Principles of Technology
Work in the Electrical
System
Work in the Electrical System
1. What is the charge on a proton?
- positive
2. What is the charge on an electron?
- negative
Work in the Electrical System
3. What is electrical current?
- flow of electrons through
conductors
4. How do electrons flow in a DC
circuit?
- electrons flow the negative
terminal through the conductors to
the positive terminal
Work in the Electrical System
5. How do we measure electric
charge?
- coulombs (q)
- one coulomb is equal to 6.25 x
1018 electrons
Work in the Electrical System
6. How do we calculate electrical
work?
- Work = Voltage Difference x
Charge moved
-W=Vxq
7. What is the unit for electrical work?
- Joules (volt-coulombs)
Work in the Electrical System
8. Can batteries be recharged?
- some can, but it takes electrical
work to do so
- to recharge a battery, electrical
current flows through the battery
opposite from the way it discharge
current
Work in the Electrical System
9. Example – A dead 12 V battery
can store 8000 coulombs of
electrical charge. How much
electrical work is done to recharge
the battery if a voltage difference of
12 V is applied?
Work in the Electrical System
- Work = voltage difference x charge
- work = 12 V x 8000 coulombs
- work = 96,000 joules
Work in the Electrical System
10. How are electrical current and
charge related?
- while coulombs cannot be easily
measured, the rate at which
coulombs move (aka current) can
be
11. How is current measured?
- amperes (I)
Work in the Electrical System
12. What is the equation for current?
- Current = Charge
Time
- I = q (or q = I x t)
t
Work in the Electrical System
13. What is the equation for charge
from current?
- charge = current x time
- q = I x t, where
- q  coulombs
- I  amperes
- t  seconds
Work in the Electrical System
14. Example – A DC electric motor
operates for two minutes on a
voltage of 12 volts while drawing a
current of 4 amperes. How much
electrical work is done?
Work in the Electrical System
1. Find q first
-q=Ixt
- q = (4 A) x (120 sec)
- q = 480 coulombs
2. Solve for work
-w=Vxq
- w = 12V x 480 C
- w = 5760 J
Work in the Electrical System
15. What are effects of electrical
work?
- you cannot see electrical work,
only the effects of it
- electrical work produces
movement, heat, light, or sound
Work in the Electrical System
16. What are some examples of how
work causes movement?
- electrical motors or switches
Work in the Electrical System
17. What are some examples of how
work causes heat?
- heaters, dryers, ovens, light bulbs,
motors
18. What are some examples of how
work causes light?
- flashlights, lamps, radio waves,
microwaves, cell phone signals
Work in the Electrical System
19. What are some examples of how
work causes sound?
- speakers, TVs, PA systems, alarm
clocks
20. What is electrical efficiency?
- ratio of useful work done (output)
to the required electrical input
Work in the Electrical System
21. What is the equation for efficiency
of an electrical device?
- Eff = W out x 100%
W in
- W out = work done by device
- W in = electrical energy required
Work in the Electrical System
22. Example – A crane lifts an I-beam
a height of 2.5 meters. The I-beam
weighs 2000 N. The crane requires
an input of 5760 J. What is the
efficiency of the crane?
Work in the Electrical System
1. Find the work done
-W=FxD
- W = 2000 N x 2.5 meters
- W = 5000 J
Work in the Electrical System
2. Find the efficiency
-Eff = W out x 100%
W in
- Eff = 5000 J x 100%
5760 J
- Eff = 86.8 %
Work in the Electrical System
23. How is electricity produced?
- coal, gas, or oil is burned to produce
heat energy
- the heat energy is used to convert water
into steam
- steam pressure is used to drive turbines
- turbines drive electrical generators to
produce electricity
Work in the Electrical System
24. Why use electricity?
- easy to transport over long distances
through wires
- clean – no by products
- convenient and easy to use; provides a
ready source of energy when you want it
- operates many machines and electronic
devices