Current, resistance, and electromotive force (emf): Chapter 25 (sec. 1-5)

• Charges (electrons) moving in a conductor • Ohm’s Law & resistance to flow of charge • Energy and power in electrical circuits

C 2012 J. Becker

Learning Goals - we will learn: • The meaning of electric • What is meant by the current resistivity , and how electric charges move in a conductor.

of a substance.

• How to calculate the resistance conductor from its dimensions and its resistivity.

• How an electromotive force ( possible for current to flow in a circuit.

• How to do calculations involving energy and power in circuits.

emf of a ) makes it

ELECTRON MOTION IN A CONDUCTOR WITH AND WITHOUT AN ELECTRIC FIELD

12 Volts

ANALOGY OF A CHARGE MOVING IN A CONDUCTOR

0 Volts

CONDUCTOR WITH

CONVENTIONAL

CURRENT MOVING FROM HIGH ELECTRICAL POTENTIAL (VOLTS) TO LOW POTENTIAL

“CONVENTIONAL” CHARGES DRIFTING IN A CONDUCTOR HIGHER POTENTIAL LOWER POTENTIAL LAMP

Which Box (A, B, or C) has the most resistance to the flow of electric charge (current)?

Each lamp has the same amount of resistance to the flow of charge. Current is the flow of charge past a point in the circuit per unit time interval.

C 1998 McDermott, et al., Prentice Hall

Which network has the most resistance flow of charge?

to the Rank the networks according to decreasing resistance.

C 1998 McDermott, et al., Prentice Hall

1. Rank the brightness of the bulbs (bright to dim).

2. A wire is added as shown below.

a) Does the brightness of bulb C increase, decrease, or remain the same?

b) Does the brightness of bulb A increase, decrease, or remain the same?

c) Does the current through the battery increase, decrease, or remain the same?

C 1998 McDermott, et al., Prentice Hall

Resistance (R) is proportional to resistivity ( r ) :

R =

r

L / A

The resistivity ( r ) depends on temperature and the physical properties of the material, so it has a different value for each material.

Temperature dependence of resistance (and resistivity) is generally linear over limited temperature ranges and is characterized by the temperature coefficient of resistivity ( a ):

R(T) = R 0 [ 1 +

a

(T-T 0 )]

r

(T) =

r

0 [ 1 +

a

(T-T 0 )]

where R or 20

o 0

and T

0

are the resistance and temperature at a standard temperature, usually room temperature C. (Measured in Lab #5)

Current – voltage relations

a) a resistor obeys

Ohm’s Law: I = V/R

with constant slope = 1/R (or D

V = I R

) b) A vacuum tube diode c) A semiconductor diode

Electric potential ( D V) rises and drops in a circuit (from previous slide)

R

CIRCUIT ENERGY and POWER

e

P = V ab I = I 2 R = V ab 2 / R

I = rate of conversion of non electrical (chemical) energy to electrical energy within the source

I 2

r = rate of electrical energy dissipation in the internal resistance of the source (battery) e

I - I 2

r = the rate at which the source delivers electrical energy to the load (headlight)

You can view a 4-minute youtube video of a smoke detector and how it works, from classmate Marjo Mallari http://www.youtube.com/ watch?v=oFUUQcpGR3k