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Class Notes on Electromotive Force, Resistors, and Kirchhoff's Rules, Study notes of Physics

These class notes cover the concepts of electromotive force (emf), resistors in series and parallel, and kirchhoff's rules. The definition of emf, its sources, and how it relates to electric potential difference. The notes also discuss the rules for calculating equivalent resistance in series and parallel resistor combinations. Lastly, the document introduces kirchhoff's rules and provides examples for their application in various circuits.

Typology: Study notes

Pre 2010

Uploaded on 08/04/2009

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Class Notes 8
Instructor: H. L. Neal
1 Electromotive Force
Electromotive force (emf) is de…ned as the amount of energy gained per unit charge that passes through
a device in the opposite direction to the electric …eld produced by that device. It is measured in volts.
Sources of emf include electric generators (both alternating current and direct current types) and batteries.
Electromotive force is often denoted by E.
R+
-E
Created usin g UNREGISTERED Top Draw 3.10 Oct 7, '106 3:17:27 PM
In the circuit Fig. 1, the emf prodiced by the battery pushes electric charge aroubd the circuit to produce
the current I. The electric potential di¤erence accross the resistor is
V=IR =E:
2 Resistors in Series and Parallel
R1
R1
R2
R
2
R1 + R2
=
=
1/R1 + 1/R2
Cr eated using UNREGIS TERED Top Draw 3. 10 Oct 7,' 106 2:46:58 P M
The resistors add and resistors in parallel divide is a good way to remember how to determine the equivalent
resistance for any combination. Note that this es exactly opposite to the rule for capacitor combinations.
1
pf3

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Class Notes 8

Instructor: H. L. Neal

1 Electromotive Force

Electromotive force (emf) is deÖned as the amount of energy gained per unit charge that passes through a device in the opposite direction to the electric Öeld produced by that device. It is measured in volts. Sources of emf include electric generators (both alternating current and direct current types) and batteries. Electromotive force is often denoted by E.

R

E

Created using UNREGISTERED Top Draw 3.10 Oct 7,'106 3:17:27 PM

In the circuit Fig. 1, the emf prodiced by the battery pushes electric charge aroubd the circuit to produce the current I. The electric potential di§erence accross the resistor is

V = IR = E:

2 Resistors in Series and Parallel

R 1

R 1

R 2

R 2

R 1 + R 2

1/R 1 + 1/R 2

Created using UNREGISTERED Top Draw 3.10 Oct 7,'106 2:46:58 PM

The resistors add and resistors in parallel divide is a good way to remember how to determine the equivalent resistance for any combination. Note that this es exactly opposite to the rule for capacitor combinations.

3 Kircho§ís Rules

Not all circuits can be reduced to sets of series and parallel resistors. (Notice that the three circuit diagrams above all describe the same circuit). How can we handle a circuit like this which is more complex than being a combination of series and parallel pieces?

Assign a current In through each resistor, so that I 1 áows through R 1 , I 2 áows through R 2 , and so on. Now apply Kircho§ ís rules:

  1. At each junction (J1, J2, J3, and J4 shown above), the sum of the currents in must equal the sum of the currents out; this is just charge conservation.
  2. Around any circuit loop (L 1 , (L 2 , and L 3 s hown below) X

i

Ei =

X

j

Ij Rj :