7. Electrical resistance¶
Electrical resistance is the opposition that shows certain components (resistors) to the passage of electric current. The greater the value of the resistance of a component, the less current it will happen through it.
The resistances have three major applications:
Generate heat. It is one of the main applications. For example, the resistances of a toaster produce heat to toast the bread. The resistance of an oven, a washing machine or an electric blanket are other daily examples.
Reduce current. Another example is found in the resistors that are connected in series with the terminals of an electric motor so that the motor start occurs gently, without abruptness.
Get intermediate voltages. In electronics this is one of the main applications of resistors. From a 5 volt voltage source, tensions can be obtained from 0 volts to 5 volts through the entire intermediate range.
In the following simulated circuit, two series resistances have been connected to achieve a lower voltage than the stack voltage. Modify the value of the resistors to be able to achieve an exact output voltage of 3.5 volts.
Ohms¶
The electrical resistance value is measured in ohms, shortened with the Greek letter omega [Ω]. In the following table, typical values of several common resistances appear in our environment:
| Component | Resistance [Ω] |
|---|---|
| Car lamp with 15 watts of electric power. | 10 ohms |
| Air electric heater. | 22 ohms |
| Current limitation resistance of an indicator LED. | 220 ohms |
| Input resistance of an audio amplifier. | 10 000 ohms |
| Input resistance of a electric multimeter. | 10 000 000 ohms |
Ohmmeter¶
An ohmmeter is a device that allows measuring the resistance of a circuit composed of resistors. The ohmmeter injects a certain amount of current in the circuit to be able to measure. For this reason, all circuit batteries must be disconnected before measuring the electrical resistance with this device.
To simulate an ohmmeter we must choose it from the menu Draw... Meters and Labels... Add Ohmmeter.
In the following simulation, add the necessary ohmmetters to measure the resistance value of R1 in series with R2 and the resistance value of R1 in parallel with R2.
Exercises¶
- What is electrical resistance?
- What applications can have an electrical resistor?
- In which units is the electrical resistance measured? Name 3 daily elements and its electrical resistance.
- What is the name of the device that measures the electrical resistance? How should you connect to resistors?