Electricity and Magnetism

 Static electricity:  stationary, non-moving electrons – produced by friction (both substances becoming electrically charged)

 Electrons removed by some substance during friction:

1) material that loses electrons = positively charged
2) material that gains electrons = negatively charged

 Law of Electrical Charges:  Like charges repel, unlike charges attract

 Grounding:  sharing electrons with a sink (something like the Earth that absorbs them)

 Insulators:  materials that greatly resist passage of electrons through them but can store electrons on surface (static electricity)

 Conductors:  materials through which electrons flow freely (silver and copper are the best conductors)

 Demo:  charged object such as comb touched to small pieces of paper

 Common examples of static electricity:

1) walking across a rug on a cold day with rubber/plastic soled shoes
2) sliding across nylon car seats
3) combing hair with rubber/plastic comb
4) clothes in dryer
5) lightning

 Lightning

 The normal fair weather charge distribution consists of a negatively charged earth's surface and a positively charged upper atmosphere. The electrical effects in thunderstorms maintains this separation of charges by lightning (otherwise in as little as an hour the positive charge would move downward and neutralize the charge at the surface.)

 The charge distribution is created by strong updrafts in the storm acting in concert with electrical interactions between cloud droplets.

 When the charge difference within the cloud becomes large enough, charge will be transferred within the cloud or from cloud to ground.

 First part of bolt is called the stepped leader. This advances in steps of about 50 meters in a time of 0.000 002 seconds per step. Between steps it is not visible. When it comes close to the ground, strong attractive forces cause an upward discharge which meets the downward moving stepped leader. This creates a conducting path between the ground and cloud.

 The average speed toward earth = 75 miles/second

 Current carried = 100-200 amperes

 Visible diameter is about 1 meter

 Actual diameter is about 2-3 cm

 Next comes the first return stroke, from ground up to cloud base, taking about 0.000 1 seconds to reach cloud base and 0.01 seconds for the negative charges to drain completely off to the ground. All parts of branches are visible.

 Next come streamers within the cloud resupplying main channel with negative charges (from other parts of cloud.) Dart leaders move rapidly down channel (1000 miles/second).

 Next comes another return stroke and then subsequent streamers, dart leaders, and return strokes. The total number is usually 4 or 5 but can go as high as 20. The total duration of the lightning flash is a few tenths of a second.

Electroscope Work:

 An electroscope is a device used to detect the presence of static electrical charges:

Conduction occurs when electrons are gained or lost through direct contact with a charged object.  Any object charged through conduction takes on the same charge as that of the original object. 

 Demonstration in class of positive and negative conduction.

 Electrostatic induction:  induction is a separation of positive and negative charges on a conductor induced by the presence of a nearby charged object.  The charge excesses which build up are called induced charges.

 Demonstration of induction charges.

 Demonstration of testing for unknown charges.

 Transition to Current Electricity:

 Wet Cells – developed in the 19^th century by Alessandro Volta  - devices that change chemical energy into electrical energy (electrochemical cell)

 à composed of strips of 2 different metals placed in a liquid that will conduct electricity

à electrode = strips of different metals (often use zinc and copper)

à electrolyte = liquid that conducts electricity (mix acid or base or salt in water, H₂SO₄ often used)