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

Lecture Outline

Chapter 17

College Physics, 7th Edition

Wilson / Buffa / Lou

© 2010 Pearson Education, Inc.

Units of Chapter 17

Batteries and Direct Current

Current and Drift Velocity

Resistance and Ohm’s Law

Electric Power

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Big Questions

• What is electric current and how does it travel?

• What causes current to move when switched on?

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.

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.

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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.

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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

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.

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17.1 Batteries and Direct CurrentThe 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.

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Let’s think about an example.

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)

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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.

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17.1 Batteries and Direct Current

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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.

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

17.2 Current and Drift Velocity

Current is the time rate of flow of charge.

SI unit of current: the ampere, A

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17.2 Current and Drift VelocityElectrons 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…

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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?

17.3 Resistance and Ohm’s LawThe 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

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17.3 Resistance and Ohm’s Law

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!]

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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.

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

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.

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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.

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.

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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)

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17.4 Electric Power

Using the figure below, find the current and the power.

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17.4 Electric Power

Electric appliances are rated in watts, assuming standard household voltage.

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17.4 Electric PowerThe electric company typically bills us for kilowatt-hours (kWh), a unit of energy.

We can reduce our energy usage by buying efficient appliances.

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1 kWh

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?