The word circuit means “closed path”.
By an electric circuit we mean an arrangement
of electrical components connected so that electric
charges can flow in a closed path.
The simplest electric circuit consists of:
1. a source of potential difference (battery)
2. a single resistor (bulb)
3. connecting wires (conducting material)
The part of the circuit containing the electrochemical cells of
the battery is the internal circuit.
The part of the circuit where charge is moving outside the
battery through the wires and the light bulb is the external
circuit.
External Circuit
Electrical energy is lost
by the charge as it does
work on circuit elements.
Internal Circuit:
Energy is supplied
to the charge.
Three requirements which must be met in order to establish an
electric circuit. The requirements are:
1. There must be an energy supply capable doing work on
charge to move it from a low energy location to a high
energy location and thus establish an electric potential
difference across the two ends of the external circuit.
2. There must be a closed conducting loop in the external
circuit which stretches from the high potential, positive
terminal to the low potential, negative terminal.
3. All connections must be made by conducting materials
capable of carrying charge.
In the diagram above, the first circuit contains the
proper elements: a power source, wire and a bulb.
But the loop is not closed, so no current flows.
In the second diagram, the wires are connected so
electricity flows.
Actual Circuit
Schematic Diagram
Electric circuits are represented by drawn schematic
diagrams like the one on the right.
In the diagram, symbols are used to represent each
part of the circuit.
Electrical symbols
are quicker and
easier to draw than
realistic pictures of
the components.
The positive terminal of a cell is the long line and the negative
terminal is the short line.
If there is a two-cell battery, then the long line on the end is the
positive terminal of the battery. The short line on the opposite
end is the negative terminal of the battery
Drawing schematic diagrams is not difficult but it takes a little
practice.
Follow these tips for best results:
1. Make sure you use the correct symbol for each
component.
2. Draw connecting wires as straight lines (use a ruler)
The wires only change direction at right angles..
3. Label components such as resistors with their values.
The orientation (vertical or horizontal) should be
consistent.
4. The positive (+) supply should be at the top and the
negative (-) supply at the bottom. The negative supply
is usually labelled 0V, zero volts.
Short circuit
Without a load, such as a bulb or other resistor, a circuit contains
little resistance to the movement of charges. This situation is
called a short circuit.
For example, a short circuit occurs when a wire is connected from
one terminal of the battery to to other by a wire with little
resistance.
When short circuits occur, the increase in current can become
unsafe. Most wires cannot withstand increased current and they
overheat. The wires insulation may even melt or catch on fire.
Another Short Circuit Example
In this case, not only would the light bulb go out,
but so much electricity would flow in the circuit
that a fire would probably start.
Short Circuit Demonstration
Electrons flow from the negative terminal to the
positive terminal.
Their energy is provided by the battery.
Convention For Current Direction In A Circuit
For current flow, the convention is to show current in the
external circuit directed away from the positive terminal and
toward the negative terminal of the battery.
This convention for current arose from a guess made about
the direction of movement of positive charge. For this reason
the direction of conventional current is the direction opposite
the electron flow.
This convention was established long before the discovery of
electrons. It turned out that movement of negative charge
(electrons) was in the opposite direction.
Yes, this is outdated, but it is still the convention when talking
about current.
Series Circuit – A circuit in which the current passes through
all components one after another because there is
only one path for charge to flow.
Each charge passing through the loop of the external circuit
will pass through each resistor in consecutive fashion.
Since there is only one pathway through the circuit, every
charge encounters the resistance of every device.
Adding more devices results in more overall resistance.
In order for the devices in a series circuit to work, each
device must work. Since current must flow through all of
the resistors, if one resistor goes out, they all go out.
The eight light bulbs are connected in series. All
electrical current in the circuit must pass through
each light bulb.
If one bulb goes out the entire string goes out.
In this animation you should notice the following things:
1. The charges are only flowing in one direction so this would be
considered direct current ( DC ).
2. The battery or source is represented by an escalator which raises
charges to a higher level of energy.
3. As the charges move through the resistor (represented by the paddle
wheel) they do work on the resistor and as a result, they lose energy.
4. By the time each charge makes it back to the battery, it has lost all the
energy given to it by the battery.
The sum of the voltage drops across the individual resistors
is equal to the voltage rating of the battery.
