# Understanding Current and Voltage

## ≡ Understanding Current

As the word current suggests, the meaning of current in electronics is like to that of the current in a river. You could think of the strength of the current in a pipe as being the amount of water passing a point in the pipe every moment. This might be measured in so many gallons per second.

In electronics, the current amount of charge carried by electrons passing a point in a wire per second we know as current. The unit of current is the ampere, abbreviated as an amp or as unit symbol A

## ≡ Understanding Voltage

The current will not flow in need of something influencing it. In a water pipe, that might be because one end of the pipe is higher than the other. To understand voltage, it can be useful to think of it as being related to height in a system of water pipes.

Just like height, it is relevant, so the height of a pipe above sea level does not determine how fast the water flows through a pipe, but rather how much higher one end of the pipe is than the other Voltage might apply to the voltage across a wire and in other situations, it might refer to the voltage from one terminal of a battery to another.

The standard feature is that for voltage to make any sense, it must refer to two points; the higher voltage is the positive voltage, marked with a +. It is the difference in voltage that makes current flow in a wire. If there is no differ‐ once in voltage between one end of a wire and another then no current will flow. The unit of voltage is the volt. An AA battery has about 1.5V across its terminals.

## ≡ The relation between Voltage, Current, and  Resistance

Use Ohm’s Law. Ohm’s Law states that the current flowing through a wire or electronic component (I) will be the voltage across that wire or component (V) divided by the resistance of the component (R). In other words:

I=

$\frac{V}{R}$

If it is the voltage that you want to calculate, then this formula can be rearranged as:

$V=I×R$

And, if you know the current flowing through a resistor and the voltage across the resistor, you can calculate the resistance using:

R=

$\frac{V}{I}$

Resistance is the capacity of a medium to resist the flow of current. A wire should have low resistance as you do not normally want the electricity flowing through the wire to be needlessly impeded.

The higher the wire, the less its resistance for a given length.

So a few feet of thin wire that you might find joining a battery to a lightbulb in a flashlight might have a resistance of perhaps 0.1Ω to 1Ω, whereas the same length of thick AC outlet cable for a kettle may have a resistance of only a couple of milliohms (mΩ).

It is extremely common to want to limit the amount of current flowing through part of a circuit by adding some resistance in the form of a special component called a resistor.