1. Current and Direct Current Circuits
Introducing
Current and Direct Current Circuits

2. Current Flow of positive charge I = Q/t
I: current in Amperes or Amps (A)
Q: charge in Coulombs (C)
t: time in seconds

3. Electron Drift Speed Much less that movement of electric field
vD = I / nqA = I / -neA = I = Q/L
N is # of charge carriers or charge q
A is cross sectional area of conductor

4. Cells
cell
battery

5. Cells Produce a voltage (called an EMF, or Electromotive Force, or e).
EMF depends on chemistry of cell. A perfect cell would produce a terminal (or actual) voltage equal to its EMF.
Terminal voltage is generally less than the EMF due to internal R.

6. Conductors Conduct electricity easily. Have high “conductivity”.
Have low “resistivity”.
Metals are examples.
Do not “want” e’s

7. Insulators Don’t conduct electricity easily. Have low “conductivity”.
Have high “resistivity”.
Rubber is an example.

8. Resistivity / Conductivity
Depends on the identity of the material, not its shape, size, or configuration.
Available in tables of data.

9. Resistors Devices put in circuits to reduce the current flow.
Built to provide a measured amount of “resistance” to electrical flow, and thus reduce the current.

10. Resisitance, R Depends on resistivity and on geometry R = L/A
Unit: Ohms ()

11. Sample Circuit V
light bulb V
cell

12. Ohm’s Law
Resistance in circuit causes potential to drop
V = IR

13. Power in General
P = W/t
P = DE/Dt
Units
Watts
Joules/second

14. Power in Electrical Circuits
P = I V
P: power (W)
I: current (A)
V: potential difference (V)
P = I2R
P = V2/R

15. Resistors V V V

16. Ohmmeter Measures Resistance
Placed across resistor when no current is flowing W

17. Ammeter
Measures Current
Series Connection
Low Resistance A

18. 1/Req = 1/R1 + 1/R2 + 1/R3 1/Req = S(1/Ri ) Resistors in parallel R1

19. Resistors in series R1 R2 R3
Req = R1 + R2 + R3
Req = SRi

20. Internal Resistance
Due to resistance to e movement, power supplies have resistance.
This causes lower EMF at battery terminals when circuit is closed.
To find terminal V, first find circuit REQ for circuit, then use with terminal V to find I.
VT = Vo – IRINT
I = ξ / R + r

21. Kirchoff’s 1st Rule Junction rule.
The sum of the currents entering a junction equals the sum of the currents leaving the junction.
Conservation of…
charge.

22. Kirchoff’s 2nd Rule Loop rule.
The net change in electrical potential in going around one complete loop in a circuit is equal to zero.
Conservation of
energy.

23. Capacitors in Circuits+Q -Q +Q -Q

24. Equivalent Capacitance
series
Charge is same on all capacitors in series arrangement.
1/Ceq = 1/C1+ 1/C2 + 1/C3

25. Equivalent Capacitance
parallel
Voltage is same on all capacitors in parallel arrangement.
Ceq = C1+ C2 + C3