Electrical Force

1. Electrical Force Unit 1.3

2. Objectives • Explain the similarities and differences between Newton’s law of universal gravitation and Coulomb’s law. • Explain how the force between two like charges and the force between two unlike charges are different. • Describe how to create an electric field. • Define electric potential difference or voltage. • Differentiate between AC and DC . • Identify the most common sources of DC voltage. • Describe how to connect DC voltage sources so that voltages will add.

3. Gravitational Force • Newton’s universal law of gravitation Every object in the universe attracts every other object with a force that is directly proportional to the mass of each body and that is inversely proportional to the square of the distance between them. m1m2 N m2 Fg= G Where G = 6.67 x 10-11 d2 kg2

4. Electric Charge • Charge – the property of an object that causes electrical force. • Positive and Negative charges exist. • Like charges repel • Unlike charges attract (opposites attract) + -

5. Gravity and Electrical Force • Both are inverse square laws • Both give magnitude of the force one object exerts on another • Gravitational force is always attractive, but electrical force can be attractive or repulsive. • Gravitational forces govern large bodies • Electrical forces govern smaller bodies

6. Origins of electrical charge • Anatomy of an atom? • Protons – Positive charge • Neutrons – Neutral charge • Electrons – Negative charge • Normal atom has no net charge • Electrons can move in some substances • Principle of conservation of charge • The net electrical charge in an isolated system never changes. (hair and comb)

7. Electrical Force • Charles Coulomb and his law • The electrical force between two charged bodies is directly proportional to the charge on each body and inversely proportional to the square of the distance between them. charges q1q2 N m2 FE= K Where K = 9.0 x 109 d2 C2 Distance SI unit = Coulomb (C) Elementary Charge - Charge on one electron or proton = 1.6 x 10-19 C

8. Gravitational and Electrical Fields • The fields are the space between two masses or charges and the effects within them. Opposite charges Positive charge Negative charge Opposite plates Like charges

9. Electrical Potential • When charges are within an electric field an electrical potential difference is created. • Volt – unit of measurement for potential difference. (electrical potential, voltage) • Voltage is the prime mover in electrical systems (like pressure in fluid system).

10. Electrical Potential Flow caused by “potential” difference Fluid system Electrical system Current – Flow

11. Sources of Difference A pump is a source of fluid pressure difference A battery is a source of electrical potential difference

12. Components of Electrical Systems • Voltage source (battery or generator) • Conductors (wires or circuit board) • Load (motor, lights, etc.) • Control element (switch) Electrical circuit Control Voltage source Electrical Load conductors

13. AC vs. DC • Direct Current (DC) – charge flows in one direction. • Batteries • Solar Panels • Alternating Current (AC) – charge flows back and forth. • Alternators • Generators

14. Batteries • Cell – a single unit housing one or more chemical. • Battery – consist of several cells • How does it work? • Chemicals react and electrons are removed creating a potential difference (voltage) • Primary Cells – one time use batteries (cannot be recharged) • Secondary Cells – Can be recharged

16. Connecting cells • Electrodes – the positive and negative terminals on a battery (cell). • Positive electrode (red) called anode. • Negative electrode (black) called cathode. • Voltages add when cells are connected in series. - + - + Battery Battery 1.5 v 1.5 v - + 3.0 v

17. A Simple DC Circuit

18. AC Circuits • An AC voltage source reverses the positive and negative terminals many times per second. • The current flows one direction and then the other then the other….. • This changing of polarity or cycling is called frequency, measured in cycles per second (hertz).

19. AC vs DC

20. Where does AC come from? • The main source of 60hz current comes from electrical power plants, through power lines and into your homes. • Generally 110 – 120 volts • Some larger electrical devices need 220 – 240 volts.

21. Summary • Newton’s law of universal gravitation and Coulomb’s law are both inverse squares laws. The magnitudes of both forces decrease with the square of the distance between the masses and the charges. • Atoms are composed of proton, neutrons and electrons. Protons are positively charged, electrons are negatively charged, and neutrons have no charge. • The flow of electrons in an electrical system is a current. • Unlike charges attract, like charges repel.

22. Summary (cont.) • An electric field is a model of the alteration of space around one or more charges. You can use the field to predict the force exerted on a charge placed in a the field. • The potential difference, or voltage, between two points in a uniform electric field is the product of the field strength and the distance between the points. • Voltage is the prime mover in electrical systems. • A battery is a source of DC voltage. It can maintain a current in an electrical circuit. • Batteries or cells can be connected in series to increase voltage.