static electricity - · pdf filestationary electric charges •protons and neutrons are...

Static Electricity

©cgrahamphysics.com 2016

Charges

• Two types of charges: positive and negative


We know

• Charge is conserved

• Charge is quantized

• Force between two point charges varies as the inverse square law of the distance between the two charges

• 𝐹 =𝑘𝑞1𝑞2

𝑅2

• 𝑘 = 𝐶𝑜𝑢𝑙𝑜𝑚𝑏 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = 8.99 × 109𝑁𝑚2𝐶−2

• 𝑘 =1

4𝜋𝜀0

• 𝜀0 = 𝑝𝑒𝑟𝑚𝑖𝑡𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑓𝑟𝑒𝑒 𝑠𝑝𝑎𝑐𝑒 = 8.55 × 10−12𝐶2𝑁−1𝑚−2


Stationary electric charges • Protons and neutrons are tightly held together in the

nucleus by the short range strong nuclear force

• Electrons are held in energy levels around the nucleus by the electromagnetic force

• Electrons can easily move under the right conditions

• A material can become electrically charged excess of electrons – negative charge deficiency of electrons – positive charge


Perspex rod is rubbed with a piece of silk


It has been found that…

• Ebonite rubbed with fur becomes negative

• Polythene rubbed with wool becomes negative

• Cellulose acetat rubbed with wool becomes positive

• Conclusion: like charges repel, opposite charges attract

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Repulsion

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Attraction

Pivot balance Pivot balance

Perspex rod Perspex rod

Perspex strip Ebonite strip


Definitions • Charging by friction redistributed charge on two

surfaces

• Law of conservation of electric charge: In a closed system the amount of charge is constant


Metals (conductors)

• Positive ions surrounded by a “sea of electrons”

• Many electrons are available for conduction

• Conductors have low electrical resistance

• If a conductors is held in hand, excess electrons will be transferred through the body to earth

• We say the conductor is earthed


Insulator • Electrons are held tightly by

atomic nuclei

• They are not free to move

• They can accumulate on the surface , but they are not conducting

• High electrical resistance

• When held, electrons remain on surface and cannot be conducted through a person

• Charge on insulator will remain for short period of time until it leaks off the surface or is discharged

Insulator Conductor


Energy band theorem • Valence or outer shell electrons are held in valence

bond that is full or partially filled with electrons. • There exist an upper energy band called conduction

band • The conduction band is empty • A forbidden energy gap exists between valence and

conduction band • For conductors, valence and conduction band overlap • For insulators, energy gap

between valence band and conduction band is large

• Electrons cannot move across the forbidden energy gap


static electricity - · pdf filestationary electric charges •protons and neutrons are...

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