1. Electrical principles - Power Power Work Energy
Objectives
Define Energy and Power
Calculate power in a circuit
Explain energy conversation and voltage drop
Discuss power supplies and their characteristics

3. Why won't this assembly spin forever, once started?
Suppose someone mechanically joined an electric motor to an electric generator, then electrically joined the two devices together in an effort to make a perpetual-motion machine:

4. Energy can be lost due to heat and friction
This will not work because neither the motor nor the generator is 100% efficient.
Useful power does what the motor is designed for

5. Work
When there is movement against a resistance by using force, work is achieved. The further the movement and the greater the resistance, the more work achieved.
The formula for work is:
Work = Force x Length
Work is measured in joules.
A joule is the work done by a force of one newton for a distance of one meter.

6. Energy
Energy is the ability to do work. The potential to do work is defined by the following formula:
Energy = Power x Time
The unit for energy is the joule (J).
Energy can exist in many forms:
magnetic
light
nuclear
chemical
sound
radiant.
Electrical energy is often large and measured in kilo watt hrs (KWh).

7. Understanding power

8. Understanding power
When somebody plugs an appliance to use electricity means that person provides electrical energy for the appliance.
The appliance usually functions by turning that electrical energy into heat, light, or work — or converts it into electrical energy again in a different form.
The current from the plug going in and out of the appliance effectively carries the power and the appliance absorbs the power.

9. Power
The power output of a lamp, resistor, or other component, is defined as the rate of change of electrical energy to heat, light, or some other form of energy.
Power is the rate at which work is done. It is calculated by Power = Work / Time
The unit for power is the watt (W).
One watt is the amount of power when one joule of energy is used in one second.

10. Power and Energy
Electric Power (P) is measure in watts (W) kilowatts (kW), megawatts (MW), and milliwatts (mW)
The amount of power consumed by an electrical device is the rate at which it dissipates energy
In a flashlight, chemical potential energy in the battery is converted to electrical energy which is passed to the bulb and dissipated in light and heat.
Energy (E) or work is measured in joules (J), so watts are joules per second, and joules are watt-seconds.

11. Energy is the capacity for doing work- ability to do work
Work is the result of conversion of energy from one form to another.
Power is the rate at which work is done.
One watt is the amount of power when one joule of energy is used in one second.
Quantity
Symbol
Unit
Unit expression
Abbreviation
Meaning
Energy E
joules J
Ability to do work
Work W
Result of energy
Power P
watt
Joules/seconds
Is the rate at which work is done

12. Power is certain amount of energy in a certain length of time

13. Calculating power in a circuit
Three equations for power in circuits that are collectively known as Watt’s law are:

14. What power is dissipated in a 27 W resistor if the current is 0.135 A?
Example
What power is dissipated in a 27 W resistor if the current is A?
Given that you know the resistance 27Ω and current, substitute the values into P =I 2R.

15. Example
What power is dissipated by a heater that draws 12 A of current from a 110 V supply?
The most direct solution is to substitute into P = IV.

16. What power is dissipated in a 100 W resistor with 5 V across it?
Example
What power is dissipated in a 100 W resistor with 5 V across it?
The most direct solution is to substitute into 2 V P R =
It is useful to keep in mind that small resistors operating in low voltage systems need to be sized for the anticipated power.

17. Resistor Power Rating
The power rating is the maximum amount of power that a resistor can dissipate without being damaged by excessive heat build-up.
It is not related to the ohmic value (resistance) but physical composition, size and shape
R=ρl/A ( l= distance; A area if cross section; ρ= resistance)

21. The Power rating of the resistor should be a bit above this value.
In a circuit there is a peak power of 3000W and a pick voltage across is 340 V. Find Power rating of the resistor.
Problem
First we find the max peak current that gives us a peak power of 3000W:
Now that we have our peak current we use the value voltage input range to find the ideal value for our resistor. Using Ohms Law: Ω
The power dissipated in the resistor is given by:
The Power rating of the resistor should be a bit above this value.

22. Which power rating of resistor is required for a 680 W current limiting resistor in series with an LED if the current flowing is 10 mA?
Any resistor of a higher rating will do, either 0.25 W or 0.5 W.

23. Resistor failures
Resistor failures are unusual except when they have been subjected to excessive heat. Look for discoloration (sometimes the color bands appear burned). Test with an ohmmeter by disconnecting one end from the circuit to isolate it and verify the resistance. Correct the cause of the heating problem (larger resistor?, wrong value?).
Normal Overheated

24. Energy W=Pt
The kilowatt-hour (kWh) is a much larger unit of energy than the joule. There are 3.6 x 106 J in a kWh. The kWh is convenient for electrical appliances.
What is the energy used in operating a 1200 W heater for 20 minutes?
P=1200 W = 1.2 kW
t= 20 min = 1/3 h
W= Pt=1.2 kW X 1/3 h =0.4 kWh

25. When there is current through a resistance, electrical energy is converted to heat energy.
This heat is caused by collisions of the free electrons within the atomic structure of the resistive material.

