1. Direct Current Electricity DC
Physics I
Direct Current Electricity DC

2. Assignment
Ch34: 696/1,2,6-10,12,15,16,19,20,26-28 AND 697/29,30,59,60,66-70,73 AND Benchmark Review on 701A
Ch35: Circuit Practice: 714/1,3,6,7,9,13,16-18 AND 716/22,24,39,40,52,55 AND Benchmark Review on 719A

3. 1. Current Electricity – charges in motion
A. Circuit – a continuous loop through which electricity can flow. It must contain
1. Energy source – battery, generator
2. Conductor – copper wire is common
3. Load – device(s) that converts electrical energy to other useful forms of energy

4. B. Circuit Types
1. Parallel Circuit - more than 1 path is provided for current to flow
2. Series Circuit - one path for current to flow

5. Schematic Symbols

6. Ohm’s Law R = V R, Resistance in Ohms, W I V, Voltage in volts, v
I, current in amperes, a
E = IR or V = IR
E, voltage in volts, for the overall circuit
V, potential drop across a section of a circuit

7. 2. Cardinal rules for Resistors
A. Voltage or Electromotive Force (across battery) is equal in parallel
Vbattery = V1 = V2 = V3 = ...
B. Current is additive in parallel
Itotal = I1 + I2 + I
Kirchhoff’s 1st Law or Junction Rule - the total current entering a junction must be equal to the total current leaving the junction.

8. C. Resistance in Parallel

9. Equivalent Resistance

10. Cardinal Rule for Req
This is the concept of equivalent resistance. The equivalent resistance of a circuit is the amount of resistance which a single resistor would need in order to equal the overall effect of the collection of resistors which are present in the circuit. For parallel circuits, the mathematical formula for computing the equivalent resistance (Req) is
1 / Req = 1 / R1 + 1 / R2 + 1 / R

11. Series Circuit – only 1 path is provided for current

12. Voltage is additive in series
Vbattery = V1 + V2 + V
Kirchoff’s 2nd Law or the Loop Rule: The algebraic sum of the potential differences that occur around a complete circuit is equal to zero.
E - (V1 + V2 + V3 + …) = 0

13. Ibattery = I1 = I2 = I3 = V / Req
Current in series
Ibattery = I1 = I2 = I3 = ...
Ibattery = I1 = I2 = I3 =   V / Req

14. Which leads us to Rt = R1 + R2 + R3 + ...
 since the current is the same

15. Cardinal Rules - Series
E = Vt = V1 + V2 + V3
I t = I1 = I2 = I3
Rt = R1 + R2 + R3
1/Ct = 1/C1 + 1/C2 + 1/C3
Or: Ct =
1/C1 + 1/C2 + 1/C3

16. Cardinal rules - Parallel
E = Vt = V1 = V2 = V3 = …
Voltages are equal in parallel
It = I1 + I2 + I3 + …
Current is additive in parallel
1/Rt = 1/R1 + 1/R2 + 1/R3
Or: Rt =
1/R1 + 1/R2 + 1/R3
Or As more resistance is added, the resistance goes DOWN!! And the current goes UP!
Circuit breaker opens

17. Joule’s Law E = I2Rt E, quantity of energy in joules
I, current in amps
R, resistance in ohms
t, time in sec
E proportional to I2

18. Power, rate of doing work
P = IV P = I2 R
Energy, E = Power x time, j/s x s = joule
P, Power: watt = joule/sec
I, Current: ampere = coulomb/sec
V, Voltage: volt = joule/coulomb
R, Resistance: ohm
E, Energy: joule

19. Sample Problem
22/1 The current through a light bulb connected across the terminals of a 125 v outlet is 0.50 a. at what rate does the bulb convert electric energy to light?
P = IV
P = (0.50 a)(125 v)
P = 63 w

20. Problem Applications

30. 15. continued

32. On the prior slide, simplification shown in the diagrams gives way to what is needed to be done mathematically using the Cardinal Rules:
Series: V is +, I is =, R is +
Parallel: V is =, I is +, R is 1/R +

34. Meters
Voltmeter – measure difference in potential between 2 points in a circuit
ALWAYS connected in parallel with the part of the circuit to be measured (acts as a load)

35. Rules for Meter Usage Handle gently Connect voltmeter in parallel
Connect ammeter in series
Polarity labels tell how to connect meters
Red (+) and Black (-)
+ should be wired directly or though other components to the + terminal of the voltage source

36. Meter Rules Continued
When taking readings of unknown amounts, start at highest range or just tap
Know meter scales BEFORE closing a circuit so accurate readings can be taken quickly
Using tapping rule when first closing a circuit so accurate readings can be taken quickly

37. Meter rules, cont. Have circuit approved before closing the circuit
Report trouble immediately
AC & DC meters are NOT interchangeable (unless so designed)
Avoid overheating
Joule’s Law Q = I2 RT / J
Q a I2

38. Ammeter – measures current
ALWAYS placed in series in a circuit
Be sure current does not exceed range of meter (use rheostat or “tap” or set meter at highest range, if possible)
The ammeter is placed in SERIES with one lead of a circuit. It must be placed around the correct way so the needle moves up-scale. 

39. Galvanometer
Used to measure weak electric currents only

40. Rheostat - variable resistor
Controls amount of current flow
Controls voltage across a load
Protects expensive instruments

41. Other methods of determining resistance
Nature of the Material
R = rL/A
r is resistivity of a conductor in Wcm
L is length of conductor, cm
A is cross sectional area of conductor, cm2
Wheatstone Bridge

42. Problem sample
What is the resistance of Copper wire 20. m long, 0.81 mm diameter at 20.0oC? (Resistivity, rCu at 20.0oC = 1.72x10-6 Wcm)
R = r L = 1.72x10-6 Wcm) (2000cm)
A p(8.1x10-2cm)2 4
R =

43. A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component.

44. R1 + R2 in parallel with R3 + Rx
Rx is unknown
R1,R2, & R3 are known. R2 is adjustable.
. If the ratio of the two resistances in the known leg (R2 / R1) is equal to the ratio of the two in the unknown leg
(Rx / R3), then the voltage between the two midpoints will be zero and no current will flow between the midpoints.
R1 + R2 in parallel with R3 + Rx

46. members.shaw.ca/roma/res_cap.html

47. Sources