Phys chapter 21 notes

Phys chapter 21 notes - CHAPTER-21 NOTES CURRENT AND...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
CHAPTER-21 NOTES CURRENT AND DIRECT-CURRENT CIRCUITS _______________________________________________________________________ 1. Current and current density: Current I = Δ Q/ Δ t, is the amount of charge flowing past a certain cross sectional area per unit time. The unit for current is the Ampere (A). (1 A = 1C/s) The current can also be expressed as I = nqv d A, where n is the number of charge carriers per unit volume, q is the carrier charge, v d is the drift velocity, and A is the cross sectional area. The current density J = nq v d . The magnitude J = I/A. 2. Ohm’s Law: V = IR. 3. Resitivity: ρ = RA/l , where R is the resistance of a conductor of length l and of uniform cross-sectional area A. The resistivity, = RA/l = VA/Il = E/J. The resistance R is a property of a given resistor whereas ρ is characteristic of the material from which the resistor is fabricated. 4. Emf, ε and terminal voltage V ab of a battery with internal resistance r. ε = V ab ir, where i is the current flowing through the circuit containing the battery. 5. Power: P = V ab I ( for a general circuit element) = I 2 R ( power into a resistor) 6. Kirchhoff’s junction and loop rules [a] Junction rule : The algebraic sum of the currents into any junction is zero Water pipe analogy for the junction rule: the flow rate of water entering a pipe equals the rate of water leaving.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
[b] The loop rule: the algebraic sum of all potential differences in a loop must be zero. Remember the following helpful conventions when analyzing dc-circuits using Kirchhoff’s junction and loop rules. In the figures below ‘travel’ is the direction in which one imagines going around the loop. This ‘travel’ direction doesn’t have to be the direction of the current in a given circuit element in the loop. Potential increase Potential drop 7. Series and parallel connections [a] resistors in series a b c d R 1 R 2 R 3 R = R 1 + R 2 + R 3 R is the net resistance The same current flows through each resistor The potential differences add: V ad = V ab + V bc + V cd [b] resistors in parallel 1/R = 1/R 1 +1/R 2 R is the net resistance The same potential difference V ab = I 1 R 1 = I 2 R 2 exists across the two resistors. The branch currents add : I = I 1 + I 2 I I 1 I 2 R 1 R 2 a b
Background image of page 2
SOLVED EXAMPLES a b c d e f 6 Ω 36 Ω 24 8 12 V 24 V I 2 I 1 I 3 12 1. Consider the circuit shown in the diagram. [a] Write down Kirchhoff’s junction rule at ‘b’ and loop equations for the loops abefa and bcdeb Solution: At junction ‘b’, - I 3 + I 1 - I 2 = 0 I 3 = I 1 - I 2 …[1] For loop abefa, - 8I 1 - 6I 3 -24I 1 + 24 = 0 or substituting for I 3 from [1] - 38I 1 +6I 2 +24 = 0 ……. .[2] For loop bcdeb - 12I 2 – 12- 36I 2 +6I 3 - = 0
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 06/02/2008 for the course PHYS 102 taught by Professor N/a during the Spring '08 term at Drexel.

Page1 / 10

Phys chapter 21 notes - CHAPTER-21 NOTES CURRENT AND...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online