway the separated electrons will leave

bull The object is now positively charged

bull Electrostatics devicesndash Electroscope the

separation of metal leaves indicates the presence of static charge

ndash Van de Graaff generator charge is delivered by a rubber belt to a metal dome

ndash Electrophorus a device used to transfer electric charge

Coulombrsquos Lawndash Calculates the magnitude

of the electric force between two charges

ndash Each charge experiences equal but opposite forces

k is a constant k = 9 x 109 Nm2C2

221

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(Since we are interested in the MAGNITUDE of the force do not include the signs of negative charges)

Coulombrsquos Law looks VERY similar to Newtonrsquos Universal Law of Gravitation

221

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221

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Differences 1 Gravitational Force is based on MASS

Coulombrsquos law is based on CHARGE2 Gravity is ALWAYS an attractive force

The Electric Force can attract and repel3 ldquoGrdquo is a tiny number therefore gravity force is a relatively

small forceldquokrdquo is a huge number therefore electric force is a

relatively large force

Both laws are INVERSE SQUARE LAWS

ldquoThe Force varies with the inverse of the distance squaredrdquo

At twice the distance d2 = 22 in denominator

= frac14 the Force

At three times the distance 32 in denominator

= 19 the Force

At half the distance (12)2 in denominator

= 4 times the Force

Now if one CHARGE q doubleshellip The Force doubles since they are directly related

221

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qqkF

Get a calculator and letrsquos practice onehellip

What is the magnitude of the electrostatic force between two charges q1 = 32 mC and q2 = -248 mC separated by a distance of 24 mm (milli = 10-3 micro = 10-6)

USE THE EXPONENT BUTTON

F = 9E9 32E-3 248E-6 divide 24E-3 2

F = 124000000 N

or for Quest 124E8

or 124e8

221

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qqkF

RememberhellipForce is a VECTOR- it always points in a specific direction If more than two charges are present we must find the VECTOR sum of the forces acting on an individual charge

+-

+

Get a clicker and calculator Log in and get ready to answer some questions

1 Electric Charge is measured in

A Amps

B Volts

C Coulombs

D Farads

2 Opposite charges

A Attract

B Repel

C May attract or repel depending on their relative size

D Neither attract nor repel

3 Objects with a neutral charge are

A Attracted only by objects with a net positive charge

B Attracted only by objects with a net negative charge

C Not attracted by any kind of charge

D Attracted by objects with either positive or negative charge

The charge of one electron = - 16 x 10-19 C

4 How much excess charge is present in an object that has 3 x 1018 more electrons than protons (Use correct sign)

5 If the distance between two charges tripled the magnitude of the electrostatic force between them would change by a factor of

A 3

B 6

C 9

D 13

E 16

F 19

221

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qqkF

6 Robert Millikan measured the elementary charge of an electron by charging and then suspending oil drops using a charged plate as shown

At the most basic level which law of physics did Millikan apply to measure the charge of an electron

A Faradayrsquos Law of mutual inductance

B Coulombrsquos Law

C The law of conservation of Energy

D Newtons 1st Law

E Zeroth Law

-- - - - - - - - - - - -

7 The development of _______ at room temerature could revolutionize the electric industry

A Conductors

B Insulators

C Semiconductors

D Superconductors

8 The force between a 150 kg dog and the Earth is 1470 N How close must an electron and a proton be to generate the same force

A 990 X 10-7 m

B 980 X 10-13 m

C 396 X 10-16 m

D 157 X 10-31 m