Lecture #25 What’s in the wall??

Last Time We discussed how charge can be studied. We discussed how charge can be studied. There are two kinds of charge There are two kinds of charge –NEGATIVE  electrons, light, mobile and easy to push around –POSITIVE  protons, located in the nucleus of atome.  “heavy”, tough and not to be messed with. Charges interact with each other. Charges interact with each other.

Results of our Study Charges can be created by mechanical rubbing or separation of bound surfaces. Charges can be created by mechanical rubbing or separation of bound surfaces. Charge cannot be created or destroyed Charge cannot be created or destroyed –It is there before we rub –All we mortals can do is move it around Like charges repel each other Like charges repel each other Un-Like charges attract each other. Un-Like charges attract each other.

So… what about this balloon trick? insulator

A Van de Graaff generator

What about our Volunteer?? This thing stores charge

This thing also has the capacity to store charge This is called a capacitor

Charging a Capacitor takes WORK The work per unit charge is called the Potential Difference

Capacitor separated charge

POTENTIAL DIFFERENCE Units are VOLTS Units are VOLTS It takes one Joule of energy (work) to move one coulomb through a potential difference of one volt. It takes one Joule of energy (work) to move one coulomb through a potential difference of one volt. We usually refer to potential differences as “the voltage” We usually refer to potential differences as “the voltage”

Another Definition If a capacitor has a potential difference V from one terminal (plate) to the other If a capacitor has a potential difference V from one terminal (plate) to the other And if the capacitor has a charge Q on the plate, And if the capacitor has a charge Q on the plate, Then Then where C is measured in Farads. where C is measured in Farads.

Example A F capacitor has 5 volts across it. How much charge is on the capacitor? A F capacitor has 5 volts across it. How much charge is on the capacitor?

Big Cap Demo

What’s Happening?

Huh? The electrons were “pushed” by the difference in potential (voltage). The electrons were “pushed” by the difference in potential (voltage). They came around to the (+) plate and neutralized the positive plate. They came around to the (+) plate and neutralized the positive plate. The two plates and the wires are said to form an electrical circuit. The two plates and the wires are said to form an electrical circuit. The movement of charge is called electrical current. The movement of charge is called electrical current.

Symbols & Definitions “Stand” at one point in a circuit where electrical current is flowing. “Stand” at one point in a circuit where electrical current is flowing. Count the amount of charge in coulombs that pass where you are standing in one second. Count the amount of charge in coulombs that pass where you are standing in one second. This amount of charge, divided by the time of passage is defined as the CURRENT. This amount of charge, divided by the time of passage is defined as the CURRENT. The current is measured in AMPERES and we use the symbol I. The current is measured in AMPERES and we use the symbol I. One ampere is a current of one coulomb per second. One ampere is a current of one coulomb per second.

The Battery The battery behaves in a way like a capacitor. The battery behaves in a way like a capacitor. As charge is withdrawn from one side of the battery the battery replaces it via a chemical reaction in the battery cell. As charge is withdrawn from one side of the battery the battery replaces it via a chemical reaction in the battery cell. The battery therefore can maintain a constant potential difference or voltage between its terminals. The battery therefore can maintain a constant potential difference or voltage between its terminals.

Electricity made a giant leap in 1800 when Volta invented the electric cell and the battery (of cells). For the first time investigators had a source of steady electric current. Alessandro Volta, ( )

Pretty Complicated Lots of sources on the internet to find out how these things really work.

The Commercial Battery

Something that we ALL Know!

The Circuit – A Thunker A I1I1 I2I2 A

Current must be the same! A I V

Ohm Sweet Ohm The more resistance, the less the current. The more resistance, the less the current. Like stepping on a garden hose. Like stepping on a garden hose. If you stepped in two spots on a garden hose, the flow would even be less. If you stepped in two spots on a garden hose, the flow would even be less. The “resistance” would double. The “resistance” would double. So let’s look at a few bulbs. So let’s look at a few bulbs.

Example 10 ohm 6 volts

Add a Bulb B C 6V 10  Total = 20  I=V/R=6/20=0.3 amperes … ½ the previous current

Parallel Circuit