Chapter 22

Current and Resistance

• Current

• Conservation of current

• Batteries

• Resistance and resistivity

• Simple circuits

Chapter 22Current and Resistance

Topics:

Sample question:

How can the measurement of an electric current passed through a person’s body allow a determination of the percentage body fat?

Slide 22-1

How can we control the flow of electric charges?

• Study the basic properties of current • Understand how our investigations of

static electricity relates to current.

Charged Parallel Plate Capacitor

• Connect the plates with a metal wire and the capacitor becomes discharged (neutral)

• The capacitor is discharged by a current in the connecting wire.

Properties of a Current

Slide 22-8

Indicators of Current in the Wire• Current is the motion of charges• Charges flow not current• Current is the flow

Charge Carriers

• Charge carriers are the charges that move in a current

• The charge carriers in metals are electrons.

Conduction Electrons in a Metal

Charge Carriers

• Insulator – does not have such free charges and cannot carry current

• Semiconductor – intermediate case – few charge carriers which can be either positive or negative ions

• Examples of ionic solutions as charge carriers – seawater, blood, intercellular fluids

Creating a Current

• We use an electric field to “push” on the electrons in in a conductor similarly to pushing a book on a table to give it kinetic energy

• When we apply an electric field the field exerts a force on the electrons and they begin to move.

Motion of electrons

• Kinetic energy of collisions in converted to thermal energy

• The metal gets warmer

Sustaining the current

• You must continue the push or the electrons will stop moving

• Maintain an electric field• Drift Velocity - Electron’s average motion

is opposite the field• If the field goes to zero then the drift

velocity goes to zero.• A current is a motion of charges sustained

by an internal electric field

Where does the electric field come from in a current carrying wire?

• A potential difference creates a current in the wire.

• Electric potential energy Uelec is energy stored in a system of charged particles due to their electrical interactions.

• The energy transfer Δ Uelec

• The potential is measured in volts

Creating Potential

• Electrons are bumped and pushed along• The E field has some strength in N/C• This is similar to the strength of a

gravitational field in N/kg. (9.8N/kg)• Think about gravitational potential energy

being mgh and electrical potential energy being qEd

Creating Potential

• Uelec = qEd energy stored in a particle• Ug = mgh energy stored by height• E d = J/C =ΔV energy independent of

charge• gh =J/kg• We care about the change in potential• Protons go from high to low potential• Electrons got from low to high potential

Creating Potential

• In reality the electrons are moving• A battery adds energy • Think about an energy elevator.• A 1.5 V battery is like a charge escalator • The battery reenergizes the particle• The volt is the unit of potential• Voltage is the difference in potential• A particle uses all of its potential in a circuit

Simple Circuits

The current is determined by the potential difference and the resistance of the wire:

Slide 22-13

I =∆Vchem

R_____

Batteries

The potential difference between the terminals of a battery, often called the terminal voltage, is the

battery’s emf.

Slide 22-12

∆Vbat = = Wchem

q____

Difference in Potential creates current

Capacitor

• The potential difference across the capacitor depends on the capacitor’s charge.

• The potential difference (and the current) steadily falls as the capacitor discharges

• When the capacitor is fully discharged there is no longer a potential difference and the current ceases

Conservation of Current• Add a lightbulb

between two capacitors

• How does the current at A compare to current at B/

• They are the same

Conservation of Current

• The number of electrons is not changed by the lightbulb

• The lightbulb cannot

destroy electrons• The lightbulb can’t store

electrons• It does use energy

Law of Conservation of Current

• The current is the same at all points in a current carrying wire

Definition of a Current

Slide 22-9

Current

• Current is a rate• The current direction in a wire is from

higher potential to lower potential (in the direction of the electric field)

• Unit is ampere • 1ampere = 1 coulomb per second = 1C/s

Conservation of Current

Slide 22-10