Open Circuits

# Open circuits

The voltage and current relationships shown above apply only to complete circuits. If the circuit is not complete (an open circuit) none of the rules for a series circuit apply. Remember, there must be two things for a voltage difference (voltage drop) to occur across a resistor, current and resistance. If there is no current flow, there will be no voltage drop. If a circuit is incomplete there is no current flow and no voltage drop across any of the resistors. If you place a volt meter across the battery you will read the battery voltage (just as in a closed circuit). However, as you move the red probe around the circuit, you will find no voltage drop as you go around the circuit. If you measure the voltage across any resistor, the voltage will read zero.

 This circuit is incomplete so there is no current flow.

 With no current flow no voltage can develop across the resistors. A voltmeter will read 0 volts across each resistor.

Now anchor the black probe at the negative terminal of the battery. Put the red probe at the positive terminal. The meter will tell you the battery voltage, which is 30 volts. When you move the red probe to the opposite side of the 10 ohm resistor you expect a lower voltage, but you get the same voltage. Why is there no voltage drop? It's because there is no current flowing through the resistor. No current flow, no voltage drop. You start with +30 volts, subtract 0 volts and you still have +30 volts. It is the same across every resistor. Once you reach the end of the circuit (where the break is) you still have +30 volts.

 If the black probe of the voltmeter is placed on the negative terminal of the battery there will be no drop in voltage measured as the red probe is moved around the circuit.

## Troubleshooting technique

Notice that when you have the red and black probes on opposite sides of the break that you see the entire battery voltage. This can be useful to find where the actual break in the circuit is. In the following circuit the break is in the middle of the circuit. Notice that when we put the red probe on the side of the break nearest the positive terminal of the battery we see the battery voltage (30 volts). However, when we put the probe on the side of the break neares the negative side of the battery we see 0 volts.

 Looking at the voltages on opposites sides of the break.

Now let's put the probes across the break. Here we see the battery voltage across the break.

 Looking at the voltage across the break.

If we put the probes across the battery we see 30 volts. If we put the probes across the break we see 30 volts. If we place the probes across any of the resistors or any other two points in the circuit (that are not on opposite sides of the break) we see 0 volts. This tells us where the break is. When we isolate the two closest points where we see the battery voltage, we have located the break in the circuit.

Let's find out what is wrong with the following circuit.

 Something is wrong here, what is it?

Notice that we see +30 volts on one side of the 15 ohm resistor and 0 volts on the other. Measuring across the resistors we see 0 volts across the 10 ohm and 5 ohm resistors, but the whole battery voltage across the 15 ohm resistor. What is happening? Just as above, we see the battery voltage across the battery and across a break in the circuit. It looks like the 15 ohm resistor has become an open circuit.

 The 15 ohm resistor is open.

## Unsticking a sticking point

I have seen people get confused when there is no voltage across a perfectly good resistor, especially in a working circuit. I think this is because they had it pounded into them that a resistor will have a voltage drop. What didn't get pounded in is that this voltage drop will not exist if there is no current. Let's say you have a 10k resistor and you measure +10.7 volts on both ends. When you put the volt meter across the resistor it reads 0 volts. Is the resistor bad? Maybe, maybe not. If you can prove that there is current flowing through the resistor then it must actually be zero ohms (if you have current but no resistance you will get zero volts). Otherwise, the lack of voltage difference only means there is no current flowing through the resistor.

Now comes an interesting question. Is there no voltage difference because there is no current or is there no current because there is no voltage difference? In the examples above there is no voltage difference because there is no current. In some circuits, for whatever reason you may get the same voltage on both sides of a resistor. With the same voltage on both sides of a resistor, showing that there is no voltage difference across the resistor, there will be no current flow through the resistor. Think of it like two people with opposite ends of a soda straw in their mouths. Each is trying to blow air through the straw. Unless one blows harder than the other the air goes nowhere. Equal pressure one opposite ends of the straw means no air flow through the straw. Equal voltage on opposite sides of a resistor means no current flow through that resistor.

Open Circuits