1. Electric Current and Resistance
3 Lecture Outline
Electric Current and Resistance

2. Electric Current and Resistance
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3. Units Batteries and Direct Current Current and Drift Velocity
Resistance and Ohm’s Law
Electric Power
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4. 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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5. Batteries and Direct Current
In a complete circuit, electrons flow from the negative electrode to the positive one. This is an arbitrary direction and can be reversed. Charges (neg and pos) flow is the opposite direction from each other.
The positive electrode is called the anode; the negative electrode is the cathode.
An ideal battery provides a constant source of voltage—it maintains a constant potential difference between its terminals.
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6. Batteries and Direct Current
The actual (real-world) 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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7. Batteries and Direct Current
When batteries are connected in series, the total voltage is the sum of the individual voltages.
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8. 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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9. Batteries and Direct Current
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10. Current and Drift Velocity
Current is the time rate of flow of charge.
SI unit of current: the ampere, A
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11. 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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12. 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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13. 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
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14. Resistance and Ohm’s Law
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15. Resistance and Ohm’s Law
Ohm’s law is valid only for ohmic materials:
This is the holy grail of electric equations!
The resistance of a particular object depends on its length, cross-sectional area, material, and temperature.
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16. Electric Power
Power, as usual, is the rate at which work is done. For work done by electricity:
Rewriting (because I=q/t),
For most materials, we can write:
You are only given P=IV on eq sheet. The others are solved for by substitution of Ohms Law V=IR.
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17. Electric Power
So, where does this power go? It is changed to heat in resistive materials – light bulbs (etc)!
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18. Electric Power
Electric appliances are rated in watts, assuming standard household voltage.
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19. Electric Power
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20. 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.
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21. Summary
A battery produces voltage; positive terminal is the anode, negative is the cathode.
Volts 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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22. Summary Ohm’s law is obeyed if the resistance is constant:
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23. Summary Power is the rate at which work is done.
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