Chapter 2.3 Notes

1. Chapter 2.3 Notes Work in Electrical Systems

2. A force does work on an object when it moves the object. A collection of charges creates an electric field. In an electrical system, when there is a difference between charges (positive and negative) a force is exerted and causes charges to move and does work.

3. There must be an electrical voltage between two points within a field so that a force can be exerted on a charge placed in the field. If you connect the two points, the force causes the charge to move from one point to the other. The presence of voltage is what causes the charge to move and to do work. Batteries are a source of voltage.

4. Electric charge is measure in coulombs (C). A single electron has a charge of 1.6 x 10 -19 C One Coulomb is equal to the charge of 6.25 billion billion electrons.

5. Electrical work = Change in Voltage x charge W = rV x q

6. A battery is being charged. During the first 15 minutes of charging, 800 C of charge are deposited in the battery by an average potential difference (voltage) of 8 V. How much electrical work is done during this 15 minutes? W = rV x q W = 8 V x 800 C W = 6400 VC = 6400 Joules

7. The rate at which charges flow is called electric current Electric current in measured in coulombs/sec which is equal to an ampere. A device measuring an ampere is called an ammeter.

8. Current = charge / change in time I = q/rt

9. A 12 volt DC motor used a current of 4 amperes. Find the electrical work done to operate this motor for 2 minutes. q = I x change in t Minutes to seconds; 2 minutes = 120 sec q = 4 A x 120 sec q = 480 C W = change in V x q W = 12 V x 480 C W = 5760 J

10. List 4 things electrical work can do: Produce Movement Create Thermal Energy Create Light Create Sound

11. Produce Movement Move fluids with pumps Electric Cars Electric fans

12. Create Thermal Energy Electric Heaters Clothes Dryers Furnaces Ovens Blow Dryers

13. Create Light Light bulbs Laser

14. Produces Sound Radios TV�s CD players Cell Phones

16. Not all electrical work is converted to mechanical Work. Some of the electrical work is lost to thermal energy (heat)

17. Efficiency = output work / input work Percent Efficiency = output work / input work x 100 %

18. The input work is 5760 Nm. The crane lifts the bean that weighs 2000 N to a height of 2.5 meters. What is the output work? What is the efficiency? What is the percent efficiency? Efficiency = output work / input work Output Work = Force x distance Output Work = 2000 N x 2.5 m Output Work = 5000 Nm Efficiency = 5000 Nm / 5760 Nm Efficiency = .87 or 87%

19. Stop after Steam Plant

20. Most electricity is produced at electrical generating stations by: Burning coal, gas, or oil to produce heat Heat converts water into steam Steam turns a turbine Turbine drives an electrical generator to produce electricity

21. The overall efficiency of this process to produce electricity is 60%. Where would they lose the majority of their efficiency? Lose the mechanical work to heat