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Reminders: Exam I coming up Mon Feb 24 th ! (a week from Monday)PowerPoint Presentation

Reminders: Exam I coming up Mon Feb 24 th ! (a week from Monday)

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Reminders: Exam I coming up Mon Feb 24 th ! (a week from Monday)

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Reminders:Exam I coming up MonFeb 24th!(a week from Monday)

HW3 due on Fri Feb 21

Conduction and Batteries!

Fixed charges

Mobile charges

−

+

−

+

Eventually …

−

Static!

Flow of charges stops!

+

Pixie ↔ electro-chemistry

EMF

Falling water analogy:

The perfect battery!

Will provide whatever current is needed.

Always maintains a fixed electric potential

across it’s terminals.

Batteries hooked in series:

Batteries hooked in parallel:

Current!

Current!

Current!

Current!

Current!

Current!

Current!

Current (conventional)!

We’ll always assume conventional current flow:

A current of positive charge moving from + to −.

The real current (electrons) going backward is

completely equivalent (for our purposes!)

Units?

Example 18-1: A current of 1.30 A flows in a wire. How many electrons are flowing past any point in the wire per second?

Ohm’s Law

Ohm’s Law

Resistance: depends on the object’s size, shape, and composition.

Units?

Standard symbol

Example: 18.7 An electric clothes dryer has a heating element with a resistance of 9.6 . (a) What’s the current in the element when connected to 240 V? (b) How much charge passes through the element in 50 min?

The water analogy again!

Large ‘resistance’

small ‘resistance’

2 Gal/min

10 Gal/min

Perfect wires!

A perfect wire has zero resistance (negligible resistance compared to other stuff)

So, no potential drop along wire

That is …. (for example)

Perfect wires!

A perfect wire has zero resistance (negligible resistance to other stuff)

So, no potential drop along wire

Whatever V is here …

Perfect wires!

A perfect wire has zero resistance (negligible resistance to other stuff)

So, no potential drop along wire

Whatever V is here …

It’s the same here!

No matter what current is flowing!

Points at the same electric potential….

Points at the same electric potential….

Every point of the same color is at the same potential!

Large ‘resistance’

Small ‘resistance’

Resistivity of a material

Resistance: depends on the object’s size, shape, and composition.

Resistivity: depends ONLY on the object’s composition.

Example: 18.13 What is the resistance of a 3.5 m length of copper wire 1.5 mm in diameter?

Example: Now, how much current will flow in that wire if it’s hooked up to a 1.5 V battery?