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### Electric Currents

Chapter 18

Batteries

- History of the battery
- Electrodes and Terminals
- Convert chemical energy to electric energy
- Flashlights
- Computers
- Watches

Summarizer Questions

- What are the functions of batteries and generators?
- Name at least two differences between batteries and generators.

Electric Current

- Flow of Electrons
- Charge per unit time
- Symbol – I
- Unit = Coulomb/second = Ampere
- Charge is conserved
- Amount of charge is equivalent everywhere in the loop

Summarizer Questions

- The charge that passes through the filament of a certain light bulb in 5 sec. is 3 coulombs.
- What is the current in the light bulb?
- How many electrons pass through the filament of the light bulb in a time interval of 1 minute?

Electric Current Cont.

- Direction of current
- Conventional vs. Flow of electrons

- Connecting wires to terminal causes an electric field

Resistance

- Current and voltage are proportional
- Impedes the movement of charge
- Voltage drop

- Symbol – R
- Unit – Ohm (Ω)
- Ohm’s Law
- Ohmic vs. nonohmic materials

Example

- The bulb in a flashlight draws 300 mA from a 1.5 V battery.
- What is the resistance of the bulb?
- If the voltage the battery produces is dropped to 1.2 V, how would the current change? Assume the resistance of the bulb stays the same.

Resistivity

- What affects the resistance of a wire?
- Length
- Cross-sectional area
- Material
- Temperature – Superconductors

- Symbol – ρ
- Units = Ω*m

Example

- You want to connect an extra set of speakers to your stereo. Each wire must be 20 m long, and that’s a long way to run speaker wire.
- What is the diameter of the wire if you want to keep the resistance to 0.10 Ω?
- If the current is 4 A, what is the voltage drop across the wire?

Example

- At 20oC, the resistance of a platinum thermometer is 164.2 Ω. When we drop it into a solution, we measure the resistance as 187.4 Ω. What is the temperature of the solution?

Summarizer Questions

- Why are resistors used in circuit boards?
- Recent discoveries have led some scientists to hope that a material will be found that is superconducting at room temperatures. Why would such a material be useful?

Electric Power

- Energy per unit time
- Symbol – P
- Unit = Watt
- Connection to the electric company

Example

- Calculate the resistance of a 40 W automobile headlight designed for a 12 V automobile electrical system.

Example

- An electric heater draws 15 A on a 120 V line. How much power does it consume and how much does it cost if it operates 3 hrs per day for 30 days if the cost is 10.5 cents per kWh?

Summarizer Questions

- If electrical energy is transmitted over long distances, the resistance of the wires becomes significant. Why?
- A 40 W light bulb and a 75 W light bulb operate from 120 volts. Which bulb has the greater current in it?

Electric Safety

- Fuses
- Circuit Breakers
- GFCI
- Grounding

Types of Current

- Direct Current
- DC
- Drift Speed

- Alternating Current
- AC
- Peak Current/Voltage
- rms Current/Voltage

Examples

- Calculate the resistance and peak current in a 1000 W hair dryer connected to a 120 V system.
a. What is the resistance of the hair dryer?

- A home stereo has the ability to deliver 100 W of power per channel. What are the rms voltage and current at 100 W if the resistance of the speakers are 8 Ω?

Summarizer Questions

- What is the difference between direct current and alternating current? Which type of current is supplied to the appliances in your home?
- In a conductor that carries a current, which is less, the drift speed of an electron or the average speed of the electron between collisions?

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