26. - + R1 R2 - - - - + - + - - -
The electrons have lost some energy in R1 resulting voltige drop and now they enter R2 with reduced energy as they flow through R2 they lose more energy, resulting in another voltage drop

27. The electrons flow through each of the resistor that are connected together to form a current path called series
Electrons have more energy when they enter a resistor than when they exit the resistor

28. Definition of the voltage is V = W/Q (energy per charge)
Q is constant so there are Wenter Wexit where Wenter bigger then Wexit so the voltage across R1 is bigger then voltage across R2
This decrease in voltage across a resistor due to loss of energy is called voltage drop.

29. What is the basic reason for energy conversation in a resistor?
Energy conversation in a resistor is caused by collisions of free electrons with the atoms in the material.
2. What is voltage drop ?
2. Voltage drop is a decrease in voltage across a resistor due to a loss of energy.

30. 3. What is the polarity of a voltage drop in relation to conventional current direction?
3. Voltage drop is positive to negative in the direction of conventional current.

31. Ampere-hour Rating of Batteries
Expected battery life of batteries is given as the ampere-hours specification. Various factors affect this, so it is an approximation. (Factors include rate of current withdrawal, age of battery, temperature, etc.)
How many hours can you expect to have a battery deliver 0.5 A if it is rated at 10 Ah?
20 h

32. Energy Efficiency
It is not something you can see or feel or sense in any direct way - it's just there. Secondly, energy does not disappear or get 'used up' but rather is converted or transferred into another form of energy. And this is where efficiency comes in.
Let's use the example of a light bulb! You don't want it to do anything, other than produce light. But the old traditional light bulbs we all should be getting rid of also produce heat, and quite a lot of it.

33. Efficiency The efficiency of a device relates to the performance.
The efficiency of an electrical machine (ie a motor) can be determined by comparing the input power against its output power.
No electrical device is 100% efficient and there will always be power losses. These losses include mechanical losses caused by friction and electrical losses caused by heat generated through current flow.
Mechanical efficiency can be calculated using the formula:
Efficiency = Power Output / Power Input x 100%

34. Example
What is the efficiency of a motor that consumes 800W and has losses of 200W?
Output power = 800W - 200W
Output power = 600W
 Efficiency = Power Output / Power Input x 100%
Efficiency = 600W / 750W x 100
Efficiency = 80%

35. Problem calculate efficiency of this bulb

36. A 60W lightbulb is put on for one minute
A 60W lightbulb is put on for one minute. A watt is equivalent to a joule per second, so it uses 3600J
The bulb produces 360J of light energy in that minute and also 3240J of heat energy. To calculate the efficiency...
Efficiency = [useful energy / total energy] x 100% = [360 / 3600] x 100% =10%

37. An energy saving bulb on the other hand is designed to produce as little heat as possible. In a similar experiment, but with an energy saving bulb, it produces only 80J of heat. So to get the same light output the bulb only needs to be J + 80J = J / 60s = 7W

38. Energy Power Joule Watt The rate of energy usage.
The ability to do work. The unit is the joule (J).
The rate of energy usage.
The SI unit of energy.
The unit of power. One watt is the power when 1 J of energy is used in 1 s.

39. A common unit of energy used mainly by utility companies.
Kilowatt-hour
Ampere-hour rating
Efficiency
A common unit of energy used mainly by utility companies.
A number determined by multiplying the current (A) times the length of time (h) that a battery can deliver that current to a load.
The ratio of output power to input power of a circuit, usually expressed as a percent.

40.  Quiz A unit of power is the a. joule b. kilowatt-hour
c. both of the above
d. none of the above 

41.  2. The SI unit of energy is the a. volt b. joule c. watt
d. kilowatt-hour 

42. 3. If the voltage in a resistive circuit is doubled, the power will be
a. halved
b. unchanged
c. doubled
d. quadrupled 

43. 4. The smallest power rating you should use for a resistor that is 330 W with 12 V across it is
a. ¼ W
b. ½ W
c. 1 W
d. 2 W 

44. 5. The power dissipated by a light operating on 12 V that has 3 A of current is
a. 4 W
b. 12 W
c. 36 W
d. 48 W 

45.  6. The power rating of a resistor is determined mainly by
a. surface area
b. length
c. body color
d. applied voltage 

46.  7. The circuit with the largest power dissipation is a. (a) b. (b)
c. (c)
d. (d) 

47.  8. The circuit with the smallest power dissipation is a. (a) b. (b)
c. (c)
d. (d) 

48. A battery rated for 20 Ah can supply 2 A for a minimum of
b. 2 h
c. 10 h
d. 40 h 

49.  a. Dividing the output power by the input power.
10. The efficiency of a power supply is determined by
a. Dividing the output power by the input power.
b. Dividing the output voltage by the input voltage.
c. Dividing the input power by the output power.
d. Dividing the input voltage by the output voltage. 

50. 11. A four-color resistor has the color-code: red-violet-brown-gold
11. A four-color resistor has the color-code: red-violet-brown-gold. If it is placed across a 10 V source, the expected power dissipated is
a. 27 mW
b. 120 mW
c. 270 mW
d. 370 mW 

52. Auto Battery, Terminal Corrosion problem
How much current we need to start a car?

53. Auto Battery, Terminal Corrosion problem
Without corrosion, the available starter current can be calculated with Ohm's Law:
The available starting power can be calculated from the power relationship.
How would these values be changed if there was a terminal resistance of 0.1 Ω?