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Relation Between Power and Resistance

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Relation Between Power and Resistance in Detail

A way of visualizing the power and resistance relation is to think of a constant voltage source or a battery. When there's a large resistance across the circuit, very little current can flow so very little power is being offered by the battery and the resistor would not get too warm because there's less power. However, if you reduce the resistance more current will flow and the resistor will get warmer because we have increased the power.

 

So, wherever there is power, surely there will be resistance. It means that both are essential factors not only in Physics but also in our real lives.

 

Power Resistance Relation

The relationship between Power and Resistance can be expressed in two following ways:

1. \[P = \frac{V^2}{R} \]

Where,

P is the Power and we measure it in watts or W. 

R is the resistance measured in ohms (Ω).

V = the potential difference applied across the ends of the conductor and is measured in Volts or simply V.

2. The second way to express the power resistance relationship is by using the following power and resistance equation:

\[ P = I^{2} R \]

Where I is the electric current measured in Ampere or A.

 

What is work?

Work is done when some force is applied to the object and it gets displaced in the direction of motion. The work done on an object by a force is equal to the product of the magnitude of the force and the distance moved by an object in the direction of the force. Work has no direction but only magnitude. Hence, it is a scalar quantity.

 

Work Formula

The work done by a force is defined as the product of the cos component of a force and the displacement of an object in the direction of the force. The formula is given as (F cos theta) d or F.d. From this, we can understand that if there is no displacement, then there is no work done irrespective of the force applied to the object. There is no work done if,

  • The force is zero

  • The displacement is zero

  • The force and displacement are mutually perpendicular to each other.

 

Unit of work

The SI unit of work is Joule (J). One joule is defined as one unit of force applied on an object to move it for one meter. 1 Joule= 1 Newton * 1 metre. Let us see an example of work done.

 

If a force of 10 Newton is applied to an object to move it for 2 metres, then the work done is 20 joules.

 

What is Power?

Power is a physical concept that has different meanings. Its definition depends on the context and available information. 


We define power as the ability of the body to do work in a unit of time. For example, person A does 30 J of work in 2 hrs and another person B does the same amount of work in 3 hrs, so here if we use the following formula:

 

Power  = Work / time

 

Case 1: 30/2 = 15 W

 

Case 2: 30/3 = 10 W

 

We can see that the capacity of person A is more than person B. So, power A is greater than the power of B.

 

However, in electrical terms, power is defined as the product of the current and the voltage.

 

P = VI

 

Where,

 

V is the potential difference and it is measured in volts.

 

I is measured in Ampere.

 

Unit of Power

Power is a scalar quantity as it only has magnitude and no direction. The SI unit of power is Joules per second which is termed, Watts. One watt is defined as the power required to do one joule of work in one second. The unit Watt is named in honour of Sir James Watt, the one who developed the steam engine. 

 

What is Resistance?

While driving a car at high-speed, we must slow down our car some distance before the speed breakers, otherwise, our car will jump with a great amount of jerk. So, here, our high-speed car is the maximum current that is flowing through the circuit (road) and the speed breaker is the resistance to avoid accidents or short circuits in our homes.

 

So, resistance is the obstruction connected across the circuit to avoid the overflow of charge through the circuit. It is measured in Ohms, where we symbolize it with omega or Ω.

 

According to Ohm's Law, we can represent resistance as voltage/current. As per this law, Resistance is constant and independent of the current. It is further written as R= ⍴ L/A, where l is the length, A is the area and rho is resistivity.

 

This shows that resistance is directly proportional to the length and inversely proportional to the area. Here, resistivity is measured in ohm meter, length in meter, and area in meter squares.

 

Applications of Ohm's Law in Daily Life 

  • We can regulate the speed of the fan using a regulator. The current flowing through the fan can be regulated by controlling the resistance via a regulator. We can calculate current, resistance, and power using Ohm's law of any input added to the appliances. 

  • The electric heaters consist of high resistance coils that generate the required amount of heat by obstructing the flow of current. This law can help to determine the power of these heaters. 

  • We can determine the input of power for electric kettle and iron to produce a sufficient amount of heat. 

 

Factors Affecting Resistance 

The electric resistance of a conductor can be affected by the following factors.

  • Length of the conductor

  • The cross-sectional area of the conductor.

  • The temperature of the conducting material

  • Conductor's material 

 

Power and Resistance Formula

We noticed that the formulas mentioned above describe the relation between power and resistance. Consider the following equation:

 

\[ P = I^{2}{R} \]


Here, we can see that the electric power is directly proportional to resistance on keeping I constant.

 

From this, we infer the following things:

  • When power increases, the resistance also increases, while keeping current I constant.

  • However, when the resistance in the circuit decreases, power in the circuit also decreases, while keeping current I constant.

 

Now, consider another following equation:

 

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


From here, we can see that the power P is inversely proportional to the resistance R. 

 

From this, we can infer the following things:

For any constant Potential difference

  • When the power in the circuit is high, resistance will be lesser.

