20.1 Current

Resistivity is used to calculate the resistance of specified configurations of material.

The change of resistance with temperature can be calculated using the thermal coefficients of resistivity.

The power dissipated by a resistor and power supplied by a power supply is calculated.

Explain why AC current is used.

Explain the effects of current on the human body.

Explain the process by which electric signals are transmitted.

Explain the effects myelin sheaths have on signal propagation.

The brain sending a message for a baby to twitch its toes, an electric train pulling its load over a mountain pass, and the flicker of numbers on a handheld calculator are just a few examples.

The basis of technology has been harnessed by humankind to improve our quality of life.

The previous two chapters focused on static electricity and the fundamental force underlying its behavior.

We will gain new insights into nature and the fact that all magnetism comes from electric current, in addition to exploring applications of electricity.

The rate at which charge flows is called electric current.

A large current, such as that used to start a truck engine, moves a large amount of charge in a small time, whereas a small current, such as that used to operate a hand-held calculator, moves a small amount of charge over a long period of time.

Many electrical appliances are rated in amperes.

Current is the rate of flow of charge.

An ampere is the flow of one coulomb through an area.

Since charge and time are given, we can use the definition of current in the equation to find the current in part a.

We use the given values of charge and current to find the time required.

A large charge is moved in a small amount of time.

The currents in the starter motor are large because of the large frictional forces that need to be overcome.

The time is less than an hour.

The large current of the truck starter takes more time than the small current of the calculator to move.
Calculators need very little energy.
It's possible to operate a handheld calculator from solar cells or use small batteries.
The technology of the calculator requires smaller currents because they don't have the same moving parts as a truck engine.

The standard schematic representation of a battery, conducting path, and load is shown in Figure 20.3.

The main features of a circuit can be visualized with schematic.
A single schematic can represent many different situations.
The analysis is the same for a wide variety of situations.
To apply the concepts and analysis to many more situations, we need to understand a few schematics.

A closed path for current to flow through is supplied by conducting wires.

A wide variety of similar circuits are represented in the schematic.

Positive charge would flow in the direction of conventional current.

Positive charges, negative charges, or both may move depending on the situation.
Negative charges move in metal wires.
Positive and negative charges move in ionic solutions.
In nerve cells, this is also true.
A Van de Graaff generator can produce a current of pure positive charges.
Benjamin Franklin, an American politician and scientist, was responsible for the fact that conventional current is taken to be in the direction that positive charge would flow.
He named the type of charge associated with electrons negative, long before they were known to carry current.
Franklin was not aware of the small-scale structure of electricity.

When a conductor in equilibrium cannot have an electric field in it, conductors carrying a current have an electric field.

To move the charges, an electric field is needed.

There are little peas that can move in the straw.

Put the straw on the table and fill it with peas.

A different pea should come out of the other end when you put one pea in.

An electric current is an analogy for this demonstration.
The supply of energy and the electrons are related.

The flow of peas is based on the physical bumping of peas into each other.

Current is the rate at which charge moves through an area.

Conventional current can be defined as moving in the direction of the electric field.
The conventional current is in the opposite direction to the negative charge.
The flow of electrons is referred to as electronic flow.

The flow of positive charge was defined in the previous example.

There are so many charged particles moving, even in small currents, that individual charges are not noticed.

They don't always keep moving forward like soldiers in a parade.
They are like a crowd of people with a general trend to move forward.

There are lots of electrons and atoms in the metal wire.

The electrical signals are moving quickly.

As soon as a switch is turned on, lights come on.

A significant fraction of the speed of light can be seen in the electrical signals carried by currents.
The individual charges that make up the current move are typically drifting at speeds on the order of.

The signal is passed on quickly because the density of charge cannot be increased easily.

The electrical shock wave travels through the system at a fast pace.
A shock wave is a rapidly propagating change in the electric field.

When charged particles are forced into this volume of a conductor, an equal number are forced to leave.

It is difficult to increase the number of charges in a volume because of the repulsion between like charges.
As one charge enters, another leaves, carrying the signal rapidly forward.

There are a lot of free charges in good conductors.

The free charges are in metals.
The distance that an individual electron can travel is very small.
The motions of atoms in a gas are similar to the electron paths.

There is an electric field in the conductor that causes the electrons to drift in the opposite direction.

There are so many free charges that the drift velocity is small.
The drift velocity is calculated if we have an estimate of the density of free electrons in a conductor.
The larger the density, the slower the current is.

electrons and atoms collide in a conductor.

The path of an electron is shown.

The drift velocity is the average speed of the free charges and it is opposite of the electric field for electrons.

A constant supply of energy is required to maintain a steady current.

Good heat conductors are also good electrical conductors.

Large numbers of free electrons can carry electrical current and thermal energy.

The atoms of the conductor receive energy from the free-electron collisions.

The electric field works in moving the electrons through a distance, but it doesn't increase the speed of the electrons.

A constant power input is needed to keep the current flowing.

An exception is found in superconductors.

Without a constant supply of energy, superconductors can have a steady current.

The supply of energy can be useful.
The supply of energy is needed to increase the temperature.

It is a good idea to look at the filament and describe it.

The number of free charges per unit volume depends on the material.

The shaded segment has a number of free charges in it.
The amount of charge on each carrier is the charge in this segment.

Since the charges move an average distance in a time, that's the magnitude of the drift velocity.

There is a wire of cross-sectional area made of a material with a free charge density.

Each carrier of the current has a charge and move with a drift velocity of magnitude.

The wire has a drift velocity of magnitude and all the charges move out in a time.

For more discussion, see the text.

Simple drift velocity is not the whole story.

The electron's speed is much higher than its drift speed.
Not all of the electrons in a conductor can move freely, and those that do might move slower than the drift velocity.
The metallic conductor has a lattice structure.
The inner electrons experience the attraction of the nuclei more than the electrons that are far away.
The electrons are free.
They can move among the atoms in a "sea" of electrons without being bound to a single atom.
When an electric field is applied, the free electrons respond by speeding up.
The conductor gets warmer as they collide with the atoms in the lattice, generating thermal energy.

The structure of the atoms does not allow for free electrons.

If there is one free electron per copper atom, calculate the drift velocity of electrons in a 12-gauge copper wire.

The density of copper is low.