Whilst you can work out the solution to this problem by using the formula I = V/R, it does not give you a feel for the interaction between the three quantities.
Introduction
Circuits
Basic Principles
Electricity
The Electron
Volts
Doing Work
in a Circuit
Resistance
Operating a light emitting diode
Conclusion
Any study of electrical items requires that we are conversant with amps, volts, ohms and power. You should all know the equations that link these terms together, but a knowledge of these terms at a conceptual level will assist your understanding of electricity and how it affects electrical devices.
Electrical circuits are often depicted as diagrams
such
as the one below. There will be a source of power, some cabling and
something
for the power to be dissipated in.
Whilst you can work out the solution to this problem
by using the formula I = V/R, it does not give you a feel for the
interaction
between the three quantities.
For instance, the classical definition of an amp is:
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 Newtons per metre of length.
What does this tell you about an ampere? It tells me nothing.
Electricity is a dynamic concept, i.e. it is in motion. Electricity flows through conductors and can be regarded as being similar to water flowing through pipes.
Let us begin our understanding with water and develop this further as we go on.
Below is a bucket of water. For argument’s sake, let us define it as having a capacity of 10 litres.
The bucket holds 10 litres of water. Now let us build a tank to hold water with a pipe leading from the bottom of the tank. To stop the water leaking out, let us put a tap in the pipe. Now we shall fill the tank using the bucket. It needs 6 buckets of water to fill the tank.
How many litres of water are in the tank?
Volume of water = 6 X 10 litres = 60 litres
Providing that the tap is closed, the water will not
leak
out of the tank. The water is static. It is not flowing through the
pipe.
Let us begin to open the tap gradually.
What will happen to the water in the tank as the tap is turned on?
The water will begin to flow past the tap and through the rest of the piping.
How can we regulate the flow of water through the piping?
The flow of water can be regulated by adjusting how much the tap is turned on. A fast flow can be achieved by turning the tap on fully; a slower rate of flow can be achieved by turning the tap partially on.
In the diagram above, let us turn the tap fully on. Now we shall allow the water to drain from the tank. We shall use a stopwatch to measure the time taken to drain the tank.
The time taken to drain the tank is 1 minute. This is sixty seconds. The tank when full holds 60 litres of water. This means that 60 litres of water flows through the pipe in 60 seconds.
How many litres of water flow through the pipe per second?
Answer = volume/ time
= 60 litres/ 60 seconds
= 1 litre per second
We have discovered the rate that water is flowing through the pipe.
Electricity is concerned with the movement of
electrons
through conductors (e.g. wires, transistors etc).
Let us now replace the water in the bucket with
electrons
(here you must allow your imagination to take over as it is really
impossible
to fill a bucket with electrons!).
Now we have a ‘bucket’ of electrons.
As before, let us fill the tank, but this time with electrons instead of water. Then we will turn the tap on and allow the electrons to flow through the pipe.
Now we have a flow of electrons through our ‘pipes’ and this can be measured by counting the number of electrons that flow past some point in the pipe per second. This is analogous to the flow of water that we discussed earlier.
Water flows in litres per second.
Electricity flows in electrons per second.
We measure the flow of electricity in Amperes. But just to confuse things, we use the symbol I to represent amps.
Thus the Ampere could be said to be the number of electrons that flow through a ‘pipe’ or conductor per second.
The electron is an extremely small particle that carries a tiny charge of electricity.
Electrical charge is measured in Coulombs.
Each electron carries a charge of 1.6 X 10-19 Coulombs.
Therefore, to make up one Coulomb of electrical charge we need: -
1/ 1.6 X 10-19
electrons which works out to be around 6.24 X 1018
electrons, in other words quite a lot of electrons!
This is around 6,241,506,000,000,000,000 electrons.
A flow of 1 ampere has been defined as a flow rate of 1 Coulomb of charge per second.
Examination of the above leads to the conclusion that 1 ampere (more commonly referred to as the amp) is a flow rate of 6.24 X 1018 electrons passing some point in a circuit per second.
1 amp = 6.24 X 1018 electrons per second.
Therefore we can say that amps are a measure of the number of electrons flowing through some device or cable per second.
The volt now needs our consideration.
To propel our electrons through a circuit, we need to apply some sort of force as electrons don’t really want to move unless they are compelled to. The force to move electrons is called the volt.
The more volts we have pushing our electrons through a circuit, the more amps will flow.
Twice as many electrons will flow through a given circuit if we apply twice as many volts to ‘push’ the electrons through it.
