Introducing Circuits
Introduction


This unit introduces you to electronic circuits and explains the meaning of current, voltage, and resistance. You will find out about Ohm's equations and about some of the components used in building electronic circuits.

Shining a light
Have you ever taken an electric torch to pieces to find out how it works? Look at Fig.1 below, which shows the arrangement of parts inside one kind of torch. Click the buttons at the lower left corner of the diagram to see the torch in action.

Figure 1.   Structure of an electric torch.
Display as fullscreen


Why did the designer of the torch choose this particular combination of materials?

  • The metal parts must conduct electric current if the torch is to function, but they must also be able to stand up to physical forces.

  • The spring holding the cells in place should stay springy, while the parts of the switch must make good electrical contact and be undamaged by repeated use.

  • The lamp and reflector make up an optical system, often intended to focus the light into a narrow beam.

  • The plastic case is an electrical insulator. Its shape and colour are important in making the torch attractive and easy to handle and use.



What problems need to be solved if the manufacturer wants to use a metal case for the torch?

A torch is a simple product, but a lot of thought is needed to make sure that it will work well. Can you think of other things which the designer should consider?

Match each description with the correct word
  • Materials which allow easy current flow
    Materials which prevent current flow
  • Click here to mark the question
Which materials used in making a torch are conductors and which are insulators?
  • Plastic
    Copper
    Tungsten (lamp filament)
    Glass (outside of lamp)
  • Click here to mark the question
Drawing a circuit diagram
A different way of describing the torch is by using a circuit diagram in which the parts of the torch are represented by symbols.

Figure 2.   Circuit diagram of an electric torch.
Display as fullscreen


In Fig.2 there are two electric cells ('batteries'), a switch, and a lamp (the torch bulb). The lines in the diagram represent the metal conductors which connect the system together.

A circuit is a closed conducting path. In the torch, closing the switch completes the
circuit
A circuit is a closed conducting path.
circuit
and allows
current
Current I is a flow of charged particles, usually electrons.
current
to flow. Torches sometimes fail when the metal parts of the switch do not make proper contact, or when the lamp filament is 'blown'. In either case, the circuit is incomplete.


Click on the figure below to interact with the model.

 Figure 3.  Torch picture circuit.



In the simulation of Fig.3, you can complete the circuit by clicking on the switch. Look carefully at the changes which take place when the switch is closed.

The diagrams show different arrangements of cells, switches, and lamps.

Figure 4.  
Display as fullscreen
Which switch in Fig.4 completes a circuit, allowing current to flow and illuminating a lamp?
  • Click here to mark the question
Current
An electric current is a flow of charged particles. Current is sometimes carried by positively charged particles, but inside a copper wire, current is carried by small negatively charged particles, called electrons. Metals, such as copper, contain free electrons, which drift in random directions as shown in Fig.5.

Figure 5.   Free electrons in a copper wire.
Display as fullscreen


When a current starts to flow, the electrons start to move in the same direction. Click the button in Fig.5 to see a simulation of this behaviour. The size of the current depends on the number of electrons passing per second.


Click on the figure below to interact with the model.

 Figure 6.  Torch circuit diagram.



Fig.6 shows the simulated circuit diagram for the torch. In the simulation, the flow of current is indicated by small arrows.

Current is represented by the symbol I, and is measured in amperes (usually shortened to amps), A.
One ampere is equivalent to a flow of 6.24 × 1018 electrons per second passing any point in a wire.
     That's more than six billion billion electrons! This is a lot of electrons, but electrons are very small and each carries a very tiny charge.



In electronic circuits, currents are most often measured in milliamps, mA, that is, thousandths of an amp.
Convert 0.2 A to the equivalent value in mA.
  • Click here to mark the question
Convert 50 mA to the equivalent value in A.
  • Click here to mark the question
Voltage
What causes the current to flow in the torch circuit?
The answer is that chemical reactions in the cells produce electrical energy. That energy provides a 'push' which makes the current flow round the circuit.
When the cells are new, enough current flows to light the lamp brightly.
     On the other hand, if the cells have been used for some time, they may be 'flat' and the lamp glows dimly or not at all.



Each
cell
A cell provides a source of electrical energy. In a circuit, cells provide the 'push' which makes current flow.
cell
provides a push, called its potential difference, or voltage. This is represented by the symbol V, and is measured in volts, V. Sometimes, you will want to measure voltages in thousandths of a volt, or millivolts, mV.

