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Ohm’s Law

  • Last Updated : 20 May, 2021

Ohm’s law states the connection between current and potential drop. Georg Simon Ohm, a German scientist was the primary to verify Ohm’s law through an experiment. Ohm’s law is one of the foremost basic and necessary laws of electrical circuits.

Ohm’s law states that the voltage across a conductor is directly proportional to the flow through it, provided all physical conditions and temperature stay constant.

Mathematically, this current-voltage relationship is often written as,

V = IR

In the equation, the constant of proportion, R is Resistance and has units of ohms, with symbol Ω.



The same formula is often rewritten to calculate the resistance:

I=\frac{V}{R}

R=\frac{V}{I}

Ohm’s law solely holds true if the provided temperature and therefore the different physical factors stay constant. Inbound parts, increasing the temperature. Associate in nursing example of this is often the filament of a light-weight bulb, during which the temperature rises because the current is inflated. During this case, Ohm’s law can not be applied. The electric-light bulb filament violates Ohm’s Law.

Water Pipe Analogy for Ohm’s Law

Ohm’s Law describes the flow through a resistance once totally different electrical potentials (voltage) square measure applied at every finish of the resistance. Since we tend to can’t see electrons, the water-pipe analogy helps us perceive the electrical circuits higher.

Here, the voltage is analogous to water pressure, it is that the quantity of water flowing through the pipe, and therefore the resistance is that the size of the pipe. A lot of water can flow through the pipe (current) once a lot of pressure is applied (voltage) and therefore the larger the pipe, (lower the resistance).

Calculating electric power victimization Ohm’s Law

The rate at that energy is regenerated from the power of the moving charges to another type of energy like mechanical energy, heat, magnetic fields, or energy hold on in electrical fields, is thought of as wattage. The unit of power is that the watt. The electric power is calculated victimization the Ohm’s law and by work the values of voltage, current, and resistance.

Formula to search out power:



When the values for voltage and current square measure are given,

P = VI

When the values for voltage and resistance square measure are given,

P=\frac{V^{2}}{R}

When the values for current and resistance square measure are given,

P = I2R

What is a Power Triangle?

The power triangle is used to see the worth of electrical power, voltage, and current once the values of the opposite 2 parameter unit are given to us. Within the power triangle, the ability (P) is on the highest and current (I) and voltage (V) area unit at rock bottom.

When the values of current and voltage are given, the formula for locating power is,

P = VI

When the values of power and voltage are given, the formula for locating current is,



I=\frac{P}{V}

When the values of power and current are given, the formula for locating voltage is,

V=\frac{P}{I}

Ohm’s Law Matrix Table

Like Ohm’s Law chart shown higher than, we are able to condense the individual Ohm’s Law equations into a straightforward matrix table as shown below for straightforward reference once shrewd associate unknown worth.

Known valuesResistance(R)Current(I) Voltage(V)Power (P)
Current and Resistance   R = I × RP = I2 × R
Voltage and Current R=\frac{V}{I}  P = V × I
Power and CurrentR=\frac{P}{I^{2}} V=\frac{P}{I} 
Voltage and Resistance I=\frac{V}{R} P=\frac{V^{2}}{R}
Power and Resistance I=\sqrt{\frac{P}{R}}V=\sqrt{Z\times R} 
Voltage and Power R=\frac{V^{2}}{P}I=\frac{P}{V}   

The main applications of Ohm’s law are:

  • To confirm the voltage, resistance, or current of an electrical circuit.
  • Ohm’s law is employed to keep up the specified fall across the electronic parts.
  • Ohm’s law is additionally employed in dc meters and different dc shunts to divert this.

Limitations of Ohm’s Law:

Following are the constraints of Ohm’s law:

  • Ohm’s law isn’t applicable for unilateral electrical components like diodes and transistors as they permit this to flow through in one direction solely.
  • For non-linear electrical components with parameters like capacitance, resistance, etc the voltage and current won’t be constant with regard to time.

Electrical Resistance

When an electrical current flows through a bulb or any conductor, the conductor offers some obstruction to this and this obstruction is understood as electric resistance and is denoted by R. each material has Associated in Nursing electric resistance and this is often the explanation why conductors provide out heat once current passes through it. Within the next few sections, allow us to study electric resistance well.

What is Electrical Resistance?

According to Ohm’s law, there’s a relationship between this flowing through a conductor and therefore the electrical phenomenon across it. It’s given by,



V ∝ I V = IR

Where,

V is that the electric potential measured across the conductor (in volts)

I is that the current through the conductor (in amperes)

R is that the constant of quotient known as resistance (in ohms)

The electrical phenomenon of a circuit is that the magnitude relation between the voltage applied to this flowing through it.

Rearranging the relation,

R=\frac{V}{I}

The unit of an electrical phenomenon is ohms.

ohm = \frac{1 volt}{1 ampere}

Electric charge flows simply through some materials than others. The electrical phenomenon measures what proportion the flow of this electrical phenomenon is restricted at intervals of the circuit.

Factors moving Electrical Resistance

The electrical phenomenon of a conductor relies on the subsequent factors:

  • The cross-sectional space of the conductor
  • Length of the conductor
  • The material of the conductor
  • The temperature of the conducting material

Electrical resistance is directly proportional to the length (L) of the conductor and reciprocally proportional to the cross-sectional space (A). It’s given by the subsequent relation.

