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

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

## 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.

• 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.

Short-Circuit

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

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