WELCOME TO PERIOD 14. Homework Exercise #13 is due today. Watch video 3 Edison s Miracle of Light for class discussion next Tuesday or Wednesday.

Transcription

1 WELCOME TO PERIOD 14 Homework Exercise #13 is due today. Watch video 3 Edison s Miracle of Light for class discussion next Tuesday or Wednesday.

2 PHYSICS 1103 PERIOD 14 What is an electric circuit? How do capacitors store energy? Why are capacitors useful? Remember to put away your phone. No calls or texting during class.

3 Tin can voltmeter Place negative charge on the inner Leyden jar V 0 V

4 Measuring voltage across a battery 1) Turn the dial to the voltage symbol V 2) Attach the two wire leads to the two outlets on the lower right of the meter. V 3) To measure voltage across a battery, touch the ends of the leads to each end of the battery. V W COM

5 Measuring voltage across a bulb 1) Turn the dial to the voltage symbol V V V W 2) Attach the two wire leads to the two outlets on the lower right of the meter. 3) To measure voltage across a bulb, connect the ends of the leads to each side of the bulb. COM

6 Voltage boosts and drops Voltage Boosts The potential energy per unit of charge, or voltage, increases when charge flows through a battery. When batteries are connected in series, charges get a voltage boost from each battery. Voltage Drops Voltage drops occur as current flows through load devices (resistors) in the circuit. The voltage boost from the battery is divided among the load devices in the circuit. The sum of the voltage boosts and drops in a closed circuit are equal. In a circuit, VOLTAGE BOOSTS = VOLTAGE DROPS

7 Electric charge does work The attraction or repulsion between charged objects results in an electrical force on the objects. Electrical forces store electrical potential energy when work is done to push charges of the same sign together. Electrical potential energy is converted into electrical energy when the charges are allowed to move apart. This electrical energy can be used to do work, such as lighting a bulb.

8 Electric circuits A closed circuit has a complete conducting pathway. A closed circuit allows current to flow through the circuit and light the bulb. Current cannot flow through an open circuit. The bulb does not light. Filament Conducting pathways Insulating ring

9 Example of an electric circuit Negative charge flows from the inner Leyden jar, through the bulb filament, and onto the positively charged outer jar. This flow of charge is an electric current.

10 Electric current Current = amount of charge moved time I Q t I = current (in amperes) Q = charge (in coulombs) t = time elapsed (in seconds)

11 Measuring current through a bulb 1)Turn the dial to the current symbol ( A ). A 10 A 2) Place one wire lead in the bottom right outlet (marked COM ). Place the other lead in the top left outlet (marked 10 A ). 3)To measure the current through the bulb, clip the ends of the leads into the circuit. COM

12 Storing charge on capacitors A capacitor stores positive and negative charge on metal plates. An insulating layer keeps the positive and negative charges separated. The charge-holding capacity of a capacitor is called its capacitance. Capacitance is measured in units of farads. The greater the capacitance, the more charge Q can be stored at voltage V Q C V Q = charge (in coulombs) C = capacitance (in farads) V = voltage (in volts)

13 A capacitor example: Leyden jars Positive charge is stored on the outer metal Leyden jar and negative charge is stored on the inner jar. The insulating plastic cup keeps the positive and negative charge separated.

14 Separating Leyden jars When Leyden jars are separated, the voltage across the jars increases. Predict what will happen to the capacitance when the jars are separated. Hint: Q = C V

15 Capacitors vs. batteries (Activity 6) b) Charge per capacitor: Q = C V = 1 farad x 1.5 V = 1.5 coul c) Battery charge per hour: 0.8 coul x 3,600 sec = 648 coul sec hour hour d) How many charged capacitors equal the energy provided by one battery in one hour? 648 coul x 1 capacitor = 432 capacitors!!! hour 1.5 coul hour

16 Charge Q (in microcoulombs) Capacitor discharge Capacitors discharge their stored charge exponentially. The amount of charge in the capacitor decreases by ½ during each time period Time (in seconds)

17 Electrical potential energy of a capacitor Electrical potential energy is the product of the amount of charge and the voltage of that charge. E pot = Q V But the electrical potential energy of a capacitor is: E cap = ½ Q V Why? The equation uses the average voltage of the charges. Initially, the voltage of the charges is very low. As the capacitor fills, the voltage increases. The factor of ½ in the equation comes from taking the average of the initial and final voltage of the charges.

18 Capacitor discharge a) Does the capacitor discharge more quickly through the rod or through the bulb? b) In which case is more energy released? c) In which case is more power produced? Power = Energy time

19 BEFORE THE NEXT CLASS Read textbook chapter 15 Complete Homework Exercise 14 Bring a blank Activity Sheet 15 to class. Watch video 3 Edison s Miracle of Light for discussion during period 15