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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?

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Electric Currents

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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?