Typically, each cell provides 1.5 V. If cells are joined together one after the other, they are said to be connected in series. Two 1.5 V cells connected in
series
Components are connected in series when they are joined end to end in a circuit, so that the same current flows through each.
series
provide 3 V, while three cells provide 4.5 V.

Figure 7.   Cells connected in series.
Display as fullscreen
Which of the arrangements in Fig.7 would make the lamp glow most brightly?
  • Click here to mark the question


Lamps are designed to work with a particular
voltage
Potential difference, or voltage V is a measure of the difference in energy between two points in a circuit. Charges gain energy in the battery and lose energy as they flow round the rest of the circuit.
voltage
, but, other things being equal, the bigger the voltage, the brighter the lamp.


Click on the figure below to interact with the model.

 Figure 8.  Torch circuit diagram.



Voltages in the simulation of Fig.8 are represented by small red bars. The height of the bars is proportional to the voltage. Click on the
battery
A battery consists of two or more cells. The cells may be connected in series or in parallel.
battery
symbol in the circuit diagram and change the voltage first to 6 V and then to 9 V.

When the battery symbol is clicked, a dotted selection box appears around it and you can edit the battery voltage to a new value. Click away from the battery symbol to run the simulation.

Check the effect on the voltage bars and on the brightness of the lamp when the switch is closed. As you increase the voltage, more current flows.

Find out what happens if you increase the voltage to 12 V.

Select the words which best complete each sentence.

  • In the simulation, when the is increased to 6 V or 9 V, more flows and the lamp shines more brightly. If the voltage is increased to 12 V, current flows and the lamp filament .
  • Click here to mark the question


Strictly speaking, a battery consists of two or more cells. These can be connected in series, as is usual in a torch circuit, but it is also possible to connect the cells in parallel, as shown in Fig.9.

Figure 9.   Cells connected in parallel.
Display as fullscreen


A single cell can provide a small current for a long time, or a big current for a short time. Connecting the cells in series increases the voltage, but does not affect the useful life of the cells. On the other hand, if the cells are connected in
parallel
Components are connected in parallel when they are joined side by side in a circuit, so that they provide alternative pathways for current flow.
parallel
, the voltage stays at 1.5 V, but the life of the battery is doubled.

A torch lamp which draws 300 mA from C-size alkaline cells should operate for more than 20 hours before the cells are exhausted.

Fig.10 shows four possible arrangements of cells.

Figure 10.   Different arrangements of cells.
Display as fullscreen
Which arrangement of cells in Fig.10 provides the highest voltage?
  • Click here to mark the question
Which arrangement of cells in Fig.10 gives the longest life?
  • Click here to mark the question
What will be the voltage of a battery consisting of four 1.5 V cells connected in series?
  • V
  • Click here to mark the question
Convert 1.5 V to the equivalent value in millivolts.
  • Click here to mark the question
Which way does the current flow?
One terminal of the battery is positive, while the other is negative. It is convenient to think of current as flowing from positive to negative. This is called conventional current. In Absorb Electronics, current arrows in circuit diagrams always point in the conventional direction. This is the direction of flow for a positively charged particle.

In a copper wire, the charge-carriers are electrons. Electrons are negatively charged and therefore move from negative to positive. This means that electron flow is opposite in direction to
conventional current
In a circuit, current is thought of as flowing from the positive terminal of the power supply towards the negative terminal. This is the direction of flow for a positively charged particle.
conventional current
.

Figure 11.   Electron flow in a copper wire.
Display as fullscreen
What happens to the free electrons in Fig.11 when current starts to flow?
  • Click here to mark the question


Current flow in electronic systems often involves charge-carriers of both types. For example, in transistors, current is carried by electrons and also by holes, which behave as positive charge-carriers.

When the behaviour of a circuit is analysed, what matters is the amount of charge which is being transferred. The effect of the current can be accurately predicted without knowing about whether the charge-carriers are positively or negatively charged.

The chemical reactions in a cell provide a steady voltage, so that current always flows in the same direction. This is called direct current, or d.c. However, electricity can also be generated by moving a
conductor
A conductor is a material which allows current to flow easily. Most metals are conductors.
conductor
in a magnetic field. When this happens, as in Fig.12, it gives rise to an alternating current, or a.c., in which the charge-carriers move backwards and forwards in the circuit.

