Physics 204, Spring 2006

DC Circuits

DC means “direct current”. DC power supplies will give us the potential difference that drives current, and little lamps will be the resistances. We will use Ohm’s law to interpret what we see. In particular, the brightness of the lamps is a qualitative measure of the power dissipated. Thus power is, in a way, the easiest thing to see, at least if it is enough to make the lamp actually glow. We will also use meters to measure the voltage difference across a lamp and the current through a lamp quantitatively.

The “meters” will actually be many meters in one, a so-called multimeter. There is a dial to select the application, effectively turning the multimeter into a voltmeter, an ohmmeter, or an ammeter. We will try it first as an ohmmeter. It is important to know what is really going on in this case. The ohmmeter has its own battery, of known voltage V, and when you connect it across a resistance, it measures the current I driven by this voltage and reports the value V/I. You can only use it on a resistance taken out of a circuit. You cannot use it on a resistance that is in a circuit carrying current, because then the voltage V across the resistance would not be the assumed one, from the battery in the meter, but rather some unknown combination of that and the voltage difference across the resistance in the operating circuit.

To use the multimeter as an ohmmeter, connect one lead to the “Ohms” contact and another to the “Common” contact, and turn the dial to one of the values marked R, for resistance. In this setting the meter is very robust, and it is not too crucial which value you choose. You can choose the appropriate range depending on what you are measuring. Touch the leads to the two sides of a lamp to measure its resistance. Measure your own resistance! Does it make a difference if your hands are wet?

There are two kinds of lamps, one small and round, and the other somewhat longer. Their characteristics are somewhat different, as we will see, and there may also be some variation within each type. There are sockets to screw them into, for easy connections to wires and meters. By the way, where ARE the contacts on a light bulb? What exactly touches what?

I. Cold resistance. Measure the cold resistance of each kind of lamp, and record it here. (Later we will determine, indirectly, the resistance of the lamps when they are glowing, and it will be different.)

II. Voltage. Now turn on the DC power supply, and adjust it to about 3 V. There is a little meter on the front of the supply, and you can also use the multimeter as a voltmeter to confirm the value. To use the multimeter as a voltmeter, keep the leads in the same places as before and just turn the selector dial to V for volts. Again, the multimeter is robust in this configuration, and you cannot hurt it. To measure the potential difference between two points, just touch the leads. Now connect the power supply across two or three lamps in series. What can you say about the power dissipated in each lamp? Why does it look like this? Measure the voltage drop across each lamp. Are the relative sizes of these drops consistent with the resistances that you measured in (I) above?

III. Current. Now measure the current through the lamps. For this purpose you must configure the multimeter as an ammeter, and in this configuration it is more fragile. There is a fuse to try to protect it in case of a mistake. In the other configurations it drew very little current, but in this configuration all the current in the circuit goes through it. Move the lead that had gone to the Volts contact to the DC Amperes contact, and set the selector dial to the maximum current, 10 A. At this setting, the meter is still well protected against current. Now connect the ammeter into the circuit so that the current passes through it as well as through the lamps. If the meter reads 0 on the selected scale, carefully reduce it to the next, more sensitive, range, and thus read out the current in amperes. Since the current that heats up the bulbs is actually an appreciable fraction of an ampere, you probably won’t need to change the scale.

IV. Hot resistance. Knowing both the voltage across the lamps and the current through them, we can find their resistance when they are hot, as R=V/I. How does the hot resistance compare with the cold resistance?

V. Power. What is the actual power dissipated in each lamp? How does this correspond to their visual appearance?

VI. Parallel configuration. Make the same kinds of careful measurements on lamps in parallel, and try to understand this configuration completely. Be sure to configure the multimeter as a voltmeter again to measure voltage drops, and be careful when using it as an ammeter not to give it too large a current for the chosen setting.

VII. Write a summary explaining how series and parallel circuits behave.