# Ohm's Law

DC Experiment 1

### Background

Resistors are the fundamental components in electrical circuits. For a resistor, the relationship between Voltage, Current, and Resistance can be mathematically described by Ohmโs Law in the following form:

Knowing any two values of the above quantities, we can use Ohmโs Law to calculate the third missing value. Ohmโs Law is used extensively in circuit analysis and calculations, so it is very important to comprehend this concept.

In the experiment part, you will follow the procedures to build simple circuits to verify Ohmโs Law. The two circuit diagrams in Figure 1.1 show the approaches to practically measure the voltage and current across the resistor, and we will use these measured values to verify Ohmโs Law experimentally.

### Procedure

**Part I: Voltage Measurement**

Pick a 1kฮฉ resistor based on its color code and measure its exact resistance using a multi-meter. Figure 1.2 illustrates how to use VEGO to measure the resistance.

Record the measured resistance for R(measured) in Table 1.1 below.

Construct the circuit on the breadboard as shown in Figure 1.3. Make sure the connectors are plugged into the holes so that the circuit is well conducted.

Turn on the power supply (MEGO) and adjust its output voltage to 5V using the screwdriver.

Plug MEGO onto the breadboard, make sure the polarities on MEGO match the marking signs on the breadboard. Your setup should look like Figure 1.4.

Now you have built the basic setup to test Ohmโs Law. While taking measurements, we use VEGO again and start off by measuring voltages.

As shown in Figure 1.5, turn VEGOโs knob to position [V]. Use the probes to measure the voltage across the 1kฮฉ resistor.

If the multi-meter gives a reasonable voltage readout, e.g. around 5V, it means the measurement is correct. Then we can continue onwards.

In Table 1.1, record the measured voltage V in column 2. You need to repeat the above steps by setting MEGO output voltage to 5V, 6V, 7V, 8V, and 9V.

Note that the reading on multi-meter is usually more precise than the number displayed on power supply, so we always use measured values to do calculations.

Calculate the current using the measured voltage and resistance, then complete column 3 of Table 1.1.

**Part II: Current Measurement**

To measure current, use the same 1kฮฉ resistor to build the circuit as shown in Figure 1.6. Note that a current meter is connected in series of the circuit so we need to reserve some extra space for the meter.

Turn MEGO output voltage to 5V and plug onto breadboard. Make sure the โ+โ and โ-โ polarities on MEGO are consistent with your breadboard.

To measure current, turn VEGOโs knob to position [mA] for a small current ammeter, and then connect VEGO in series of the 1kฮฉ resistor. See Figure 1.7.

If the current reading is around 5mA, it means the setup is correct. Then we can continue on wards.

In Table 1.1, record the measured value of IR in column 4. We also need to set MEGO output voltage to 5V, 6V, 7V, 8V and 9V respectively.

Once Table 1.1 is completed, turn off both devices and disconnect MEGO from the breadboard.

### Exercises

For Table 1.1, compare the measured currents and current obtained by Ohmโs Law, are the differences sufficiently small to verify Ohmโs law?

Plotting Ohmโs law

a. Using the data of Table 1.1, plot an I-V characteristic curve on the graph paper below. Use the measured current for IR and measured voltage for VR.

b. Using graphical approach to estimate the voltage and current in the missing fields of Table 1.2.

The slope of I-V curve is related to the resistance by

What do you predict the slopes to look like for a superconductive material vs. an insulator?

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