(electronic whirring) - [Instructor] Hey there guys, Paul here from theengineeringmindset.com. In this video we're going
to be looking at DC series circuits. We'll cover voltage, current, resistance, and power consumption, as
well as using a multimeter and a chance for you to test
your knowledge at the end. When we connect components
in electrical circuit, we can connect them either
in series, or parallel, or we can combine these to
make a series parallel circuit. We're going to start with the series type, which is the most basic. We will cover the other
types in other tutorials, do check those out, links down below. So if we place two components
in a line end to end, or with some wires in between,
then these are connected in series, the electrons only
have one path they can take, so they would all flow through
each of the components. By the way in these
animations I use electron flow which is from negative to positive. You might be used to seeing
conventional current, which is from positive to negative. Electron flow is what's
actually occurring. Conventional was the original theory but it's still taught because
it's easy to understand. Just be aware of the two,
and which one we're using. Resistance in series circuits. Each component will have
a certain resistance, the resistance opposes
the voltage being applied. We measure resistance in a unit of ohms. In series circuits, we
find the total resistance for the circuit by simply adding together all resistances. We label each resistor with a capital R, and number them R1, R2, R3, et cetera. The total resistance is
shown with a capital letter R and a subscript T, which
represents the resistance total or the total resistance. To calculate the total
resistance of a series circuit is super easy. You simply add together the
resistance value of each resistor. Let's say we have a circuit
with a single resistor, that's our R1, and this
has a value of 10 ohms, so what is our total resistance? Well that's easy, the total
resistance is just 10 ohms, if we then add in the second resistor, R2, with five ohms of
resistance into the circuit, the total resistance is now 15 ohms. That's because 10 ohms plus five ohms. If we added another five ohm resistor, then the total resistance is now 20 ohms, in reality the wires too
will add some resistance but this is very small, you
might need to account for this depending on how accurate
your design needs to be. Current in series. Current is the flow of
electrons, it's like water which flows through a pipe,
the higher the current, the more electrons are flowing. We measure current in the unit of Amperes, but engineers tend to
shorten this to just amps. Now we have covered current in detail in our previous video, do check
that out, links down below. We measure current by placing
an ammeter into the circuit for the electrons to flow through. This is like a water meter,
in the sense that water must pass through it for us to measure it. We can connect a
multimeter into the circuit to also read the current. The multimeter must be
placed into the circuit for us to take a reading,
because the current will flow through this. The meter will add some
resistance to the circuit but it's such a small
amount that we can usually just ignore this. If you don't have a multimeter yet then I highly recommend you
get one, they are essential for troubleshooting and also
building your understanding. I'll leave some links down
below for which one to get and from where. We can calculate the total
current of the circuit by dividing the voltage by the resistance. So if we connect a 10 ohm
resistor to a nine volt battery, nine volts divided by 10
ohms give us 0.9 amps. If we added another five
ohm resistor to the circuit that gives us 15 ohms of resistance, so nine volts divided by
15 ohms equals 0.6 amps, and if we added another five ohm resistor that gives us 20 ohms of resistance, so nine volts divided by
20 ohms equals 0.45 amps. So we can see that as we add
more resistance to the circuit the current reduces, so
less electrons are flowing and that means we can do less work. We can visualize that by connecting an LED with a resistor into a circuit. The higher the resistance,
the dimmer the LED will be. We can also use resistors
to protect components in the circuit. If I use a 100 ohm resistor
with a nine volt battery, the current will be around
0.09 amps, or 90 milliamps, and that will be too much
it will blow the LED. If I use a 450 ohm resistor
the current will be around 0.02 amps or 20 milliamps,
so the LED should be okay. If I use a 900 ohm resistor,
the current will be 0.01 amp or 10 milliamp, and the
LED will be very dim then. In a series circuit the
current is the same throughout the entire circuit, that's very important so do remember that. If we place the meter
here or here we will get the same reading. That's because there is only one path for the electrons to flow, and
they would all move together in the same direction, so
the current must be the same. It doesn't matter where we
measure or where we place the resistor, even if we swap
the order of the resistors, the current will be the same
anywhere in a series circuit. Voltage in series. Remember voltage is the
pushing force of electrons. It's like pressure in a
pipe, the higher the pressure the more water can flow,
the higher the voltage the more electrons can flow. We can see that by varying
the voltage to a lamp as illustrated here. The lamp increases in brightness
as the voltage increases. When we measure voltage,
we're measuring the difference or potential difference
between two points. If we read across a 1.5 volt battery, we get a reading of 1.5 volts. But if we try to measure the same side we wouldn't read any voltage, we can only measure the
