How rapidly is work being done in an electrical circuit?

POWER = WORKTIME

What electrical unit is most closely associated with work (energy)?

What electrical unit involves time?

What is the unit of POWER?

If an electric iron draws 5 Amperes from a 120v line,

at what rate is it using power?

(1) 6 Watts (2) 12 Watts(3) 120 Watts (4) 240 Watts(5) 600 Watts (6) 720 Watts

Which has a greater resistance, Which has a greater resistance,

a 100 Watt lightbulb or a 100 Watt lightbulb or

a 25 Watt lightbulb?a 25 Watt lightbulb?

1) the 25 W bulb1) the 25 W bulb

2) the 100 W bulb2) the 100 W bulb

3) the same for both3) the same for both

Which carries the greater current?Which carries the greater current?

1) the 25 W bulb1) the 25 W bulb

2) the 100 W bulb2) the 100 W bulb

3) the same for both3) the same for both

A A 1440 W1440 W microwave oven is microwave oven is

designed to operate using a designed to operate using a

120 V120 V power source. What is power source. What is

the resistance of this appliance?the resistance of this appliance?

1) 0.1 1) 0.1

2) 10 2) 10

3) 72 3) 72

4) need to know the current 4) need to know the current 5) none of these5) none of these

At age 67, in his autobiography,

Einstein wrote vividly of his recollection of a new toy from his father:

a compass.

Thales of Miletus 600 BC

“Lodestone”

Lodestone first discovered near the ancient city of Magnesia in Asia Minor

The mineral ore is called “magnetite”

Electrostatics Magnetism- attracts dust, hair, plastic, paper scraps, dried grass, leaves, (small bits of anything)

- attracts only metals of iron, nickel, & cobalt

- amber or glass charges when rubbed

- lodestone always exhibits this “charge”

- CONDUCTION can charge other objects

- stroking (in one direction) will magnetize metals

- induced charges are long-lived

- induced magnetism is short-lived

'When troops encountered gloomy weather or dark nights, and the directions of space could not be distinguished, they made use of the south-pointing fish to identify the directions. A thin leaf of iron is cut into the shape of a fish two inches long and half an inch broad, having a pointed head and tail. This is then heated in a charcoal fire, and when it has become thoroughly red-hot, it is taken out by the head with iron tongs and placed so that its tail points due north. In this position it is quenched with water in a basin, so that its tail is submerged for several tenths of an inch. It is then kept in a tightly closed box. To use it, a small bowl filled with water is set up in a windless place, and the fish is laid as flat as possible on the water surface so that it floats, whereupon its head will point south.'

Tseng Kung-Liang, Compendium of Important Military Techniques 1044 AD

1296 Petrus Peregrinus (French physicist)

Any magnet has two “poles”

opposite ends at which all the magnetic attraction seems to be concentrated.

like poles repel

unlike poles attract

NS

Magnetic Dipole Electric Dipole

How do the magnets exert forces on each other?

via the magnetic field

-

Magnetic Monopoles?• Does there exist an isolated magnetic charge?

– we would call this a magnetic monopole

– it would have a N or S magnetic “charge”

• How might we find this isolated magnetic charge?

Simple, right? Cut a bar magnet in half:

No attempt yet has been successful in finding magnetic monopoles in nature

All you getis a bunch

of little magnets!

The North pole of a small magnet

(compass) points towards geographic

North because Earth’s magnetic

South pole is up there!!

Magnetic north is not exactly geographic north.

and occasionally reverse polarity!

The magnetic poles drift

Magnetic declinationor “dip angle”

allows your lattitude to be determined.

writes a philosophical essay predicting a connection between electricity & magnetism

1820 Oersted discovers while STATIC CHARGE has no effect

on a magnet (compass) nearby CURRENT twists a

magnet sideways!

1813 Hans Christian Oersted Danish physician and researcher

+ -

We saw that

A B-field must surround the current carrying wire like

A current travels out toward you

The B-field surrounding this wire is best represented by

through a conducting wire

The B-field surrounding this wire is best represented by

A current travels out toward you

through a conducting wire

Looking down a length of wire, the magnetic field lines show that the current must be

A. coming out of the screen at you.B. zero.C. going into the screen away from you.

The B-field at POINT points

1. up. 2. down. 3. left. 4. right.

5. into the screen. 6.out of the screen.

7. is zero.

CurrentCurrent Effect Effect Fatal?Fatal?

0.001 Ampere mild shock no

0.005 Ampere painful shock no

<0.010 Ampere tingles no

0.010 Ampere paralysis of motor muscles no

0.020 Ampere threshold of pain (cannot let go!) no

0.030 Ampere breathing becomes laboredbreathing becomes labored nono

0.100 Ampere 0.100 Ampere death due to fibrillationdeath due to fibrillation seconds seconds

0.200 Ampere no fibrillation, but severe seconds0.200 Ampere no fibrillation, but severe seconds

burns and burns and breathing stopsbreathing stops

>0.200 Ampere heart stops>0.200 Ampere heart stops seconds seconds

1.000 Ampere serious burns1.000 Ampere serious burns instantly instantly

Galvani was correct in so far as our central nervous system is in fact

a complex electrical circuit:

Neurons fire electrical pulses along synapses.Muscle fibers contract in response to these signals.

(5) 600 WattsQUESTION 1 P = IV

Greater resistance: (1) the 25 W bulbGreater resistance: (1) the 25 W bulb

QUESTION 2 Since P = V2/R, P 1/R.

QUESTION 3

The greater current: (2) 100 W bulbThe greater current: (2) 100 W bulbThe bulb with the smallest resistance must draw the greater current!Also: since P = IV, P I.

(2) 10 (2) 10 QUESTION 4

101440/)120( 2 WV

Some Answers

QUESTION 5

QUESTION 6

QUESTION 7

QUESTION 8

QUESTION 9,10,11

There can’t possibly be any way nature distinguishes the wire’s top from bottom, left side from right.

Imagine just rolling the wire on its side. Current still flows in the same direction; the magnetic field must remain unchanged.

Which we’ll remember by a hand rule!

We certainly expect the magnetic fieldto weaken with distance from the wire!

A. coming out of the screen.To curling the fingers of your right hand in the direction of the field lines, your thumb must point away from the screen.

6.out of the screen.Just ride your closed hand along the curve in the wire, your thumb pointing along the current’s direction. Your curled fingers come out of the loop at all three points.