Ohm’s Law Current, Voltage and Resistance | PPT | web4study

Ohm’s Law Current, Voltage and Resistance | PPT



Ohm’s Law Current, Voltage and Resistance

Ohm’s Law

3-1: The Current I = V/R

3-2: The Voltage V = IR

3-3: The Resistance R = V/I

3-4: Practical Units

3-5: Multiple and Submultiple Units

3-6: The Linear Proportion between V and I

3-7: Electric Power

3-8: Power Dissipation in Resistance

3-9: Power Formulas

3-10: Choosing a Resistor for a Circuit

3-11: Electric Shock

3-12: Open-Circuit and Short-Circuit Troubles

 

Ohm’s Law Formulas

  • There are three forms of Ohm’s Law:
  • I = V/R
  • V = IR
  • R = V/I

where:

  • I = Current
  • V = Voltage
  • R = Resistance

The Current I = V/R

  • I = V/R
  • In practical units, this law may be stated as:
  • amperes = volts / ohms

Practical Units

The three forms of Ohm’s law can be used to define the practical units of current, voltage, and resistance:

  • 1 ampere = 1 volt / 1 ohm
  • 1 volt = 1 ampere × 1 ohm
  • 1 ohm = 1 volt / 1 ampere

Multiple and Submultiple Units

Units of Voltage

  • The basic unit of voltage is the volt (V).
  • Multiple units of voltage are:
  • kilovolt (kV)
    1 thousand volts or 103 V
  • megavolt (MV)
    1 million volts or 106 V
  • Submultiple units of voltage are:
  • millivolt (mV)
    1-thousandth of a volt or 10-3 V
  • microvolt (μV)
    1-millionth of a volt or 10-6 V

Units of Current

  • The basic unit of current is the ampere (A).
  • Submultiple units of current are:
  • milliampere (mA)
    1-thousandth of an ampere or 10-3 A
  • microampere (μA)
    1-millionth of an ampere or 10-6 A

Units of Resistance

  • The basic unit of resistance is the Ohm (Ω).
  • Multiple units of resistance are:
  • kilohm (kW)
    1 thousand ohms or 103 Ω
  • Megohm (MW)
    1 million ohms or 106 Ω

The Linear Proportion between V and I

  • The Ohm’s Law formula I = V/R states that V and I are directly proportional to any one value of R.
  • Fig. 3.5: Experiment to show that I increase in direct proportion to V with the same R. (a) Circuit with variable V but constant R. (b) Table of increasing I for higher V. (c) Graph of V and  I values. This is a linear volt-ampere characteristic. It shows a direct proportion between V and   I.

When V is constant:

  • I decrease as R increases.
  • I increase as R decreases.

Examples:

  • If R doubles, I is reduced by half.
  • If R is reduced to ¼, I increase by 4.
  • This is known as an inverse relationship.

Linear Resistance

  • A linear resistance has a constant value of ohms. Its R does not change the applied voltage, so V and I are directly proportional.
  • Carbon-film and metal-film resistors are examples of linear resistors.

Nonlinear Resistance

  • In a nonlinear resistance, increasing the applied V produces more current, but I do not increase in the same proportion as the increase in V.
  • Example of a Nonlinear Volt–Ampere Relationship:
  • As the tungsten filament in a light bulb gets hot, its resistance increases.
  • Another nonlinear resistance is a thermistor.
  • A thermistor is a resistor whose resistance value changes with its operating temperature.
  • As an NTC (negative temperature coefficient) thermistor gets hot, its resistance decreases.

Electric Power

The basic unit of power is the watt (W).

  • Multiple units of power are:
  • kilowatt (kW):
    1000 watts or 103 W
  • megawatt (MW):
    1 million watts or 106 W
  • Submultiple units of power are:
  • milliwatt (mW):
    1-thousandth of a watt or 10-3 W
  • microwatt (μW):
    1-millionth of a watt or 10-6 W
  • Work and energy are basically the same, with identical units.
  • Power is different. It is the time rate of doing work.
  • Power = work / time.
  • Work = power × time.

