OHM’S LAW :
OHM’S LAW By: Mr. Augosto M. Directo Voltage, Current and Resistance :
Voltage, Current and Resistance Voltage is the amount of energy per charge move electrons in one point to another in a circuit. It is measured in volts (v)
Current is the rate of charge flow and it is measured in amperes (A). Its unit is (I).
Resistance is the opposition of current and it is measured in ohms (W) DC Current vs. AC Current :
DC Current vs. AC Current The current from a battery is always in the same direction.
One end of the battery is positive and the other end is negative.
The direction of current flows from positive to negative.
This is called direct current, or DC. DC Current vs. AC Current :
DC Current vs. AC Current If voltage alternates, so does current.
When the voltage is positive, the current in the circuit is clockwise.
When the voltage is negative the current is the opposite direction.
This type of current is called alternating current, or AC. Series and Parallel Circuit :
Series and Parallel Circuit In a series circuit , the current through each of the components is the same, and the voltage across the components is the sum of the voltages across each component.
In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through each component. Series Vs. Parallel :
Series Vs. Parallel Series Circuits
A series circuit is a circuit in which the current can only flow through one path.
Current is the same at all points in a series circuit
Parallel Circuits
In contrast, in a parallel circuit, there are multiple paths for current flow.
Different paths may contain different current flow. This is also based on Ohms Law
Total resistance will be less than the smallest resistor Slide 7:
In Series circuit; the total current is equal
Rtotal = R1 + R2 + R3
V total = V1 + V2 + V3
In Parallel Circuit; the total voltage is equal
1 = 1 + 1 + 1 + 1
Rtotal R1 R2 R3 Rn
I total = I1 + I2 + I3 By Analogy: Series Vs Parallel :
By Analogy: Series Vs Parallel E R2 R1 I E R1 R3 R2 I1 I3 I2 Advantages of parallel circuits :
Advantages of parallel circuits Parallel circuits have two big advantages over series circuits:
1. Each device in the circuit sees the full battery voltage.
2. Each device in the circuit may be turned off independently without stopping the current flowing to other devices in the circuit. Ohms Law :
Ohms Law States that the voltage is directly proportional to the current in the circuit.
It is discovered by the German Physicist George Simon Ohm.
Ohm’s law is:
V = IR ,
I = V/R,
R= V/I Unit Modifiers for Reference :
Unit Modifiers for Reference Smaller
Deci = 10-1
Centi = 10-2
Milli = 10-3m
Micro = 10-6
Nano = 10-9
Pico = 10-12p
Fento = 10-15 Larger
Kilo = 103k
Mega = 106
Giga = 109
Tera = 1015 Series Circuit Analysis :
Series Circuit Analysis Example #1
A 4v battery is placed in a series circuit with a 2 resistor.
What is the total current that will flow through the circuit? I = ? 2 4v Solution :
Solution Given:
V = 4v
R = 2
I = ?
I = V/R
= 4v/2
I = 2A Slide 14:
Example#2
A 110v supplies a load with a resistance of 3,5, and 7 respectively, find the current in the circuit? I = ? 110v 3 7 5 Solution :
Solution Given:
V= 110 v
R1 = 3
R2 = 5
R3 = 7
I = ?
Rtotal = R1 + R2 + R3 I = V/R
= 3 + 5 + 7 = 110/12
= 12 = 9.17 A Parallel Circuit Analysis :
Parallel Circuit Analysis Example # 3
A 220v is connected in parallel with the load. It has a resistance of 5ohms and 10ohms. Find the Total current and the I1 and the I2 220v 5 10 Solution :
Solution Given:
V=220v
R1 = 5
R2 = 10
I1 = V/ R1 I2 = V/ R2
= 220 / 5 = 220 / 10
= 44 A = 22 A
RT = R1 R2 / R1 + R2 IT = V / RT
= (5 x 10) / 15 = 220/3.33
= 3.33 = 66.06A Slide 18:
Example #4
Find the total voltage and the total resistance of the load if the total current is 15A and it has a R1 of 6 ohms and R2 of 2 ohms. V=? 6 2 IT =15 Solution :
Solution Given:
R1 = 6
R2 = 2
IT =15 A
RT = R1 R2 / R1 + R2 VT = IT / RT
= (6 x 2) / 8 = 15 A x 1.5
= 1.5 = 22.5 v
I1 = V/ R1 I2 = V/ R2
= 22.5 / 6 = 22.5 / 2
= 3.75 A = 11.25 A Electric Power, AC, and DC Electricity :
Electric Power, AC, and DC Electricity The watt (W) is a unit of power.
