CHAPTER 33 – ELECTRIC FIELDS & POTENTIALS

1. CHAPTER 33 – ELECTRIC FIELDS & POTENTIALS

2. 33.1 – ELECTRIC FIELDS • Action forces at a distance (no contact) are explained using fields • Mass creates gravitational fields • Charges  electric fields • These permeate all space • Mass & charge interact with fields & experience forces

3. How do charges sense other charges? • Michael Faraday suggested something called a field • To him it was real • He imagined “invisible lines of force” • It is these that cause coulomb attract/repulsion • Charges interact with electric field • Very much like masses w/I Earth’s gravitational field • This mechanism is responsible for enigmatic “action at a distance”

4. The electric field (E) is what causes forces on charges • A vector: magnitude and direction • Determined by positive “test charge” placed all around another charge • Then mapping the force strength & direction • The value = how much force a test charge experiences

5. The strength is determined using small positive test charge • Direction of field = dir. of force • Positive charge  field points away, negative  towards • The closer the lines are together, the stronger the electric field • E – field lines emanate from pos. charge & terminate on neg. (or infinity) • Parallel plates create a uniform electric field (constant force)

6. 33.2 – electric field lines

7. 33.3 – electric shielding • Charges placed on a conductor try to spread out as much as possible (they are repelled) • Any imbalance causes an E-field to be created, moving the charges • They evenly distribute on surface to minimize the repulsion & cause a cancelling of the E-field inside

8. A nonuniform object has nonuniform charge dist. • Charge tends to accumulate in regions of high curvature – like points • Since E-field inside conductor is zero, we can block electric signals inside – called shielding • Why radio reception can be poor

9. 33.4 – electrical potential energy • Recall: doing work against gravity ↑ potential energy • Gravity is a field (force) – causes a force on masses • In the same way, work is done pushing charges around on E-field • Gives charges potential energy • Stored energy can be converted into other energy (KE, Q)

10. 33.5 – electric potential • Rather than figuring energy, we find the energy per charge • The electric potential, or voltage (V) • Voltage = amount of energy each coulomb of charge has • A conductor of 1000 V req. 1000 J of energy to bring 1 C of charge from ∞ to it

11. A large voltage ≠ large energy • Usually a tradeoff  • high voltage, but few charges or • Low voltage & a lot of charges

12. 33.6 – electrical energy storage • Stored in a device – a capacitor • A battery or voltage supply does work, “pulls” e from one plate & deposits them on the other plate • As charge slowly accumulates, energy is stored in the E-field between plates until V = battery voltage • Is released all at once (discharged) • Energy ↑ when: plates size ↑, plates move closer together, or material placed between plates

13. 33.7 – The van de graaff generator • Used to generate high voltages • Friction between rubber belt and roller causes e to accumulate on belt • e are carried to metal dome & deposited on surface (E-field = 0 inside) • Buildup ↑ (millions of volts), until discharged through air • V ↑ by: ↑ radius of dome or surround by pressurized gas