Figure 12.   Alternating current in a copper wire.
Display as fullscreen


Because it is generated in this way, the domestic mains provides a constantly changing voltage which reverses in polarity 50 times per second (UK domestic mains), or 60 times per second (US domestic mains).

For safety reasons, you must never connect circuits to the mains supply.


Select the words which best complete each sentence.

  • Conventional current flows from the terminal to the terminal of the power supply. This is to the direction of flow of electrons.
  • Click here to mark the question
Resistance
If a thick copper wire is connected from the positive terminal of a battery directly to the negative terminal, you get a very large current for a very short time. In a torch, this does not happen.

Part of the torch circuit limits, or resists, the flow of current.
Most of the circuit consists of thick metal conductors which allow current to flow easily. These parts, including the spring, switch plates, and lamp connections, have a low resistance.
     The lamp filament, on the other hand, is made up of very thin wire. It conducts much less easily than the rest of the circuit and has a higher resistance.

Figure 13.   Lamp filament.
Display as fullscreen


The flow of current through the filament causes it to heat up and glow white hot. Lamp filaments are usually made of the metal tungsten because of its very high melting point. In air, the filament would quickly oxidize. This is prevented by removing all the air inside the glass of the lamp and replacing it with a non-reactive gas.

Which part of the torch circuit has the highest resistance to the flow of current?
  • Click here to mark the question
The resistance, R, of the filament is measured in ohms, Ω.
If the battery voltage is 3 V (2 C-size cells in series) and the lamp current is 300 mA (0.3 A), what is the resistance of the filament?
     This is calculated from:



where R is resistance (in ohms), V is the voltage across the lamp (in volts), and I is current (in amps).



In this case, 10 Ω is the
resistance
Resistance R limits current flow.
resistance
of the lamp filament once it has heated up. Its resistance is less when cold and there will be a surge of current, more than 300 mA, when the torch is first switched on.

Resistance values in electronic circuits vary from a few ohms, Ω, to values in kilohms (thousands of ohms), kΩ, and megohms (millions of ohms), MΩ. Electronic components designed to have particular resistance values are called resistors.
Convert 120 Ω to the equivalent value in kilohms, kΩ.
  • Click here to mark the question
Ohm's equations
The relationship between current, voltage, and resistance was discovered by Georg Ohm, who published his results in 1827.

Figure 14.   Georg Ohm, 1789–1854.
Display as fullscreen
Ohm made his own wires and was able to show that the size of an electric current depended upon their length and thickness. The current was reduced by increasing the length of the wire, or by making it thinner. Current was increased if a shorter thicker wire was used. In addition, larger currents were observed when the voltage across the wire was increased.

From experiments like these, Ohm found that, at constant temperature, the ratio of voltage to current was constant for any particular wire, that is,

Ohm's Law states that, at constant temperature, the electric current flowing in a conducting material is directly proportional to the applied voltage, and inversely proportional to the resistance.

Rearranging the formula gives two additional equations:



and



These simple equations are fundamental to electronics and, once you have learned to use them effectively, you will find that they are the key to a wide range of circuit problems. You are going to need these equations, so learn them now.

Absorb Electronics includes interactive versions of important equations and formulae. These allow you to rearrange the equation or formula by clicking on the term you would like to see on the left-hand side.

Match each symbol with the correct word:
  • V
    I
    R
  • Click here to mark the question
What is the resistance of a lamp filament if a current of 150 mA flows when the lamp is connected to three cells in series (4.5 V)?
  • Ω
  • Click here to mark the question
Ohm's equation calculator
Fig.15 is an animation which allows you to make calculations using any version of
Ohm's equation
Current, voltage, and resistance are related according to Ohm's equations:

        
 
Ohm's equation
.

