Static Electricity - · PDF fileStationary electric charges •Protons and neutrons are...
Transcript of Static Electricity - · PDF fileStationary electric charges •Protons and neutrons are...
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
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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
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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
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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
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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
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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
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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
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