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DC Circuits

DC Circuits. Currents. Resistors. Batteries. Kirchhoff’s Loop Rules . Power. Examples of Circuits. Current: flow of charge. Average current: I av = Charge  Q flowing across area A in time  t. Instantaneous current: differential limit of I av.

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DC Circuits

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  1. DC Circuits Currents. Resistors. Batteries. Kirchhoff’s Loop Rules. Power

  2. Examples of Circuits

  3. Current: flow of charge Average current: Iav= Charge Q flowing across area A in time t Instantaneous current: differential limit of Iav Units of current: Coulomb/second=Ampere (A)

  4. Direction of the current Direction of current is in direction of flow of positive charge or, opposite direction of flow of negative charge

  5. Current density J (1) J: current/unit area points in the direction of the current Units A/m2

  6. Current density J (2) If the ara is not perpendicular to the current  : angle between normal to A and current An area or surface perpendicular to the direction of the current

  7. Why does current flow? If an electric field is set up in a conductor, charge will move (making a current in the direction of E) Note that when current is flowing, the conductor is not an equipotential surface (and Einside0)!

  8. Microscopic picture (1) Drift speed is velocity forced by applied electric field in the presence of collisions, it is typically 4x10-5m/s, or 0.04 mm/s! To go one meter at this speed takes about 10 hours Thermal velocity is around 103 km/s ! How can this be?

  9. Microscopic picture (2) vd : drift velocity n : number of carriers per unit volume q : charge of each carrier (normally e) A : suface perpendicular to vd

  10. Conductivity and resistiviy Ability of current to flow depends on density of charges & rate of scattering. Two quantities summarize this: : conductivity : resistivity

  11. Microscopic Ohm’s Law  And  depend only on the microscopic properties of the material, not on its shape

  12. The voltage drops in a resistor The electric field brings about a voltage drop in a resistor

  13. Ohm’s Law (1) What is the relationship between V and current?

  14. Ohm’s Law (2) R has units of ohms ()=Volt/Amp Then, units of  :m and  : -1m-1

  15. Examples of Circuits

  16. Symbols for circuit elements

  17. Electromotive force -Battery Moving from the negative to positive terminal of a battery increases your potential Think: Ski Lift

  18. Internal resistance Real batteries have an internal resistance, r, which is small but non-zero Terminal voltage: Real battery=ideal battery in series with a resistance

  19. Sign conventions - Resistor Moving across a resistor in the direction of current decreases your potential Voltage drop: Think: Ski Slope Voltage decreases in the direction of the current

  20. Sign conventions - Capacitor Moving across a capacitor from the negatively to positively charged plate increases your potential

  21. Resistors in series The same current I must flow through both resistors or

  22. Resistors in parallel Voltage drop across the resistors must be the same or

  23. Measuring V, I, R

  24. Measuring Potential Difference A voltmeter must be hooked in parallel across the element you want to measure the potential difference across Voltmeters have a very large resistance, so that they don’t affect the circuit too much

  25. Measuring Current An ammeter must be hooked in series with the element you want to measure the current through Ammeters have a very low resistance, so that they don’t affect the circuit too much

  26. Measuring Resistance An ohmmeter must be hooked in parallel across the element you want to measure the resistance of Here we are measuring R1 Ohmmeters apply a voltage and measure the current that flows. They typically won’t work if the resistor is powered (connected to a battery)

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