Download Chapter 17 Electric Current and Resistance

Chapter 17
Electric Current and
Resistance
What is electric “current”?
17.1 Batteries and Direct Current
Electric current is the
flow of electric charge.
A battery is a source of
electric energy—it
converts chemical energy
into electric energy.
17.1 Batteries and Direct Current
“Redox” Reaction
Cathode – Oxidation
Electrons lost
Anode – Reduction
Electrons gained
17.1 Batteries and Direct Current
“Redox” Reaction
Electrolyte
Contains free ions
Salt Bridge
Allows flow of ions
Allows continued use
Typical “Alkaline” Batteries
Potassium hydroxide serves as the electrolyte
(an alkaline, not acid)
17.1 Batteries and Direct Current
Potassium hydroxide (do not eat)
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.
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.
17.1 Batteries and Direct Current
The actual terminal voltage of the battery is always less than
the emf, due to internal resistance. Usually the difference is
very small.
17.1 Batteries and Direct Current
When batteries are
connected in series, the
total voltage is the sum of
the individual voltages.
17.1 Batteries and Direct Current
When batteries of equal
voltage are connected in
parallel, the total voltage does
not change; each battery
supplies part of the total
current.
17.1 Batteries and Direct Current
Battery Clean-Up
• Anode / Cathode Mix-Up
– Ben Franklin first described the movement of current under
an assumption that the electron carried positive charge.
– Today, we still refer to this as “current” or “conventional
current”
– The actual direction that electrons move is opposite of this
and called “electron flow”
17.2 Current and Drift Velocity
Battery Clean-Up
• Different Voltages
– In series, the voltages add, but the current will be the
lowest of the batteries
– In parallel, the current adds, but the voltage will be the
lowest of the batteries
• Probably will lead to something heating up and breaking
Battery Clean-Up
• Battery Naming (A, AA, AAA, B, C, D)
– 1.5 Volts is a standard value used for portable electronics.
– 1.5 V is a good multiple
• Need 3V? Use two in series!
– Letter refers to size
• Started with A and went up
• Advent of miniature electronics demanded establishment of AA and AAA
17.2 Current and Drift Velocity
Electrons do not flow like water in a pipe. In the absence of
voltage, they move randomly at high speeds, due to their
temperature.
When a voltage is applied, a
very small drift velocity is
added to the thermal motion,
typically around 1 mm/s; this
is enough to yield the
observed current.
17.2 Current and Drift Velocity
Current is the time rate of flow of charge.
SI unit of current: the ampere, A
How long would it take a circuit with
2.0 A of current to deliver 3.6x10-3 C of
charge?
17.3 Resistance and Ohm’s Law
If there is a potential difference across a conductor, how
much current flows?
The ratio between the voltage and the current is called the
resistance.
SI unit of resistance: the ohm, Ω
17.3 Resistance and Ohm’s Law
It is helpful to think of resistance as… resistance to
current moving
No resistance? FULL SPEED AHEAD!
So resistance is bad… right?
(Hint: no)
17.3 Resistance and Ohm’s Law
Resistance captures energy from
electrons and lets us do stuff with it!
How?
Official Physics
Terminology
17.3 Resistance and Ohm’s Law
An ohmic material is one whose resistance is
constant.
17.3 Resistance and Ohm’s Law
Ohm’s law is valid only for “ohmic” materials:
The resistance of a particular
object depends on its length,
cross-sectional area, material,
and temperature.
What is the resistance in a circuit with
0.2 A of current and a difference in
potential electric difference of 12 J/C?
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.
17.3 Resistance and Ohm’s Law
In this table, you can easily see the differences between the
resistivities of conductors, semiconductors, and insulators.
The speaker wires for the rear speakers in this
room are approximately 8 m in length and have a
cross-sectional area of 10-6 m2. Identify the
resistance of these wires.
17.3 Resistance and Ohm’s Law
For many materials, the temperature dependence of the
resistivity is approximately linear, as long as the temperature
change is not too large.
The constant α is called the temperature coefficient of
resistivity. Some values of α are listed in the table on the
previous page.
Find the resistivity of platinum when the
temperature is increased to 35ºC.
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. They are impractical for everyday
home use, however, as they must be cooled to
cryogenic temperatures.
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:
17.4 Electric Power
So, where does this power go? It is changed
to heat in resistive materials.
17.4 Electric Power
Electric appliances are
rated in watts,
assuming standard
household voltage.
17.4 Electric Power
17.4 Electric Power
The electric company
typically bills us for
kilowatt-hours (kWh), a
unit of energy.
We can reduce our energy
usage by buying efficient
appliances.
Chapter 17
Quiz Thursday
Review of Chapter 17
A battery produces emf; positive terminal is the
anode, negative is the cathode.
emf is measured in volts; it is the number of joules
the battery supplies per coulomb of charge.
An electric current can exist only in a complete
circuit.
Resistance:
Review of Chapter 17
Ohm’s law is obeyed if the resistance is
constant:
The resistance of an object depends on its
length, cross-sectional area, and resistivity.
Review of Chapter 17
Power is the rate at which work is done.