R = \frac{ρL}{A}

Where wherever is that the resistance of the fabric (measured in Ωm, ohm meter)

Resistivity could be a qualitative activity of a material’s ability to resist a flowing electrical phenomenon. Obviously, insulators can have a better worth of resistance than that of conductors. The resistivity of some materials is given below for a comparison. Materials with an occasional worth of resistance conduct electricity very well.

MaterialResistivity
Silver1.00×10−8
Copper1.68×10−8
Aluminum2.82×10−8
Wood1.00×1014
Air2.30×1016
Teflon1.00×1023

What Is Resistivity?

Electrical resistance is outlined because the ohmic resistance offered per unit length and unit cross-sectional space at a selected temperature and is denoted by ρ. ohmic resistance is additionally referred to as specific ohmic resistance. The SI unit of electrical resistivity is ohms meter. Following is that the formula of electrical resistivity:

ρ= \frac{E}{J}

Where,

  • ρ is that the electrical resistance of metal Ω.m
  • E is that the electric field in V.m-1
  • J is that the current density in A.m-2

Sample Questions

Question 1. What does Ohm’s law state?



Answer: 

Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

Question 2. What can Ohm’s Law be used for?

Answer: 

Ohm’s law is employed to validate the static values of circuit parts like current levels, voltage provides, and voltage drops.

Question 3. If the resistance of an electric iron is 60 Ω and a current of 4.5 A flows through the resistance. Find the voltage between two points.

Solution:

If we tend to ask to calculate the worth of voltage with the worth of current and resistance given to us, then cover V within the triangle. Now, we tend to left with I and R or additional exactly I × R.

Therefore, we tend to use the subsequent formula to calculate the worth of V:

V = I × R

Substituting the values in the equation, we get

V = 4.5 A × 60 Ω = 270 V

V = 270 V

Question 4. An EMF supply of 12.0 V is connected to a strictly resistive electrical appliance (a light bulb). An electric current of 3.0 A flows through it. Consider the conducting wires to be resistance-free. Calculate the resistance offered by the electrical appliance.

Solution:

When we are asked to search out the worth of resistance once the values of voltage and current are given, then we tend to cowl R within the triangle. This leaves us with solely V and that I, a lot of exactly \frac{V}{I}.

Substituting the values in the equation, we get

R = \frac{V}{I}

R = \frac{12 V}{3 A} = 4 Ω

R = 4 Ω



Question 5. What is the reciprocal of resistivity?

Answer:

The reciprocal of resistivity is conductivity.

Question 6. What happens to the resistance of pure metals with an increase in temperature?

Answer:

As the temperature will increase, the resistance of pure metals increase. The rationale behind this is often the rise within the variety of electrons within the physical phenomenon band that reduces the quality thereby increasing the resistance.

Question 7. What happens to the resistance of insulators with a rise in temperature?

Answer:

Because the temperature will increase, the resistance of insulators decreases. The rationale behind this is often the lepton movement from the physical phenomenon band to the valence band will increase because the energy gap between these two bands is massive. Therefore, the electrical phenomenon will increase and also the resistance decreases.

Question 8. If 0.7A current flows through a resistor. The voltage of the two points of the resistor is 14V. What is the resistance of the resistor?

Solution:

Here, Current, I = 0.7A

Potential difference or Voltage, V = 14V

Resistance, R =?

According to ohms law questions we know,

V = IR

Or, R = \frac{V}{I}

\frac{14V}{0.7A}

= 20 Ω

Question 9. Resistance of an electric iron 60 Ω. 3.2A current flows through the resistance. Find the voltage between two points.

Solution:

Here, Resistance, R = 60 Ω.

Current, I = 3.2 A

Voltage, V = ?

From Ohm’s law,

V = IR

= 3.2 × 60

= 192V

Question 10. Determine the number of electrons representing one coulomb of charge.

Answer:



One electron acquires a charge of 1.6 ×10-19C, i.e., 1.6 ×10-19C of charge is enclosed in 1 electron.

∴ 1 C of charge is enclosed in 1/1.6 × 10-19 = 6.25 × 1018 = 6 × 1018

Therefore, 6 × 1018 electrons compose one coulomb of charge.

Question 11. Let the potential drop across the two ends of the element decreases to half of its former value whereas the resistance of an electrical element remains constant. What modification can occur within the current through it?

Solution:

According to Ohm’s law

V = IR

⇒ I= \frac{V}{R}  …                   (1)

Now Potential difference is decreased to half

∴ New potential difference Vʹ= \frac{V}{2}

Resistance remains constant

So the new current Iʹ = \frac{V'}{R}

= \frac{\frac{V}{2}}{R}

= \frac{1}{2}\times \frac{V}{R}

= \frac{1}{2}\times I

= \frac{I}{2}

Question 12. With an increase in temperature what occurs to the resistance of insulators?

Answer:

Because the temperature will increase, the resistance of insulators decreases. The rationale behind this is often the lepton movement from the physical phenomenon band to the valence band will increase because the energy gap between these two bands is massive. Therefore, the electrical phenomenon will increase and also the resistance decreases.

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