Chapter 18

Electric Currents

Units of Chapter 18

• The Electric Battery

• Electric Current

• Ohm’s Law: Resistance and Resistors

• Resistivity

• Electric Power

Units of Chapter 18

• Power in Household Circuits

• Alternating Current

• Microscopic View of Electric Current

• Superconductivity

• Electrical Conduction in the Human Nervous System

ObjectivesAfter studying the material of this chapter, the student should be able to:1. Explain how a simple battery can produce an electrical current.2. Define current, ampere, emf, voltage, resistance, resistivity, and temperature coefficient of resistance.3. Write the symbols used for electromotive force, electric current, resistance, resistivity, temperature coefficient of resistance and power and state the unit associated with each quantity.4. Distinguish between a) conventional current and electron current and b) direct current and alternating current.5. Know the symbols used to represent a source of emf, resistor, voltmeter, and ammeter and how to interpret a simple circuit diagram.6. Given the length, cross sectional area, resistivity, and temperature coefficient of resistance, determine a wire's resistance at room temperature and some higher or lower temperature.7. Solve simple dc circuit problems using Ohm's law.8. Use the equations for electric power to determine the power and energy dissipated in a resistor and calculate the cost of this energy to the consumer.9. Distinguish between the rms and peak values for current and voltage and apply these concepts in solving problems involving a simple ac circuit.

ObjectivesAfter studying the material of this chapter, the student should be able to:

1.Explain how a simple battery can produce an electrical current.2.Describe the concepts in an electrical circuit including electric potential energy, electric potential, voltage, current, and resistance.3.Describe conditions that create current in an electric current.

Recap• In ch. 16, Electrostatics, electric field must be 0

inside a conductor and charges did not move.• When charges move in a conductor, there is

usually an electric field present.• An electric field is needed to put charges in

motion.• The flow of charge can be controlled using

electric fields and electric potential.• Therefore, in order to have a current in a wire, a

potential difference is needed.• That difference can be provided by a battery.

18.1 The Electric Battery

Volta discovered that electricity could be created if dissimilar metals were connected by a conductive solution called an electrolyte.

This is a simple electric cell.

Mechanical Universe The Electric Battery

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

BATTERY BASICS INTRODUCTION

• 1800 – Alessandro Volta discovered the chemical battery by creating a portable electricity source known as a “Voltaic Pile”.

• A Voltaic Pile is a device using pieces of silver and zinc separated by moist cloth soaked in an electrolyte (in Volta’s case, sea water) solution.

• Humphry Davy later proved that the electricity from voltaic piles was caused by the chemical reaction, and not the different metals, as first assumed.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.

BATTERY BASICS Voltaic Pile

• In the lemon experiment, the lemon juice allows the metal plates to gain or lose electrons. Then, those electrons travel over to the other plate (via the electrolyte solution, lemon juice), forming a redox reaction.

• The electrolyte is electrically the same on both sides, but the reaction creates a different electrical potential on the two different plates, so connecting them shows a voltage difference.

18.1 The Electric Battery

A battery transforms chemical energy into electrical energy.

Chemical reactions within the cell create a potential difference between the terminals by slowly dissolving them. This potential difference can be maintained even if a current is kept flowing, until one or the other terminal is completely dissolved.

18.1 The Electric Battery

Several cells connected together make a battery, although now we refer to a single cell as a battery as well.

What does a battery do?

• Static discharge is the moving of electrons from one atom to another.

• In order to keep the electrons moving through the circuit, there has to be something that causes a push, or a voltage difference- a battery does just that…

How do batteries work?

• Batteries create an uneven level of electrons which causes the electrons to move from a high concentration to a low concentration…

• This is also known as the voltage difference..

Parts of the Dry Cell

• Positive End • Plastic Insulator • Moist Paste • Carbon Rod • Zinc Container • Negative Terminal

How does a Dry Cell Work?

• When the circuit is closed, and the battery is connected, a chemical reaction starts the process.

• The chemical reaction with zinc and several other chemicals occurs in the moist paste.

How does a Dry Cell Work?

• The carbon rod acts as a conductor and transfers electrons.

• The carbon rod is not part of the reaction happening in the moist paste.

• But…the chemical reaction in the moist paste does cause the carbon rod to become charged.

• This charge on the carbon rod creates a positive end.

• The negative end is made by the Zinc.

How does a Dry Cell Work?

• The voltage difference between the positive and negative ends causes the current to flow.

• By connecting more batteries you increase the voltage difference.

18.2 Electric Current

In order for current to flow, there must be a path from one battery terminal, through the circuit, and back to the other battery terminal. Only one of these circuits will work:

18.2 Electric Current

By convention, current is defined as flowing from + to -. Electrons actually flow in the opposite direction, but not all currents consist of electrons.

18.2 Electric Current

Electric current is the rate of flow of charge through a conductor:

(18-1)

Unit of electric current: the ampere, A.

1 A = 1 C/s.

18.2 Electric Current

A complete circuit is one where current can flow all the way around. Note that the schematic drawing doesn’t look much like the physical circuit!

Electric CircuitElectric Circuit – continuous conductingpath

Electric Current – flowof charge from one terminal to the other

Diagram of Electric Circuit

Remember: Electric Potential Energy- Two Unlike Charges

Higher Potential Energy

Lower Potential Energy

+

-

•To cause movement of a charge, there must be a potential difference.

While the switch is open:• Free electrons (conducting electrons) are

always moving in random motion.

• The random speeds are at an order of

106 m/s.

• There is no net movement of charge across a cross section of a wire.

http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/imgele/micohm.gif

What occurs in a wire when the circuit switch is closed?

What occurs in a wire when the circuit switch is closed?

• An electric field is established instantaneously (at almost the speed of light, 3x108 m/s).

• Free electrons, while still randomly moving, immediately begin drifting due to the electric field, resulting in a net flow of charge.

• Average drift velocity is about 0.01cm/s.

Closing the switch establishes a potential difference (voltage) and an electric field in the circuit.

• Electrons flow in a net direction away from the (-) terminal

towards

the (+)

terminal.

High PotentialLow

Potential

Conventional Current• By tradition,

direction in which “positive charges” would flow.

• Direction is opposite of electron flow.

Question:

What is required in order to have an electric current flow in a circuit?

Answer:1. A voltage source.

2. The circuit must be closed.

Homework

• Questions p. 514 2-5

Due by Friday – collected and graded