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Materials for this lesson:

  • A Multimeter (Digital by preference)
  • A 3v power supply or 2-1.5v batteries.
  • A 3v light bulb
  • A socket for the light bulb
  • Electric tape
  • Three wires

After this lesson you should be able to measure voltage and current in a circuit. You should also know what is an electric circuit and how it works.

Look around you; every electric device you see operates under one basic principle, the electric circuit. A circuit is the path in which an electric current flows. It consists of three things: the power, the current, and the resistance. Without one of these three things a circuit cannot be completed and nothing exciting would happen.

Electric current always flows from negative (-) to positive (+). Current is the motion of the electrons through a conductor. It is measured in Amps (A) and in electronics it is represented by the leter (I). When a circuit is closed or complete, a current develops from the negative (-) side of the power supply, passes through the resistance, and continues its way to the positive side of the battery.

All circuits have a resistance. A resistance most of the times is the functional part of the circuit. It can be something as simple as a light bulb or as complex as the mother board of a computer. Resistance is always measured in ohms (), and in electronics it is represented by the letter (R).

The last thing to mention is the power or voltage. Voltage is also called the electromotive force. It is measured in volts (v) and it is represented by the letter (V). Notice that voltage is represented by the capital V and volts is represented by a small v. Do not mix these two prefixes, each is a different thing.

Symbols to remeber

1 2 3

  1. Battery
  2. Switch
  3. Lamp

Notice how a circuit is formed

This is an open circuit. Notice there is no current flow through the circuit and therefor the light is not on. Open circuit
This is a closed circuit. Notice how the current is attracted from the negative side to the positive side the same way a magnet works. Closed circuit

In an electric circuit we can find the value of one of the three components of a circuit by just knowing two. For example: if you have the value of the voltage and the value of the resistance, you can find the value of the current needed for the circuit to work. To do this, you can use a formula. This is possible by using the "Ohms law," doesn't this sound familiar to you?

Ohms Law

V=Voltage
I=Current
R=Resistance
To find the voltage:V=I*R Voltage=Current times Resistance
To find the current:I=V/R Current=Voltage divided by Resistance
To find the resistance:R=V/I Resistance=Voltage divided by Current

Can you see the pattern?. Just remember the triangle!

Practice 1

Wire up the following circuit

  1. Set the Multimeter in DC voltage. If your multimeter is not auto-range, set it at a rage of more then 3 volts.
  2. With the red terminal touch the point B of the circuit. Use the black terminal to touch the point A. Record your voltage reading.
    V=?
  3. Set your multimeter in ohms () within a rage of 0 and 1K . Don't worry about this if your MM is autorange.
  4. Touch the E and F points with each of the terminals. Record the resistance value.
    R=?
  5. Now can you use the current formula to find out the current of the circuit? Yes, go ahead. I=V/R
    I=?
  6. Now that you have estimated the current of the circuit, we are going to measure it with the multimeter. Go ahead and set the MM to DC current with a rage higher than 300 mA (milli Amps). If necessary change the input connection of the red lid of the MM to Amps.
  7. Connect the red terminal to pointC of the circuit, and the black terminal to point D. Now read the display. To measure current in a circuit, you always break the circuit flow and connect the MM as a bypass for the current as shown in the diagram of the circuit. Never try to measure current directly from the power supply's output. It could damage your multimeter, the power supply or both. Now record your reading.
    I=?
  8. Was the reading you got close to the one you got from the equation? If not, check the other readings and the circuit and make sure everything is correct. The reading should be pretty close to the value you calculated.
  9. Try to calculate the voltage and resistance alternating the known values. Use the Ohms law for each of the missing readings. Then check your answers with the readings of the multimeter.

Introduction | Back to menu | Lesson 2, Resistance