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?