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Electricity

Electricity. Chapter 1. Newspaper Demonstration. Before the strips of newspaper were in contact with the plastic bag : they hung straight down while touching each other After sliding the plastic bag over them : they REPELLED each other WHY?

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Electricity

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  1. Electricity Chapter 1

  2. Newspaper Demonstration • Before the strips of newspaper were in contact with the plastic bag: • they hung straight down while touching each other • After sliding the plastic bag over them: • they REPELLED each other • WHY? • Rubbing the plastic bag charges the strips with like charges. Like charges REPEL (try to get as far away from each other as possible)

  3. Atom • Smallest unit of material that still has the characteristics of that material • What makes up an atom? • Protons: positive • Neutrons: neutral • Electrons: negative • What is the overall charge on an atom? NEUTRAL (ZERO)

  4. Electric Charge Attraction • Particles with unlike charges attract - pull on each other • Particles with like charges repel – push each other away • Can be 2 positive or 2 negative charges Repulsion

  5. Static Charge • A buildup of electric charge in an object caused by the presence of many particles with the same charge • Materials can have: • Positive charge: more protons than electrons • Negative charge: more electrons than protons • Electrons move more easily than protons – charges result from their movement • 2 Types of static charge: • Charging by contact • Charging by induction

  6. Charging by Contact At first, a glass rod and balloon each have balanced, neutral charge When they touch, electrons move from the rod to the balloon Afterwards, the balloon has a negative charge and the rod has a positive charge

  7. Van de Graaff Generator • How does it work? • Builds up STRONG electric charge through contact • As the sphere takes on negative charge electrons spread over the students skin and hair • Hairs now all have same charge and repel one another

  8. How materials affect static charge Materials higher on the list tend to give up electrons to materials lower on the list

  9. Charging by Induction At first, the rod has a negative charge and the balloon has a balanced charge When the rod come close to the balloon, electrons in the balloon move away from the rod When the rod is removed, electrons in the balloon spread out evenly as before

  10. Revisiting the Newspaper Demo • Draw a diagram of what is occurring between the newspaper strips and the plastic bag.

  11. Balloon – Wall Demonstration • What is occurring in this situation? • Before the charged balloon comes near the wall, the atoms in the surface of the wall are neutral • As the balloon nears the wall, the electrons move deeper into the wall leaving the wall closest to the balloon with a positive charge

  12. Charges can move from one place to another Section 1.2

  13. What causes a charge to move? • The force of attraction or repulsion between charged particles.

  14. Electrical Potential Energy (EPE) • Energy a charged particle has due to its position in an electric field • Like charges repel: EPE increases when particles with like charges are pushed closer together • Opposite charges attract: EPE decreases when particles with like charges move away from one another • EPE is measured in volts

  15. Fluorescent Bulb Demonstration • What happened in the bulb when the spark jumped? • The bulb lit up briefly • How might you explain this observation? • A static charge moved from the balloon to the bulb and lit the bulb

  16. How Lightning Forms Charge Separation: Collisions between particles in storm clouds separate charges. Negatively charged particles collect at the bottom of the cloud Charge Buildup: The negatively charged bottom of the cloud induces a positive charge in the surface of the ground. Static Discharge: The charge jumps through the air to the ground. The energy released by the discharge causes thunder and lightning.

  17. Materials Affect Charge Movement • Conductors • Material that allows electric charge to pass through easily • Examples: iron, steel, copper, aluminum • Insulators • Material that does not allow electric charge to pass through it • Important in electrical safety because they keep charge away from the body • Examples: plastic, rubber

  18. Resistance • Materials resist the movement of a charge in different amounts • Electrical resistance: the property of a material that determines how easily a charge can move through it • Units: ohms (Ω) • Factors that affect resistance: • Type of material – conductors have LOW resistance; insulators have HIGH resistance • Amount – a long wire has more resistance than a short wire • Shape – a thin wire has more resistance than a thick wire

  19. Grounding • Providing a harmless, low resistance path (a ground) for electricity to follow. • In many cases this path actually leads to the ground • EXAMPLE: • Protect buildings from damage by lightning : • Most building have a lightning rod located high up on the building, which is made of a material that is a good conductor • The rod is connected to a conductor cable, which is connected into the ground. • Lightning hits the rod and passes harmlessly through the cable into the ground

  20. Electric current is a flow of charge SECTION 1.3

  21. Electric charge can flow continuously • Static charge cannot make your TV or other electronics play. • Static charge contains a limited amount of charge • Static charges moves from higher to lower potential • Electric current: flow of charge • Some electrical pathways receive a continuous supply of charge (the difference between the 2 ends of the pathway are the same)

  22. Electric Current • Can only flow if it has a pathway to follow – a material to conduct it • Charge flows from higher to lower potential • Charge that flows steadily has a certain rate of flow. • Standard unit of measure: ampere (amp) • Amount of charge that flows past a given point per unit of time

  23. Resistance • Recall: measured in ohms (Ω) • Current decreases as resistance increases

  24. Ohm’s Law Current = ____Voltage______ I = __V__ Resistance R

  25. Example Problem What is the current in an electrical pathway with an electrical potential of 120 volts and a resistance of 60 Ω?

  26. LIST V = 120 V R = 60 Ω I = ?

  27. Plan I = _V_ R

  28. Work I = 120 V 60Ω I = 2 A

  29. YOU TRY! What is the voltage of a circuit with 15 amps of current and toaster with 8 ohms of resistance?

  30. Electrochemical cells • Produces electric current using the chemical or physical properties of different materials • Example: batteries • 2 Types: • Primary cells • Storage cells

  31. Primary cell • Produce electric current through chemical reactions • Reactions continue until the chemicals are used up • Example: dry cell battery

  32. Storage cells • Produce current through chemical reactions that can be reversed inside the battery • Example: Car Battery • Discharging: when storage cells are producing current • Battery discharges to operate the motor • Operate lights when car is off • Charging: sending current through the battery in the opposite direction (reverses reaction) • When car is running, battery is continually being charged • Car’s alternator produces the current. Current from alternator runs through battery in reverse to recharge

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