Unit 13/Chapter 18 Electric Currents

1. 0.0 0.0 Durable Unit 13/Chapter 18Electric Currents

2. “Motion” of Positive Charges • Many texts define current as the direction that the “positive” charges move; however, there are not actually any “positive” charges that contribute to current flow. • The “positive” charges are actually holes vacated by electrons which do actually move and form current. • These holes may be thought of as positive charges; however, they do not move as they are “anchored” within the material. • Electrons move when a potential difference is applied across a conductor. • As an electron moves leaving behind a hole, another electron occupies the hole left behind by the first electron. • When this electron occupies this hole, it leaves behind another hole and the process repeats. • The positive charges appear to move; however, it is the electrons that actually move. • However, due to a long standing tradition in physics, we will select the opposite direction as that which the electrons actually move to be our current direction..

3. Conduction Electrons • In organic compounds, electrons are bound to specific atoms. • In metallic compounds, some of the electrons are not bound to a specific atom. • They are free to move throughout the metal. • These electrons are called conduction electrons. • If a potential difference is placed across the wire (like when you connect the wire to a battery), then the electrons will move. • As they move, the electrons collide with the metallic atoms. • Depending upon the number of collisions an electron has, it may move faster or slower through the metallic structure. • Remember, moving electrons in a wire are known as current.

4. Durable Batteries and Current • A complete circuit is one that connects a battery to an electrical component back to a battery. • Remember, electrons flow from the negative end of a battery through the light bulb (resistor) and back into the positive end of the battery. • By our standard, the current flows in the opposite direction. • The symbol “I” is used to denote current. • Current is the number of electrons passing a certain point in a circuit per unit of time. I

5. Durable Resistors • Resistors are used to control the amount of current flowing through a circuit. • Resistors impede the flow of electrons (current). • They impede this flow because certain electrical items have maximum limitations on the current they can handle. • Consider the Light Emitting Diode (LED) in the figure to the right. • The battery supplies too high a current to the LED. • As a result, the LED is damaged by the current. • When a resistor is placed into the circuit, the current is reduced to a level appropriate for use with the LED, and the LED is not damaged.

6. Ohm’s Law • Ohm’s Law gives us a mathematical expression relating the voltage (V), Current (I), and Equivalent Resistance (R) of a circuit. • The three forms of Ohm’s Law are listed below. • In this equation, R is the Resistance in Ohms (), I is the Current in Amps (A), and V is the Voltage in Volts (V).

7. Resistance and Resistivity • The resistance of a material is dependant on the resistivity of the material. • This relationship is expressed by Pouillet's law. • In this equation, R is the resistance in Ohms (),  is the resistivity in Ohm meters (m), L is the length in meters (m), and A is the cross sectional area in square meters (m2). • The values of resistivity for different materials may be found on page 501 of your textbook.

8. Resistance and Resistivity • Suppose a wire of length L and area A (assume a circular cross section) has a resistance of R1. • How much resistance (R2) would a second wire, made from the same material, have if its length was 2L?

9. Resistance and Resistivity • Suppose a wire of length l and area A (assume a circular cross section) has a resistance of R1 (in terms of the wire’s diameter). • How much resistance would a second wire, made from the same material, have if its diameter was halved (d2 = d1/2)?

10. Power Equation • Power is the rate at which electrical energy is consumed. • The equation used to determine power consumption is as shown. • This equation is used to calculate the Power consumed in Watts (W) by a resistor when the current and the voltage drop across the resistor are known. • Using Ohm’s Law, derive two more forms of the power equation.

11. 0.0 0.0 Durable Direct Current v. Alternating Current • In a direct current circuit (one with a battery) the electrons flow in one and only one direction. • In an alternating current circuit (like the electricity from your wall outlet), the electrons repetitively change directions. • Either way, the light remains lit because the electrons still move through it.

12. Direct Current • Batteries produce direct current (DC). • The graph below shows a plot of a direct current produced by a battery.

13. Alternating Current • Your home uses Alternating Current (AC). • This current is produced by generators in electrical power plants. • The graph below shows a plot of a alternating current.

14. Electrical Safety: Fuse • Two devices are used in household and vehicle applications in order to ensure electrical safety: a fuse and a circuit breaker. • When the current through the fuse pictured below exceeds a certain value, the metallic ribbon melts and opens the circuit. • A fuse must be replaced after preventing an electrical overload.. Metallic Ribbon

15. Electrical Safety: Circuit Breaker • When the current through a circuit breaker exceeds a certain value, it opens the circuit. • A circuit breaker can be flipped back on and does not need to be replaced. Bi-Metallic Strip

16. Durable Durable Durable Durable Sure Start Batteries • Batteries come in many shapes and sizes that have a variety of voltage and currents. • Identify the voltages of the four common battery types shown below.

17. Durable Batteries • This slide will explain how a battery provides electricity for use in your small electrical appliances. • Free electrons and “holes,” which are the absences of electrons, are produced within the battery due to electrochemical reactions.

18. Durable Durable Durable Durable Series Battery Configuration WS 6 #15-18 • Batteries, like other electrical components, may be connected in series or parallel configurations. • In a series configuration like the one below, the electrons flow from one battery into the next battery where they remain. • As a result, the current produced by combining batteries in series is equal to the current produced by just one of the batteries. • However, when batteries are connected in series, the voltages (electric potentials) are added. • What is the voltage produced by the four AA batteries shown below? • These four batteries each produce 1.5 V; therefore, the total voltage produced by these batteries is 6.0 V.

19. Durable Durable Durable Durable Parallel Battery Configuration WS 6 #15-18 • Notice what happens to the current when batteries are connected in parallel. • The current produced by parallel batteries combines resulting in a higher current. • AA batteries typically provide 500 mA. • What is the total current produced by the batteries below? • 2000 mA. • What is the total voltage of these four batteries? • The voltage is the same as that of a single battery. • The schematic diagram for these batteries would be as follows.

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22. Metallic Ribbon