chapter 22 current and resistance. current conservation of current batteries resistance and...
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• 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
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