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.

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.



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?

A different way of describing the torch is by using a circuit diagram in which the parts of the torch are represented by symbols.



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.




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.

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.



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.




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




In electronic circuits, currents are most often measured in milliamps, mA, that is, thousandths of an amp.

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.



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.




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.



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.



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.

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.



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.



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).

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.




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.




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.

The relationship between current, voltage, and resistance was discovered by Georg Ohm, who published his results in 1827.

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.

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.



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.


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.


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.

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.



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.



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:

          

Click on the figure below to interact with the model.