Relationships

Youd be right in thinking that there must be some form of relationship between this pushing power in volts and the rate of electron flow in amps. After all, the higher the voltage, the more pushing power the electrons have behind them so the faster they should flow. The relationship was first discovered by a scientist called Ohm, and so is commonly known as Ohms law. It may be summarised by the expression:

where the constant depends on the substance through which the current flows and the voltage is applied across. Figure 1.5 gives an example of a substance which is connected to a cell. The cell has a voltage of 2 V, so the voltage applied across the substance is also 2 V. The current through the substance is, in this case, 0.4 A. This means, from Ohms law, that the constant for the substance is:

The constant is commonly called the substances resistance (because it is, in fact a measure of the amount the substance resists the flow of current through it) and is given the unit: Ω (pronounced ohm not omega after the scientist, not the Greek letter its symbol is borrowed from). So, in our example of Figure 1.5, the resistance of the substance is 5 Ω. In some literature the letter R is used instead of Ω. Different substances may have different resistances and may therefore change the current flowing.

Cells voltage is 2 V, and a current of 0.4 A flows

Figure 1.5 Cells voltage is 2 V, and a current of 0.4 A flows

Take note: This is a vitally important concept probably the most important one in the whole world of electronics and yet it is often misunderstood. Even if it is not misunderstood, it is often misinterpreted.

Indeed, this is so important, lets recap it and see what it all means: If a voltage (V measured in volts) is applied across a resistance (R measured in ohms), a current (I measured in amps) will flow. The voltage, current and resistance are related by the expression (1):

  (1)

The importance of this is that the current which flows depends entirely on the values of the resistance and the voltage. The value of the current may be determined simply by rearranging expression 1, so that it gives (2):

  (2)

So, a voltage of say 10 V, applied across a resistance of 20 Ω, produces a current of:

Similarly, if we have a resistance, and a current is made to flow through it, then a voltage is produced across it. The value of the voltage may be determined by again rearranging expression 1, so that it now gives (3):

  (3)

Hint: Ohms law

A simple method to help you remember Ohms law: remember a triangle, divided into three parts. Voltage (V) is at the top. Current (I) and resistance (R) are at the bottom: it doesnt matter which way round I and R are the important thing to remember is V at the top. Then, if you have any two of the constants, cover up the missing one with your finger and the formula for calculating the missing one will appear. Say you know the voltage across a resistor and the current through it, but you need to know the resistance itself. Simply cover the letter R with your finger:

and the formula to calculate the resistance is then given as: V/I

Thus, a current of, say, 1 A flowing through a resistance of 5 Ω, produces a voltage of:

across the resistance.

These three expressions which combine to make Ohms law are the most common ones youll ever meet in electronics, so look at em, read em, use em, learn em, inwardly digest em just dont forget em. Right? Right.

Take note:

And another thing. See the way weve said throughout, that a voltage is applied or produced across a resistance. Similarly a current flows through a resistance. Well lets keep it like that! Huh? Just remember that a voltage is across: a voltage does not flow through. Likewise, a current flows through: it is not across.

There is no such thing as a flow of voltage through a resistance, and theres no such thing as a current across a resistance.

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