❑ Simple electric circuits that contain

batteries, resistors, and capacitors

in various combinations

28.1 Electromotive Force

• When the potential difference at the batteryterminals is constant in a particular circuit, thecurrent in the circuit is constant in magnitudeand direction and is called direct current.

• A battery is called either a source ofelectromotive force or, more commonly, asource of emf.

• The emf e of a battery is the maximumpossible voltage the battery can providebetween its terminals.

Terminal Voltage

Load Resistance

Internal Resistance

28.2 Resistors in Series and Parallel

Potential Divider

7

𝑽𝒙 =𝑹𝒙

𝑹𝒆𝒒. 𝑽𝒎𝒂𝒊𝒏

Potential Divider

This technique is not restricted to only two resistors in series, but may be applied to any number. For

example, if there were three resistors in series, then the p.d. across each may be found from

8

The Slidewire Potentiometer

• This instrument is used for the accurate measurement of small voltages.

9

Potentiometer, Pot• A potentiometer, informally a pot, is a three-

terminal resistor with a sliding or rotatingcontact that forms an adjustable voltagedivider. If only two terminals are used, one endand the wiper, it acts as a variable resistor orrheostat.

• Potentiometers are commonly used to controlelectrical devices such as volume controls onaudio equipment

10

Current Divider When the I is known

12

Current Divider

The current divider theory is not limited to only two resistors in parallel. Any number can be

accommodated. However, with three or more parallel resistors the current division method can be

cumbersome to use, and it is much easier for mistakes to be made.

13

Current Divider

14

𝑰𝒙 =𝑹𝒆𝒒

𝑹𝒙. 𝑰𝒎𝒂𝒊𝒏

❑Use nodes for naming the

equivalent resistance of the

segments of the circuit.

✓ Rab, Rbc, Rac

Problems

I=600 m

A

𝑅𝑙𝑎𝑚𝑝

0.5 A

𝐼

𝐼𝑎𝑏 =2.94

25. 1.93 = 0.227 𝐴

𝑉𝑎𝑏 = 25𝑥0.227 = 5.68 𝑉

𝐼𝑎𝑏

𝐼

𝑉𝑎𝑏 = 1.93𝑥2.94 = 5.68 𝑉

𝐼𝑎𝑏

𝐼𝑎𝑏 =5.68

25= 0.227 𝐴

S is open

S is in position a

S is in position b

𝐼

𝐼

∆𝑉0.750= 0.750𝑥2,67 = 2.0 𝑉

𝑃3 =2.0 2

3=1.33 W 𝑃1 =

2.0 2

1=4.00 W