The potential drop ( - potential difference) across the
resistor is the same as the potential rise ( + potential
difference) across the battery.
This demonstrates that a charge can only do as much
work as was done on it by the battery.
Resistance In A Series Circuit
Equivalent resistance – the total resistance of the
circuit.
The equivalent resistance in a series circuit is the
sum of all the circuit’s resistances.
Req = R1 + R2 + R3 + ...
The equivalent resistance of a series circuit is
always greater than any individual resistance.
.
You Try!
=
100 Ω
Current In A Series Circuit
The current in a series circuit is the same everywhere.
Use Ohm’s Law to calculate the current in a
series circuit.
Determine the current in the circuit shown above.
Rtotal = 3Ω + 10Ω + 5Ω = 18Ω
V = 9v (voltage of battery)
I = 9/18 = .5 amps
Batteries in Series
Vtotal = V1 + V2
Vtotal = 1.5 V + 1.5 V = 3.00
When batteries are wired in series, the voltage is additive.
NO
YES
Voltage In A Series Circuit
Voltage drop is the reduction in voltage in an electrical circuit
between the battery (source) and load. The amount of the drop
Is given by Ohms Law.
So the voltage drop across a circuit measures how much
energy would be dispersed by a unit charge going through
that circuit. The voltage drop across the circuit is equal to the
voltage of the battery.
Similarly, a voltage drop across a resistor in a circuit is the
energy dispersed by a unit charge going through a single
resistor in the circuit.
.
The battery simply supplies the energy to increase
the potential energy of a charge that moves from
the negative terminal to the positive terminal.
Once the charge has reached the positive terminal
(the high potential terminal), it will naturally flow
through the wires to the (negative terminal) low
potential terminal.
Fill In The Blanks In The Circuits Below.
40
5
Use Ohm’s Law to calculate the voltage drop in a
series circuit as well as the total voltage in the
circuit.
I = 1 amp
Calculate the voltage drop for the 4Ω resistor.
V = I x R (value of the single resistor here)
V= 1A x 4 Ω = 4V drop across the 4Ω resistor
Calculate the voltage drop across the entire circuit.
V = 1 amp x 9 volts (sum of all resistances here)
V = 9 volt drop across the entire circuit.
(*voltage drop across the circuit = voltage of the battery*)
Other Examples Of
Electrical Circuits
AC's alternating nature has a greater tendency to throw the
heart's pacemaker neurons into a condition of fibrillation,
whereas DC tends to just make the heart stand still.
Once the shock current is halted, a "frozen" heart has a
better chance of regaining a normal beat pattern than a
fibrillating heart.
This is why "defibrillating" equipment
used by emergency medics works: the jolt
of current supplied by the defibrillator unit
is DC, which halts fibrillation and gives
the heart a chance to recover.
In either case, electric currents high
enough to cause involuntary muscle
action are dangerous and are to be
avoided at all costs.
Plasma Membrane Of A Cell
When we call a circuit a "closed circuit" we mean
A. There is only one path for current to flow through..
B. The current is blocked and cannot flow.
C. The pathway is complete with no gaps and
current can flow.
D. There is only one device connected to the power
source
When two light bulbs are connected in series, which
is true?
A. The current through both is the same.
B. The current through one plus the current through
the other equals the line current.
C. Together they have less resistance than either
would alone.
D. There is a danger of overloading the circuit.
When you add devices to a series circuit, what do you
know about the current?
A. Adding more devices increases the amount of current
through the battery.
B. Adding more devices decreases the amount of
current through the battery.
C. More devices change the resistance of the circuit, but
not the current.
If you have a strand of Christmas tree lights and one
bulb is not working but the remaining bulbs continue
to burn, then you know the lights are wired using...
A. A series circuit
B. A parallel circuit
Three 3-ohm resistors are connected in series. What is the
total resistance?
A. 1 ohm
B. 3 ohms
C. 9 ohms
The voltage across every circuit element is the same
in a series circuit?
A. True
B. False
In a series circuit, if one bulb goes out:
A. They all go out.
B. All other bulbs stay lit.
C. It is impossible to tell in advance.
If two 700 ohm resisters are wired in series, the
equivalent resistance of the combination is
_____________ ohms.
In the circuit shown above, the total resistance
of the two resistors is _________ ohms.
The End