  • However, if the power is low, the resistance will be high.

 

Power Resistance Formula

Deriving the Power and Resistance formula will help us in understanding the concept of the power and resistance relation.

 

In physics, power and resistance can be related using two formulas, which we will discuss in this article in detail.

 

We know that the electric power or P is the measure of the electric current I with q coulombs of charge passing through a potential difference of V (in volts) in time t seconds. Mathematically, we can express this statement as:

 

P = Vq/t = VI…..(1)

 

We know that Ohm’s law is stated as: 

 

V = IR

 

Where V is directly proportional to the current.

 

Now, applying this law in equation (1), we get:

 

\[ P = I \times IR = I^{2} R \]


Also, 

\[ I = \frac {V}{R}\]

 

Now, placing the value of equation (2) in equation (1) we get:

 

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


From the above derivations, we got the following conclusion:

 

Power and Resistance in Electronics

In electronics, we define power as the rate of doing work. So, what kind of work is being done in electronics? Is it the normal work that we do daily or something else?  Let’s describe it with a simple statement:

 

We define resistance as the opposition offered against the flow of electrons in the circuit. It means that the more is the obstruction, the more is the work done per unit time to make them flow, i.e. the more is the power required to make their flow easy.

 

From the above statement, we cannot deny the fact that the relation between power and resistance is proportional. 

 

Moreover, both power and resistance are important factors to consider for making electrical appliances that should not be neglected in any case. 

FAQs on Relation Between Power and Resistance

1. What is the fundamental relationship between electric power and resistance in a circuit?

The relationship between electric power (P) and resistance (R) is not fixed; it depends on whether the voltage or current is held constant.

  • When the current (I) is constant, power is directly proportional to resistance (P ∝ R). More resistance leads to more power dissipated.
  • When the voltage (V) is constant, power is inversely proportional to resistance (P ∝ 1/R). More resistance leads to less power dissipated.
This dual nature is crucial for understanding how different electrical components work.

2. What are the two main formulas that connect power, voltage, and resistance?

The two primary formulas derived from Ohm's Law that describe the relation between power and resistance are:
1. P = I²R: This formula is used when the current (I) flowing through the resistor is known. It shows that power increases with the square of the current.
2. P = V²/R: This formula is used when the voltage (V) across the resistor is known. It shows that power decreases as resistance increases for a constant voltage.

3. Does increasing resistance always decrease the power in a circuit?

No, this is a common misconception. Increasing resistance does not always decrease power. It depends entirely on the circuit's conditions:

  • In a constant voltage circuit (like home wiring): Yes, increasing resistance will decrease the power, as per the formula P = V²/R. For example, a 100W bulb has less resistance than a 60W bulb designed for the same voltage supply.
  • In a constant current circuit: No, increasing resistance will actually increase the power dissipated, according to the formula P = I²R.

4. How is the relationship between power and resistance applied in an electric heater?

An electric heater is a perfect real-world example of the power-resistance relationship. The heating element is a coil made of a material with very high resistance (like nichrome). When electric current flows through this high-resistance coil, a large amount of heat is generated according to the principle of Joule heating (P = I²R). The goal is to maximize power dissipation as heat, so high resistance is intentionally designed into the appliance.

5. How does the power consumed by resistors differ in series and parallel circuits?

The power consumption rule reverses depending on the circuit configuration:

  • In a series circuit, the current (I) is the same through all resistors. Since P = I²R, the resistor with the highest resistance will consume the most power and glow brightest or get hottest.
  • In a parallel circuit, the voltage (V) is the same across all resistors. Since P = V²/R, the resistor with the lowest resistance will consume the most power.

6. What is the key difference between electric power and electric energy?

While related, electric power and energy are distinct concepts.

  • Electric Power is the rate at which electrical energy is consumed or converted into another form (like heat or light). Its SI unit is the Watt (W), which means Joules per second.
  • Electric Energy is the total amount of work done by the electric current over a period. Its SI unit is the Joule (J). Your electricity bill is based on the total energy consumed (measured in kilowatt-hours), not just the power of your appliances.

7. Why do high-power devices like electric kettles have high-resistance coils but the connecting wires have very low resistance?

This design is intentional and based on controlling where power is dissipated. The heating coil is designed with high resistance to purposely convert electrical energy into a large amount of heat (P = I²R). This is its function. Conversely, the connecting wires are made with very low resistance (e.g., copper) to minimize power loss during transmission. If the wires had high resistance, they would also get hot, wasting energy and creating a safety hazard.

8. How does Ohm's Law provide the foundation for understanding the power-resistance relationship?

Ohm's Law, which states that Voltage = Current × Resistance (V = IR), is the fundamental principle that connects the three basic electrical quantities. The formulas for power (P = VI) are combined with Ohm's Law to derive the specific relationships with resistance. By substituting V = IR or I = V/R into the main power formula, we get P = I²R and P = V²/R, which are essential for analysing power in circuits.