Practical Units of Power and Work:

  • The rate at which work is done (power) equals the product of voltage and current. This is derived as follows:
  • First, recall that:
  • 1 volt = 1 joule / 1 coulomb
  • 1 ampere = 1 coulomb / 1 second

Power = Volts × Amps, or  P = V × I

Kilowatt Hours

  • The kilowatt-hour (kWh) is a unit commonly used for large amounts of electrical work or energy.
  • For example, electric bills are calculated in kilowatt hours. The kilowatt-hour is the billing unit.
  • The amount of work (energy) can be found by multiplying power (in kilowatts) × time in hours.

To calculate the electric cost, start with the power:

  • An air conditioner operates at 240 volts and 20 amperes.
  • The power is P = V × I = 240 × 20 = 4800 watts.
  • Convert to kilowatts:

4800 watts = 4.8 kilowatts

  • Multiply by hours: (Assume it runs half the day)

energy = 4.8 kW × 12 hours =  57.6 kWh

  • Multiply by rate: (Assume a rate of $0.08/ kWh)

cost = 57.6 × $0.08 = $4.61 per day

Power Dissipation in Resistance

  • When current flows in a resistance, heat is produced from the friction between the moving free electrons and the atoms obstructing their path.
  • Heat is evidence that power is used in producing current.
  • The amount of power dissipated in a resistance may be calculated using any one of three formulas, depending on which factors are known:
  • P = I2×R
  • P = V2 / R
  • P = V×I

Power Formulas

  • Combining Ohm’s Law and the Power Formula
  • All nine power formulas are based on Ohm’s Law.
  • Substitute IR for V to obtain:
  • P = VI
  •     = (IR)I
  •     = I2R

Combining Ohm’s Law and the Power Formula

  • Substitute V/R for I to obtain:
  • P = VI
  •   = V × V/ R
  •       = V2 / R

Choosing a Resistor for a Circuit

  • Follow these steps when choosing a resistor for a circuit:
  • Determine the required resistance value as R = V / I.
  • Calculate the power dissipated by the resistor using any of the power formulas.
  • Select a wattage rating for the resistor that will provide an adequate cushion between the actual power dissipation and the resistor’s power rating.

Maximum Working Voltage Rating

  • A resistor’s maximum working voltage rating is the maximum voltage a resistor can withstand without internal arcing.
  • The higher the wattage rating of the resistor, the higher the maximum working voltage rating.

Maximum Working Voltage Rating

  • With very large resistance values, the maximum working voltage rating may be exceeded before the power rating is exceeded.
  • For any resistor, the maximum voltage which produces the rated power dissipation is:
  • Vmax =
  • Maximum Working Voltage Rating
  • With very large resistance values, the maximum working voltage rating may be exceeded before the power rating is exceeded.
  • For any resistor, the maximum voltage which produces the rated power dissipation is:
  • Vmax = rating × R

Electric Shock

  • When possible, work only on circuits that have the power shut off.
  • If the power must be on, use only one hand when making voltage measurements.
  • Keep yourself insulated from earth ground.
  • Hand-to-hand shocks can be very dangerous because current is likely to flow through the heart!

Open-Circuit

An open circuit has zero current flow.

 

Open-Circuit 

Short-Circuit

  • A short circuit has excessive current flow.
  • As R approaches 0, I approach ¥.

 

Short-Circuit

 

 


3 thoughts on “Ohm’s Law Current, Voltage and Resistance | PPT”

  1. Ꮋi! Would you mind if I share your blog with my twitter group?
    There’s a lot օf folks that I think woսld really
    aρpreciate your content. Please let me know. Ϲheers

  2. all the time i used to read smaller articles or reviews that
    also clear their motive, and that is also happening with this piece
    of writing which I am reading at this time.

Comments are closed.