unit 8 : part 1 electric current and resistance. outline batteries and direct current current and...
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Unit 8 : Part 1Electric Current and
Resistance
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Outline
Batteries and Direct Current
Current and Drift Velocity
Resistance and Ohm’s Law
Electric Power
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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.
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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.
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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.
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Batteries and Direct CurrentThe actual terminal voltage of the battery is always less than the emf, due to internal resistance. Usually the difference is very small.
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Batteries and Direct Current
When batteries are connected in series, the total voltage is the sum of the individual voltages.
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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.
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Batteries and Direct Current
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Current and Drift Velocity
Current is the time rate of flow of charge.
SI unit of current: the ampere, A
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Current and Drift Velocity
Historically, the direction of current has been taken to be from positive to negative; this is opposite to the way electrons flow. However, this seldom matters.
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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.
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Resistance and Ohm’s LawIf 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
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Resistance and Ohm’s Law
An ohmic material is one whose resistance is constant.
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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.
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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.
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Resistance and Ohm’s LawIn this table, you can easily see the differences between the resistivities of conductors, semiconductors, and insulators.
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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.
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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.
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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:
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Electric Power
So, where does this power go? It is changed to heat in resistive materials.
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Electric Power
Electric appliances are rated in watts, assuming standard household voltage.
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Electric Power
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Electric PowerThe electric company typically bills us for kilowatt-hours (kWh), a unit of energy.
We can reduce our energy usage by buying efficient appliances.
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Review
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:
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Review
Ohm’s law is obeyed if the resistance is constant:
The resistance of an object depends on its length, cross-sectional area, and resistivity.
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Review of
Power is the rate at which work is done.