Ch. 27 – Current Electricity

1. Ch. 27 – Current Electricity

2. e- e- e- Sears Diehard Current = flow of charge • If we connect the ends of a wire to a car battery, electrons will continuously flow in the wire from the negative side of the battery to toward the positive side. . . around and around • The flow of electrons is called a current and this set-up represents an electric circuit!

3. electron flow Electric Current, (I) • Current is a physics term that measures the rate of electron flow over or through a material

4. Electricity happens instantly? • When you turn on a light switch, the light comes on almost instantaneously. You might think electrons are moving at incredible speed • Actually, the electric field created by the car’s battery happens near the speed of light, but individual electrons travel through the circuit very slowly

5. Electrons undergo repeated collisions with the atoms in the conductor—a complicated zigzag motion occurs Actual speed of electrons in current in typical wiring (“drift speed”) is about 1 linear foot per hour Electrons transfer energy to the atoms during the collisions, which increase the random thermal energy of the atoms… the conductor gets warmer! atoms electron Speed of charges in a current low voltage side high voltage side

6. Think of the motion of electrons in a circuit like balls in a pinball machine • Pinballs “drift” downward over time, because of gravity, even though their motion is erratic and random • Electrons “drift” from high voltage toward low voltage because of the electric field

7. Electric circuits • A closed circuit provides a continuous path for current to circulate around • Electrons transfer some of their energy as they pass through a resistor like the light bulb (the filament heats up) • The battery is a pump that reenergizes the electrons each time they pass through it

8. e- e- “current” Electron flow vs. “current” • The electrons flow from the negative side of the battery to the positive side • The “conventional current”flows in the opposite direction of the electrons (this is Ben Franklin’s “fault”) Franklin described current as the flow of positive charge 100 years before it was realized that it was the electrons that were actually in motion, not the protons!

9. Voltage, (ΔV) • A voltage difference is what causes electrons to flow through a conductor • It is analogous to water pressure in a pipe, which is necessary for water to flow • A battery (or other power supply) is necessary to provide the voltage difference. It is like a pump, giving the energy needed for electrons to flow

10. A water analogy Water flow = Pressure difference = Thickness of water hose = High potential energy Flow of water Low potential energy

11. Street trolleys draw current from overhead power lines (which are at different voltages)

12. 2 things are needed to have a current • You can have a (1)voltage difference, but without a (2) connection path between the high and low voltage locations, there wouldn’t be a current yet ΔV = 120 volts Path is completed if wires touch

13. Not too bright Homer’s body is “grounded” V2 = 0 volts V1 = 120 volts at the slot Paper clip

14. A good insulator is just a poor conductor… • …but can still conduct electricity nonetheless! • Even an insulator (like air) can be the path for current, but this is not usual. A very large voltage difference is required first High voltage cloud An enormous, but very brief “current” zero voltage ground

15. 2000 V 2000 V • The amount of resistance in the circuit • Good conductors (metals) have low resistance • Poor conductors (glass, plastic, . . .) have high resistance What does current depend on? • The amount of voltage difference • That’s why birds don’t get shocked on an uninsulated wire • No voltage difference means no current!

16. Ohm’s law

17. Δv ΣF = = a a t m Formulas are your friends Factors which determine magnitude Definition

18. If a ghetto blaster draws 2 amps of current when on (powered by eight 1.5-volt batteries), what is the resistance of the circuit? Example 1 • If a 120-volt circular saw has a resistance of 24 ohms, how much current will it draw?

19. Ohm’s law, I = ΔV / R • Ohm’s law is not really a fundamental law of nature, but more like a useful rule • Materials which follow this linear rule are called “ohmic” as opposed to “non-ohmic” Ohmic (I = ΔV/R) Voltage Non-ohmic I  ΔV/R Resistance

20. What does resistance depend on? • The amount of resistance in a wire depends on four factors: • Material - resistivityρ (“rho”) • Length of conductor L • Cross-sectional area A • Temperature

21. Resistivity,r • Resistivity is a property of the material. It describes how much a material resists the flow of electrons • Copper = low resistivity this is why it’s used for wires and extension cords • Nichrome = high resistivity (relatively speaking, for a conductor) used for toasters heating coil and light bulb filaments • Glass = high resistivity

22. ρL = R A ρ(.3 m) = .143 Ω π(0.001 m) 2 = - ρ 1.50 10 Ωm 6 × Example 2 • A voltage difference of 1.5 V is found to produce a current of 10.47 amperes • What is the resistance of the wire? • The wire is 30 cm long and is 1 mm thick. What is the resitivity of the wire? R = .143 Ω

23. filament support wires Inside a light bulb • QUESTION • Which part of the conducting path has the greatest resistance? • ANSWER • Thefilament is thinner and more difficult for electrons to move through. Therefore, electrons lose more energy traveling through the filament → it gets hot

24. Filament thickness QUESTION Which has more resistance, a thinorthickfilament?

25. Resistance question QUESTION Which has more resistance, 300 inches of extension cord or 30 inches of filament?

26. Electrons = energy delivery • The electron’s role in a circuit is to transport energy from the battery to the electrical devices

27. Power • The amount of energy delivered per second is called power • This is the same definition as from earlier in the year (remember racing up the bleachers?)

28. Brighter light bulbs! More work done in same amount of time 25-W bulb vs. 100-W bulb 100 joules of energy converted into light per second Only 25 joules of energy converted into light per second

29. Power, P • An alternate way to think of power is to consider the factors which would convert more energy in less time • The power dissipated by a light bulb (or any other electrical device) depends on the voltage difference across it and the current through it

30. Thick filament = brighter • A thick filament has less resistance. Therefore, more current flows through it for a given voltage difference • This means more power output → brighter 100 watt bulb 25 watt bulb

31. Example 3 • How much current is drawn by a 60-W light bulb when connected to a 120-volt power line? • What is the resistance of the bulb?

32. Joule = watt • sec Energy used, in kilowatt·hours • A joule is too small to measure your household energy consumption, so HECO uses units of kiloWatt·hours instead 1kWh = 3,600,000 joules

33. My energy bill

34. Example 4 • How much energy does a 550-W toaster use in the morning if it is in operation for a total of 10 min? (Give answer in kWh.) • At a cost of 12 cents/kWh, how much would this add to your monthly electric energy bill if you made toast four times per week?