Figure 15.   Ohm's equation calculator.
Display as fullscreen


Initially, the calculator is set up to work out resistance for a voltage (measured in volts) and a corresponding current (measured in milliamps). All you need to do is to type in values in the relevant boxes.
What is the resistance of a lamp when the voltage across the lamp is 9 V and the current flowing is 60 mA? (Type '9' in the Voltage box and '60' in the Current box and then click the 'equals' button.)
  • Click here to mark the question


Rearrange the calculator's equation by clicking on a term in the green equation box in Fig.15. The calculator can also be used to find:

  • Voltage (when you know the resistance and current)

  • Current (when you know the resistance and voltage)



Use the Ohm's equation calculator to answer the following questions. Be sure to check that the units selected in the calculator match those of the question.
What is the voltage across a 100 Ω
resistor
A resistor is an electronic component with a particular resistance values. Resistors limit current.
resistor
when the current flowing is 50 mA?
  • V
  • Click here to mark the question
What is the current flowing through a 3.9 kΩ resistor if the voltage across the resistor is 9 V? Give your answer correct to one decimal place.
  • mA
  • Click here to mark the question
What resistance limits the current flowing to 10 mA if the voltage across the resistor is 6 V? Convert your answer to a value in Ω.
  • Ω
  • Click here to mark the question


The Ohm's equation calculator is a useful resource. You can use it in any unit of Absorb Electronics by selecting it from the 'Tools' menu.
Did you know …? Light bulbs
The filament lamp was first invented in 1860 by a British physicist, Sir Joseph Swan. When electric current passes through a thin filament of a conducting material, the filament heats up and, if the current is large enough, the filament becomes first red hot and then white hot, or incandescent. In air, this effect is short-lived because the filament burns up and breaks. Swan had the idea of enclosing the filament in a glass container, preventing oxidation by removing the air inside the container using a vacuum pump.

These early experiments suggested that a useful light source was possible, but Swan did not have a sufficiently powerful vacuum pump. Years later, Swan tried again using a better vacuum pump. In 1878, he was successful in demonstrating a true incandescent light bulb.

The American Thomas Edison demonstrated a similar lamp in 1879. However, his real contribution was to develop not just the light bulb but the whole concept of electric power into a practical, safe, and economic system. In September 1882, the first commercial power station went into operation, providing light and power to customers in part of Manhattan. The electric age had begun.

Figure 16.   Edison with a light bulb.
Display as fullscreen


Edison tested thousands of different filament materials. The first commercial lamps had filaments made of carbon. This was later replaced by tungsten, a metal with a particularly high melting point.

In a modern filament lamp, a very fine tungsten wire is coiled in a tiny spiral. This spiral is coiled again to make a 'coiled coil'. This arrangement concentrates the heat produced as current passes through the wire, causing the filament to heat up and reach incandescence much more quickly. The space inside the lamp is filled with a non-reactive gas, usually an argon/nitrogen mixture.

The spectacular success of electric lighting is evident from night-time satellite photographs of the earth from space.

Figure 17.   City lights from space.
Display as fullscreen


The outline of most parts of the world is identified clearly by city lights. Zoom in to where you live.

Only recently have people started to worry about all the energy used in lighting and how it affects global warming. The filament lamp is not very efficient and converts just 10 per cent of its energy into light. The rest is wasted as heat. Energy efficient light bulbs use a different technology and use three to four times less energy for the same light output. Every home should have them!

The race is on for lighting manufacturers to find ways of making lighting more energy efficient. Huge savings could be made. It's possible that in a few years you will be able to light your house using super-efficient giant LEDs (light-emitting diodes). (You can find out more about LEDs in the unit, Diodes.)
Summary


A circuit is a closed conducting path.

Current, I, is a flow of charged particles, usually electrons.

Voltage, V, is the 'push' which makes current flow.

Resistance, R, limits current flow.

The behaviour of circuits is described by Ohm's equations:

          
Exercises

Click on the figure below to interact with the model.

 Figure 18.  

1. Which switches should be closed in Fig.18 to light lamp 1 only?
  • Click here to mark the question
2. Which switches should be closed in Fig.18 to light lamp 2 only?
  • Click here to mark the question
3. Which switches should be closed to light both lamp 1 and lamp 2 in Fig.18?
  • Click here to mark the question
4. What happens to the component parts of the circuit if all three switches in the simulation are closed at the same time?
  • Battery
    Lamps 1 and 2
    Switch 1
    Switches 2 and 3
  • Click here to mark the question
5. Hold the cursor over one of the wires in the simulation until the small green measurement box appears. Use the measurements obtained to help you to choose the alternatives which best complete each sentence.

  • With one lamp illuminated, the current flowing is . With two lamps illuminated, the current flowing is . When the lamps are connected in series, the resistance in the circuit and the current .
  • Click here to mark the question
Well done!
Try again!