A battery may be thought of as an ‘electron pump’. It is a source of electrons and can ‘push’ electrons through a circuit.
To force the electrons through a circuit we need to create a potential difference between the opposite ends of the circuit. The potential difference is measured in volts.
If we use a 1.5 volt battery to propel electrons
through
a circuit as shown below, the potential difference between the opposite
ends of the circuit will be 1.5 volts.
The potential difference of 1.5 volts will ‘push’ a
certain
number of electrons through the circuit per second. Unfortunately, the
number of electrons that flows is so high that it is too cumbersome to
use electrons as our unit of measurement. We measure the flow of
electrons
in amps.
Therefore the force of 1.5 volts causes a flow of electrons through the circuit. The flow of electrons is known as current, measured in amps.
When we create an electrical circuit, we want it to do some job for us. For instance we could replace the resistor in the circuit above with a light bulb. When we turn the current on, the light bulb shines because electrons are flowing through it.
If we put too few electrons through the bulb, it will glow dimly, if at all. If we put too many electrons through the bulb, the wire that glows will get too hot and melt.
Manufacturers build electrical components and specify the optimum current that is needed to make the component function at its best. This means that there is an optimum number of electrons per second that the device will require.
The previous discussion of volts should have told you that we will need a certain number of volts to force the electrons through the bulb.
Too few volts will cause too few electrons to flow. Conversely, too many volts will cause too many electrons to flow through the bulb with the dire consequence of a blown bulb!
The unit of resistance is the ohm. It is a measure of how difficult it is for electrons to pass through a certain component. Let us reconsider the water circuit from earlier.
We measured the rate of flow of water in litres per second. Suppose we took a big hammer and hit the pipe, bending it so that it became crimped and impeded the flow of water.
Would this increase or decrease the rate of flow of
water
through the pipe when the tap was turned on?
Of course it would slow the flow of water down to a trickle. This is because we have reduced the path for the water to flow through. This has made it more difficult for the water to flow through the pipe.
If we hit the water pipe again, it would make the pipe even narrower, increasing the resistance to the flow of water.
Similarly we can make it more difficult for electrons to flow through a circuit by making the cable thinner. Alternatively we could use a material such as carbon that electrons find it harder to travel through. Carbon forms the basis for common resistors.
By making an obstacle for the electrons to pass, we have created a resistance to the flow of electrons. The higher the value of the resistance, the fewer electrons can pass through a circuit.
The unit of resistance is the ohm.
To calculate the current (number of electrons per second) that will flow through a circuit, we use the formula
Amps = Volts/ Resistance
[1]
You can easily get this result by covering up the
quantity you require - in this example we need Amps (symbol is I) so by
covering the I you are left with V/R
It should be possible to see from the equation above that we can increase the flow of current (amps) by raising the force pushing the electrons, the volts.
Alternatively we could decrease the resistance to the flow of electrons, i.e. lower the resistance.
Operating a Light Emitting Diode (LED)
The current required to light a particular LED is 35 milliamps (mA). This means that the LED requires a particular flow of electrons per second to function properly.
LEDs do not put up much resistance to the flow of electrons, so if we just connect the LED across the terminals of a 12 volt battery, it will fail, more than likely popping the top of the LED off in the process!
Therefore we need to regulate the number of electrons that our battery (electron pump) can pass through the LED. We can do this by calculating the value of resistance that we need to place in the circuit to slow the flow of electrons to 35mA.
We need to rearrange the equation relating amps volts and ohms to make ohms the subject of the equation.
Resistance = V/ I
We now substitute our values into the equation.
Resistance (ohms) = 12/ 35 X 10-3 = 343 ohms
We will need a resistor having a value of 343 ohms to reduce the flow of electrons to 35mA and therefore supply the LED with the correct number of electrons per second to function properly.
Amperes (amps) is a measure of the number of electrons flowing past a certain point in a circuit per second or the rate of flow of electrons.
Each electron carries a small amount of charge. This is measured in coulombs. The number of coulombs passing a point in a circuit per second is known as current.
Current is measured in amps.
To force the electrons through the circuit, they need a force. This force is referred to as the potential difference and it is measured in volts. A battery may be thought of as an ‘electron pump’.
The more force (volts) we apply to our electrons, the more electrons will flow per seconds, so the current (amps) will increase.
Resistance is a measure of how difficult electrons find it to pass through a component. The more difficult it is for electrons to pass through a component, the higher a resistance it has. The unit of resistance is the ohm.
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© MM Clements 2001