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 fault (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.

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:

          Current = Voltage / Resistance           or           I = V /R

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

for Volts: V=I R

and for Resistance: R = V / I

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

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 several people are being allowed to enter at a time.

High resistance is where people are being allowed through one by one

Infinite resistance would be like where the entrance has been closed.

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.

Measuring Resistance:

Resistance is measured with a Ohmeter. If you are using a multimeter, you will need to switch the measurement setting to Ohms or Ω.

To measure resistance, the component may need to be taken out of circuit if there is a possibility that the resistance of other components may affect the reading.

IMPORTANT:

  • You must disconnect the power to the circuit before measuring Resistance. The meter will supply its own power to get the reading. NOTE: Low batteries on your multimeter can affect the reading!
Schematic Symbol  Description
 Symbol-Ohmmeter

Ohmmeter

Fig9-Ohmmeter-Connection-Schematic

No harm should come to your meter or component if you are in the wrong resistance range. You will either get an infinite resistance reading (or OL on a digital meter) if the range is too high, or a Zero reading if the range is too low. Select a different resistance range to get a more specific result.

As resistance readings do not require the components to be in circuit, it can be a good way to check their values, before connecting them to the circuit.