difference between two points. If we place a nine volt
battery into the circuit we apply nine volts to the circuit, we can increase this by wiring
the batteries in series. So if we place two nine
volt batteries in a circuit in series, then we get 18 volts. Three nine volt batteries
will give us 27 volts. Let's take a nine volt battery, and add an R1 resistor of
10 ohms to the circuit. If we use a multimeter to
measure across the resistor, we get a voltage reading of nine volts. If we add another 10 ohm resistor, we get a reading of nine volts
across the two resistors, but we get a reading of 4.5
volts if we measure across either of the resistors individually. So the resistors are dividing the voltage. If we replace the R2 resistor
with a five ohm resistor, the total voltage would
again be nine volts, and that's what we see if we measure across the two resistors. But if we measure across
the 10 ohm resistor, we see a voltage of six
volts, and if we measure across the five ohm
resistor, we see three volts. We'll look at why that is just shortly. If we added another
resistor, R3, with five ohms into the circuit, we again
get a total voltage drop of nine volts across the three resistors. Across the R1 10 ohm
resistor, we read 4.5 volts. Across the R2 five ohm
resistor, we read 2.25 volts. And across the last R3
five ohm resistor we again see 2.25 volts. We can combine these
readings to find the voltage at different parts of the circuit, for example if we measure
from the battery across R1, we see 4.5 volts. If we measure from the
battery across R1 and R2, we get 6.75 volts, because
4.5 plus 2.25 volts. So unlike current where it's the same throughout the circuit, the
voltage will be different throughout a series circuit. This shows us the voltage
is reduced by each resistor, so the resistor creates a voltage drop, that's the purpose of the
resistor, to reduce the voltage or the pressure. What's happening is the resistor
creates a more difficult path for the electrons to flow through, and as they flow through
it they will collide with other electrons. This collision will convert
the energy into heat, the same amount of electrons
will enter and exit the resistor, they will just
have less energy or pressure as there's been a voltage drop. We can calculate the voltage
drop across each resistor individually, by multiplying
the total current in the circuit, by the
resistance of each component. Remember in a series circuit,
the current is the same anywhere in the circuit. The total voltage drop
will be the total of all the individual voltage drops combined. The first circuit there was
a 10 ohm resistor by itself, the circuit had a current of 0.9 amps, so 0.9 amps multiplied by
10 ohms, equals nine volts. The voltage drop across
the resistor is therefore nine volts, and that's the
same as the voltage source. The second circuit had the 10
ohm, and a five ohm resistor together, so the first resistor
voltage drop is 0.6 amps multiplied by 10 ohms,
which gives us six volts. The second resistor was five ohms, and the current was the same, so 0.6 amps multiplied by five ohms,
equals three volts. The total voltage drop
is therefore six volts, plus three volts which
gives us our nine volts. The third circuit has a 10 ohm
and two five ohm resistors, the circuit had a current of
0.45 amps, so R1 is 0.45 amps multiplied by 10 ohms,
which gives us 4.5 volts. R2 and R3 will be 0.45 amps
multiplied by five ohms which gives us 2.25 volts on each. The total voltage drop
is therefore nine volts which is 4.5 plus 2.25, plus 2.25. Power consumption in series circuits. How do we measure power
consumption of a circuit? Well we can use the following equations. We can either use power, which is watts, equals voltage squared,
divided by resistance, or we can use power equals
voltage, multiplied by current. You might be wondering how
can a resistor consume power? Well as the resistor is
creating a voltage drop, the electrons are losing some energy, where is this energy going. Well the electrical energy
is being converted into heat, and if we look at some resistors
under a thermal imaging camera we can see the
heat is being generated. So the power consumption
is actually the heat being dissipated from the circuit. So in this circuit the
resistance is 10 ohms, the battery's providing nine volts, the current is 0.9 amps, and the circuit consumes 8.1 watts of power,
how do we calculate that? Using method one, nine volts squared or nine multiplied by nine is 81, divided by 10 ohms, is 8.1 watts. Alternatively, nine volts
multiplied by 0.9 amps equals 8.1 watts. In the circuit with a 10 ohms
and the five ohm resistor, the total resistance were 15 ohms, and the current was 0.6
amps, so nine volts squared is 81 divided by 15 ohms is 5.4 watts, or nine volts multiplied by
0.6 amps equals 5.4 watts. In the circuit with a 10 ohm
and the two five ohm resistors the total circuit resistance was 20 ohms, and the current was 0.45 amps. So nine volts squared is
81, divided by 20 ohms gives us 4.05 watts, or alternatively, we could use nine volts
multiplied by 0.45 amps which equals 4.05 watts. Okay so now it's time for
you to test your knowledge. So this LED can't exceed a
maximum current of 0.02 amps, or 20 milliamps, otherwise
it will burn out. So if we connected it to these resistors and a nine volt battery, what
will the approximate current in the circuit be, I will leave a link in the video description
down below for the answer. Okay guys that's it for this video, but to continue your
learning then checkout one of the videos on screen
now, and I'll catch you there for the next lesson. Don't forget to follow
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