# Slide Iowa State University by nikeborome

VIEWS: 24 PAGES: 10

• pg 1
```									                           II. Electric current
1. Definition
Conventional        Electron
current             flow
Units: [ I ] = 1A = 1 C/s

Example: 1020 electrons passed through the electric conductor during 4 seconds.
Find the electric current through this conductor.

Example: The electric current of 0.5 A is flowing through the electric conductor.
a) What electric charge is passing through the conductor during each second?
b) What electric charge will pass through the conductor during 1 minute?

a)

b)
2. Ohm’s Law                                    Nonohmic device

I                                      I

V                                       V

3. Resistance

Definition:        Units: [ R ] = 1Ω = 1 V/A      Ohm’s Law:
4. Resistivity
Definition:
I            A
L

Example: What is the resistance of 1 m of nichrome wire of 2 mm diameter ?

Temperature dependence of resistivity

T
Example: Two cylindrical resistors, R1 and R2, are made of identical material.
R2 has twice the length of R1 but half the radius of R1. They are connected to a
battery V as shown. Compare the currents flowing through R1 and through R2.

A. I1 < I2               B. I1 = I2               C. I1 > I2

I1       I2
V
5a. Resistors in series
Two resistors R1 and R2 are in series when they are connected
one after the other

I       R1                R2                              I     Req

V1                V2
V                                              V
5b. Resistors in parallel
Two resistors R1 and R2 are in parallel when they are connected to the
same potential difference

I1   R1                                            I    Req
I                         I
I2   R2                                                  V
Example: A 14-A current flows into a series combination of a 3.0-Ω and a 4.0-Ω
resistors. What is the voltage drop across the 4.0-Ω resistor?
A) 38 V;     B) 42 V;           C) 56 V ;       D) 98 V.

Example: A 22-A current flows into a parallel combination of 4.0 Ω, 6.0 Ω, and
12 Ω resistors. What current flows through the 12-Ω resistor?
A) 18 A;       B) 11 A;           C) 7.3 A;       D) 3.7 A.
Symbols for circuit elements

Conductor (wire) with negligible resistance

Resistors

- +
Source of emf (for example, a battery)

Switch

Ground

Voltmeter (should be connected in parallel)

Ammeter (should be connected in series)
6a. Ammeter (used to measure current)
G
A                                         A        =
r
r is small shunt resistance or shunt
G is galvanometer

• Ammeter should be connected in series
• Ammeter has very small (ideally zero) internal resistance,
so it does not affect the current to be measured

6b. Voltmeter (used to measure voltage or potential difference)
R
A                            V                  G
=
V
B                                             R is large

•   Voltmeter should be connected in parallel
•   Voltmeter has very large (ideally infinite) internal resistance,
so that very little current is diverted through it.
6. Voltmeter and Ammeter
•   Voltmeter should be connected in parallel
•   Ammeter should be connected in series
•   Voltmeter has very large (ideally infinite) internal resistance
•   Ammeter has very small (ideally zero) internal resistance

Example: What connection is not correct?

A

C
B

```
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