The word circuit means “closed path”.

By an electric circuit we mean an arrangement of electrical components connected so that electriccharges can flow in a closed path.

1. a source of potential difference (battery)

2. a single resistor (bulb)

3. connecting wires (conducting material)

The simplest electric circuit consists of:

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.

Internal Circuit:Energy is suppliedto the charge.

External CircuitElectrical energy is lost by the charge as it does work on circuit elements.

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 schematicdiagrams like the one on the right.

In the diagram, symbols are used to represent eachpart 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.

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.

Follow these tips for best results:

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 fromone 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.

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.

Convention For Current Direction In A Circuit

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 talkingabout current.

Series Circuit – A circuit in which the current passes throughall components one after another because there isonly 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 ofthe 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.

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.

In this animation you should notice the following things:

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.

Equivalent resistance – the total resistance of thecircuit.

The equivalent resistance in a series circuit is thesum of all the circuit’s resistances.

The equivalent resistance of a series circuit isalways greater than any individual resistance.

Req = R1 + R2 + R3 + ...

.

Resistance In A Series Circuit

= 100 Ω

You Try!

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

Vtotal = V1 + V2

Vtotal = 1.5 V + 1.5 V = 3.00

When batteries are wired in series, the voltage is additive.

Batteries in Series

NO YES

Voltage In A Series Circuit

Voltage drop is the reduction in voltage in an electrical circuitbetween the battery (source) and load. The amount of the dropIs given by Ohms Law.

So the voltage drop across a circuit measures how muchenergy would be dispersed by a unit charge going throughthat circuit. The voltage drop across the circuit is equal to thevoltage of the battery.

Similarly, a voltage drop across a resistor in a circuit is theenergy dispersed by a unit charge going through a singleresistor 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.

Calculate the voltage drop for the 4Ω resistor.

V = I x RV= 1A x 4 Ω = 4V drop across the 4Ω resistor

(value of the single resistor here)

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*)

I = 1 amp

Other Examples OfElectrical Circuits

Neuron

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

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 we call a circuit a "closed circuit" we mean

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 throughthe other equals the line current.

C. Together they have less resistance than eitherwould alone.

D. There is a danger of overloading the circuit.

When you add devices to a series circuit, what do youknow about the current?

A. Adding more devices increases the amount of current through the battery.

B. Adding more devices decreases the amount ofcurrent 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 samein 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, theequivalent resistance of the combination is_____________ ohms.

In the circuit shown above, the total resistanceof the two resistors is _________ ohms.

The End