Resistance and Ohms Law Basics

When current flows around a circuit, it will probably encounter resistance along the way, making it harder for all the electrons to complete the circuit.

The filament in our bulb creates deliberate resistance against the current, making it heat up so much that it glows white hot giving out light. We say the bulb acts as a load on the circuit, as it uses up electrical power.

We ultimately lose power (use up electrons) through resistance, as the electrons are blocked or lost (energy transferred into light or heat), resulting in a reduction of current. The voltage is also affected by being reduced proportionally, being more difficult to push the current when it’s facing resistance in the circuit.

Although the battery in our simple bulb circuit has plenty more current available (we earlier measured a potential of 15 times more current available in the battery than the bulb uses), the bulb’s resistance prevents the rest of that available current from flowing to the rest of the circuit.

The unit for resistance is called an Ohm, and the symbols used are the Greek capital letter Omega for values, and R for equations.

No Resistance found in a circuit would either indicate a short circuit (where the electrical flow has bypassed the components), or a closed circuit with no components. The electrons can move around the circuit with virtually no restriction. A circuit with a no resistance could measure around 0 Ohm.

Low resistance would describe a circuit with a small load, such as our bulb, which measures around 1.1 Ohms. As we start to add certain components, the resistance will increase with the resistance value moving towards medium resistance from tens of Ohms to thousands of ohms.

High resistance would be a circuit that has a large load, the load uses up a lot of the available current, with very little to spare further on in the circuit. A circuit with a high resistance could measure in the 100’s of Kilo-Ohms or in Mega-Oms.

Infinite resistance would describe an open circuit, as the circuit has a break somewhere. As its difficult for the electrons to jump across through the air (unless we are using very high voltage), they simply don't make it to the other side of the break.


Ohms Law - Resistance

The relationship between current, voltage and resistance is proportional, giving us Ohms Law, where “electric current is proportional to voltage, and inversely proportional to resistance”. More simply, we can explain that in the formula:

      Formula: Current=Voltage/Resistance          or           Formula:I-V/R

Current is the quotient (result of a division) of voltage divided by resistance.

So long as we know two of the factors of this equation, we can always calculate the third:

Volts is the product of multiplying the current with the resistance:      Formula:V=IR

and the quotient of Resistance is obtained by dividing voltage by current:      Formula:R=V/I


Ohms Law and Power

Going off topic with regard to resistance, but elaborating further on Ohms Law: In the last tutorial (Measuring Current, Voltage and Power with a multimeter), Ohms Law was used to calculate power from a voltage and current reading. As with resistance, Ohms Law states, "electric current is proportional to power, and inversely proportional to voltage". Again, as we know two factors of the equation, we can always calculate the third.


Resistors

A component called a resistor deliberately creates resistance in a circuit. Its main purpose is to control the flow of electrons in certain parts of the circuit. That control reduces the current, and proportionally reduces the voltage.

Turnstiles work in a similar way to resistors by limiting flow. Source: http://i.dailymail.co.uk/i/pix/2015/06/06/21/29679C4B00000578-3113517-image-a-1_1433624305283.jpg
Turnstiles representing how resistors control the flow in the circuit

 

 

 

 

 

An entrance with turnstiles represents how a resistor works very well. A mass of people want to enter an event (like the current from the battery wanting to get through our bulb). The turnstiles control the flow of how quickly people can enter.

  • Low resistance is as if the turnstiles left open and people can walk straight through
  • Medium Resistance is where people are being allowed to enter a few at a time
  • High resistance is where people are being allowed through one at a time
  • Infinite resistance would be like where the entrance has been closed - The event has been cancelled and the current won't make it any further.

The reduction in current lowers the voltage, our turnstiles have had the additional effect of reducing the momentum of the crowd slowing them down.

Resistors are covered further detail in the Resistor Basics section.

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