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Resistance

We have established that current always flows from higher voltage to lower voltage, but the electrons flow from lower voltage to higher voltage.

As current moves through a wire into a resistor, some of the energy is transferred to the resistor. A resistor helps to regulate the amount of current going through a circuit.

Every resistor is designed to have a specific resistance. Resistance is the tendency for a material to resist flow of electrons and to convert electrical energy into other forms of energy, such as thermal energy.

  • Resistance is caused by internal friction as the electrons flow through a conducting material.
  • As the electrons flow through the circuit, they collide with atoms within the device.
  • Some of the kinetic energy is transferred at heat.

circuit board

Resistors on an electronic circuit board.



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Examples of Resistance

Almost all materials have some resistance.

  • Resistance is demonstrated when your cell phone begins to get hot after you talk on it for extended lengths of time.
  • This also happens in televisions, stereos, refrigerators and all other electric devices. Turn them on for a little while and then touch the side of it. You will feel the heat!
  • The wire used in circuits also has some resistance.

Resistance determines how bright a light bulb will be.

  • The resistor of a light bulb is the filament.
  • A 40 W light bulb has a higher resistance than one that is 75 W.


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Review

Before we dive into Ohm's Law, see if you remember definitions for the following:

  • Resistance

  • Current

  • Potential Difference

Ohm's Law

Now that you've refreshed your memory, let's discuss how Ohm's Law allows us to calculate resistance, current and potential difference.

Ohm's Law shows that the electric potential difference between two points on a circuit (ΔV) is equivalent to the product of the current between those two points (I) and the total resistance of all electrical devices present between those two points (R).

The formula for Ohm's Law is:

Current = potential difference / resistance or I = V / R

The SI unit of resistance is the ohm (Ω), which is equal to one volt per ampére.



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Ohm's Law Calculation

Let's try an example problems to solve using the Ohm's Law formula.

Problem: The current in a resistor is 6.2 A and the voltage across the resistor is 110 V. What is the resistance of the resistor?

To solve:

  1. Identify known and unknown values.

    I = 6.2 A

    V = 110 V

    R = ?


  2. Since we are solving for R, we need to rearrange the equation.
    • Start with the original formula.

      I = V / R

    • Divide both sides by V.

      (V) / I = (V / R) / (V)

      On the right side of the equation, V is canceled out.

      V / I = R

      R = V / I

  3. Input the values into the rearranged equation and solve.

    R = V / I

    R = (110 V) / (6.2 A)

    R = 18 Ω

  4. Answer: The resistance of the resistor is 18 Ω.


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