Download presentation

1
**College Physics, 7th Edition**

Lecture Outline Chapter 17 College Physics, 7th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc.

2
**Units of Chapter 17 Batteries and Direct Current**

Current and Drift Velocity Resistance and Ohm’s Law Electric Power © 2010 Pearson Education, Inc.c

3
**Big Questions What is electric current and how does it travel?**

What causes current to move when switched on?

4
**17.1 Batteries and Direct Current**

What is required to produce a flow of charge? Voltage Energy is required to move electrons. 2 examples Any device that can produce and maintain a potential difference (voltage) is called a power supply.

5
**17.1 Batteries and Direct Current**

A battery is a source of electric energy—it converts chemical energy into electric energy. Let’s discuss how the battery works. © 2010 Pearson Education, Inc.c

6
**17.1 Batteries and Direct Current**

In a complete circuit, electrons flow from the negative electrode to the positive one. The positive electrode is called the anode; the negative electrode is the cathode. A battery provides a constant source of voltage—it maintains a constant potential difference between its terminals. © 2010 Pearson Education, Inc.c

7
**17.1 Batteries and Direct Current**

A battery can do work on, and transfer energy to, the electrons in the wire, which then delivers that energy to other circuit elements. Energy can be converted into different forms: Heat Light Mechanical

8
**17.1 Batteries and Direct Current**

The potential difference between the battery terminals when the battery is not connected to anything is called the electromotive force, emf. Name is misleading is because not a force. Represents work done per charge. J/C Represents maximum potential difference across terminals. © 2010 Pearson Education, Inc.c

9
**17.1 Batteries and Direct Current**

The actual voltage of the battery is always less than the emf, due to internal resistance. This “operating voltage,” is called terminal voltage. Usually the difference is very small. Let’s think about an example. © 2010 Pearson Education, Inc.c

10
**17.1 Batteries and Direct Current**

When batteries are connected in series, the total voltage is the sum of the individual voltages. (Positive ends are connected to negative ends) © 2010 Pearson Education, Inc.c

11
**17.1 Batteries and Direct Current**

When batteries of equal voltage are connected in parallel, the total voltage does not change; each battery supplies only a fraction of the total current. The voltage is the same for all batteries. (+’s connected to +’s) © 2010 Pearson Education, Inc.c

12
**17.1 Batteries and Direct Current**

© 2010 Pearson Education, Inc.c

13
**17.1 Batteries and Direct Current**

The figure to the right shows a circuit diagram that represents 2 identical batteries connect in parallel to a lightbulb. It is assumed that the wires have no resistance. What happens to the voltage when S1 is opened? a.) The voltage remains the same as that before the switch was opened. b.) The voltage drops to half because only one battery is now connected. c.) The voltage drops to zero.

14
**17.2 Current and Drift Velocity**

To sustain an electric current requires a _________ source and a ________ circuit. A complete circuit is… Open switch Closed switch

15
**17.2 Current and Drift Velocity**

Current is the time rate of flow of charge. SI unit of current: the ampere, A © 2010 Pearson Education, Inc.c

16
**17.2 Current and Drift Velocity**

Electrons move away from… This is opposite of the current’s direction. Current is one in which positive charges flow. A circuit draws current from a battery. A battery can only deliver current in 1 direction. 1 directional current is called… © 2010 Pearson Education, Inc.c

17
**17.2 Current and Drift Velocity**

Suppose there is a steady current of 0.50A in a flashlight bulb lasting for 2.0 min. How much charge passes through the bulb during this time? How many electrons does this represent?

18
**17.3 Resistance and Ohm’s Law**

The greater the voltage, the greater the current. However, resistance influences current as well. Resistors The ratio between the voltage and the current is called the resistance. SI unit of resistance: the ohm, Ω © 2010 Pearson Education, Inc.c

19
**17.3 Resistance and Ohm’s Law**

20
**17.3 Resistance and Ohm’s Law**

Ohm’s law states that when resistance is constant over ranges of voltages a material is said to be ohmic. [Key here is that resistance is constant!] © 2010 Pearson Education, Inc.c

21
**17.3 Resistance and Ohm’s Law**

Suppose a person steps out of a shower and inadvertently touches an exposed 120-V wire with a finger this creating a complete circuit. The human body when wet can have an electrical resistance as low as 300 ohms. Using this value, estimate the current in that person’s body.

22
**17.3 Resistance and Ohm’s Law**

Resistance arises when electrons collide with the atoms that make up the material. There are 4 factors that influence resistance: Type of Material Length Cross-Sectional Area Temperature

23
**17.3 Resistance and Ohm’s Law**

As expected, the resistance is proportional to the length and inversely proportional to the cross-sectional area (why?): The constant ρ is called the resistivity, and is characteristic of the material. © 2010 Pearson Education, Inc.c

24
**17.3 Resistance and Ohm’s Law**

Suppose an electric eel touches the head and tail of a long approximately cylindrically shaped fish, and applies a voltage of 600 V across it. If a current of 0.80 A results, estimate the average resistivity of the fish’s flesh, assuming it is 20 cm long and 2.0 cm in diameter.

25
**17.3 Resistance and Ohm’s Law**

Some materials exhibit a curious phenomenon: at a very low temperature called the critical temperature, their resistivity drops abruptly to zero. These are called superconductors; they have a number of unique properties. Many more studies and research are being done right now to implement more superconductors into everyday appliances to make items more efficient. © 2010 Pearson Education, Inc.c

26
17.4 Electric Power Power, as usual, is the rate at which work is done. For work done by electricity: Rewriting, For ohmic materials, we can write: (3 ways) © 2010 Pearson Education, Inc.c

27
17.4 Electric Power Using the figure below, find the current and the power. © 2010 Pearson Education, Inc.c

28
17.4 Electric Power Electric appliances are rated in watts, assuming standard household voltage. © 2010 Pearson Education, Inc.c

29
17.4 Electric Power The electric company typically bills us for kilowatt-hours (kWh), a unit of energy. 1 kWh We can reduce our energy usage by buying efficient appliances. © 2010 Pearson Education, Inc.c

30
17.4 Electric Power Consider 2 appliances that operate at the same voltage. Appliance A has a higher power rating than Appliance B. How does the resistance of A compare to B? Larger Smaller The Same A computer system includes a color monitor with a power requirement of 200W, whereas a countertop toaster oven is rated at 1500W. What is the resistance of both if each run at 120 V?

Similar presentations

© 2024 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google