Chapter 32 – Electric Forces

1. Chapter 32 – Electric Forces

2. 32.1 – Electric Forces & Charges • Electrostatics – nonmoving electric charges • All atoms composed of protons – positively charged, electrons – negatively charged & neutrons – no charge • Electrons (e) held on atom because of attraction to positive protons in nucleus • mproton~ 2000melectron • Neutral atom  # protons = # electrons

3. The Simple Model of the Atom

5. Electricity has attraction & repulsion (forces) • Like charges  repulsive, unlike  attractive • Pith balls and the electroscope are often used to demonstrate this

6. 32.2 – Conservation of Charge • Atoms (& objects) become charged when they gain or lose electrons • Gain e  negative charge (ion) • Lose e  positive charge (ion) • Objects become charged because of exchanges of e, gain of e means loss somewhere else – conservation of charge • Therefore, objects become charged as multiples of e (fundamental electric charge)

7. The outer e (valence) in atoms are not “held” very tight • Different materials can hold onto e stronger than others (a larger affinity, ie: rubber, plastic) • They can strip away e of another material (fur, glass) – giving them extra e & negative charge • The other material has absence of e, & is positive • Protons do not move to charge objects positively, they are “locked inside the nucleus

9. Triboelectric Series

10. 32.5 – Charging by Friction & Contact • Objects become charged in two fundamental ways: • By conduction – friction & contact • Induction- no contact • Rubbing two materials together can cause e to be pulled from one and placed on the other, ie: charged • Once an object is charged, touching something will cause some of the charge to transfer

11. 32.3 – Coulomb’s Law • Charles Coulomb discovered the math. Relationship between charges – another inverse square • q  magnitude of electric charge, unit = coulomb (C) • k  constant of proport., k = 8.99 x 109 Nm2/C2 • This makes electrical forces HUGE, 1 C is HUGE • Can be attractive or repulsive

12. Since everyday objects are neutral (protons = electrons), there are no electrostatic forces • At the atomic level, atoms bond together because of giving up/taking e (ionic) or sharing e (covalent & metallic)

13. 32.4 – Conductors & Insulators • Some materials allow e to flow easier than others – conductors – e are held weakly to nucleus • Generally metals • Others materials, e have difficult time moving – insulators – e held strongly to nucleus • Rubber, glass • All charges placed on conductors spread out evenly • Those placed on insulators stay in location of placement

14. Some materials are good insulators & then when energy is added to the material, become conductors – semiconductors • Used to make transistors that act as tiny switches or amplify electric signals

15. 32.6 – Charging by Induction • Because of electrostatic forces, charged objects brought near conductors move charges on surface – they are induced to move • Object is still neutral, but charge is nonuniform on surface • Contact with another object causes charges to leave first – giving a charge to it

16. Charge can leave one side of induced object by grounding it – providing a path for charges to, go to/come from, earth • Can be a wire connected to earth or touching it • Clouds can become charged (friction) inducing charge on the surface - Franklin • Also discovered charge collects at points – lightning rod • Allows for easier discharge from surface to cloud • Lightning strikes rod instead of building

17. 32.7 – Charge Polarization • It is possible for an insulator to pretend to become positive • Since charges are not allowed to move, they can only realign • Sides of atoms/molecules have a temporary charge – electrically polarized • Some molecules come polarized (water) – electric dipoles