Power is the rate at which energy moves or is used.
Since energy is measured in joules, power is measured in joules per second.
One joule per second is equal to one watt. Power in electric circuits :
Power in electric circuits One watt is a pretty small amount of power.
In everyday use, larger units are more convenient to use.
A kilowatt (kW) is equal to 1,000 watts.
The other common unit of power often seen on electric motors is the horsepower.
One horsepower is 746 watts. Formula of Power :
Formula of Power Power (watts) P = VI Voltage (volts) Current (amps Slide 23:
Example # 5
A light bulb with a resistance of 1.5Ω is connected to a 1.5-volt battery in the circuit shown at right.
Calculate the power used by the light bulb. Solution :
Solution Given:
V = 1.5v
R = 3
I =V / R P = V I
= 1.5 / 3 = 1.5 x 0.5
= 0.5 A = 0.75 watts Paying for electricity :
Paying for electricity Electric companies charge for the number of kilowatt-hours used during a set period of time, often a month.
One kilowatt-hour (kWh) means that a kilowatt of power has been used for one hour.
Since power multiplied by time is energy, a kilowatt-hour is a unit of energy.
One kilowatt-hour is 3.6 x 106 joules. Exercises: :
Exercises: I. Identification:
Direction: Identify the following questions.
1. It is the amount of energy per charge move electrons in one point to another in a circuit. It is measured in volts?
2. It is a type of current which the current from a battery is always in the same direction?
3. It is the opposition of current?
4.It is the rate of charge flow?
5. It is the rate at which energy moves or is used? Slide 27:
6. One horsepower is equivalent to how many watts?
7. In series circuit the total _______ is equal /the same.
8. In parallel circuit the total _______ is equal/the same.
9. A circuit where the voltage across each of the components is the same, and the total current is the sum of the currents through each component.
10. It States that the voltage is directly proportional to the current in the circuit.
11. A circuit where the current through each of the components is the same, and the voltage across the components is the sum of the voltages across each component.
12. Ohm’s Law is discovered by whom a German Physicist. Slide 28:
13. what does kwh means?
14. How many joules are there in a kwh?
15. The other common unit of power often seen on electric motors is?
II. Problem Solving:
1. A 115 volt is connected in parallel with the load. It has a I1 3 A and R2 15 ohms. Find the IT and the RT. I1 115v I1 = 3A 15 Slide 29:
2. Resistors 1, 2 ,3 has a resistance of 4 , 8 and 12 are connected in series with a total current of 25 A. Find the total voltage and the V1 , V2 , V3.
3. A motor has a voltage of 220 volts and a resistance of 35 . Find the current and the power. V1 VT = ? 4 8 12 IT = 25A Key Answers :
Key Answers I. Identification
1. voltage
2. direct current
3. resistance
4. current
5. power
6. 746watts
7. current
8. voltage Slide 31:
9. Series Circuit
10 Ohm’s Law
11. Parallel Circuit
12. George Simon Ohm
13. kilo watt hour
14. 3.6 x 106 joules
15. horsepower
II. Problem Solving
1. Given:
V = 115 v R2 = 15
I1 = 3A
R1 = V / I1 I2 = V/ R2 RT = R1 + R2 IT = V/ RT
= 115/ 3 A = 115/15 = 15 + 38.33 = 115 / 53.33
= 38.33 = 7.67A = 53.33 = 2.16A Slide 32:
2. Given:
R1 = 4
R2 = 8
R3 = 12
IT = 25A
V1 = IT R1 V2 = IT R2 V3 = IT R3
= 25 x 4 = 25 x 8 = 25 x 12
= 100 v = 200v = 300v
RT = R1 +R2 +R3 VT = IT RT
= 4 + 8 + 12 = 25 x 24
= 24 = 600v Slide 33:
3. Given:
V= 220v
R = 35 ohms
Find: I? P?
I = V/R P = V I
= 220 / 35 = 220 x 6.29
= 6.29A = 1382.86 watts Thank You :
Thank You God Bless