22 February, 2020

# Electric current Electric power is indispensable in our society. It is therefore very important that we understand how it works and how it can be applied. The material about electric current can be summarized in a number of subjects: the basis, the Ohm's law, resistors and circuits.

## Base

To understand electrical current it is important that the basics are properly understood. There is positive and negative charge. Electrons are carriers of negative charge and protons are carriers of positive charge. Equal charges repel each other, but negative and positive attract each other. Electrical conduction takes place in metals through free electrons. In liquids by ions. The unit of loading is coulomb (C). Charge of one electron is -1.6x10 ^ -19 C. It follows that -1 coulomb is the charge of 6.25x10 ^ 18 electrons.
Examples of voltage sources are: battery, battery and a power supply device. The plus pole is always 'higher' than the minus pole. One of the two poles is grounded. For example, with a 9V battery with the plus pole grounded, the minus pole is 9V lower. The minus pole is therefore -9V. There are also alternating voltage sources such as the alternator and the socket voltage. In that case, one pole is usually 0V, the other alternates between positive and negative.
There are various options for conducting electrical measurements. The voltage can be measured with the voltmeter. Connect the voltmeter between the points between which you want to measure voltage. The amperage can be measured with the ammeter. Connect the ammeter in the wire where you want to measure the current.

## Ohm's law

U = IxR where U is the voltage in volts, I is the current in amps and R is the resistance in ohms, the formula is ohms. This formula always applies even if R is not constant. When looking at a circuit, the formula can be used for the entire circuit, but also for a part of the circuit that you 'zoom in' to. I is the result of the combination of U and R. I always adapts to the circumstances U and R. The resistor R is called an ohmic resistor. Some examples of ohmic resistors are:

### Carbon resistors

Often used in radios, TVs and other electronic equipment. The external size of the resistor does not say anything about the number of ohms, but about the capacity that resistor can absorb without burning.

### Metal wire resistance

At a constant temperature the following applies: R = Px (l / A).
• P is the resistivity with unit Ohm.
• l is the length of the wire in meters
• a is the size of the flow area in square meters

The resistance of a copper power cord is very small, with an increase in temperature P increases and as a result the resistance also increases.

## Non-ohmic resistors

Non-ohmic resistors are resistors with a variable value. Below some examples
• Carbon microphone, consists of many small particles of carbon in an enclosed space, closed by a membrane.
• PTC, a resistor with Positive Temperature Coefficient. This means that the resistance increases with temperature rise.
• NTC, a resistance with Negative Temperature Coefficient. That means that the resistance decreases with temperature rise.
• LDR, Light Dependent Resistor. A light sensitive resistor: the more light falls on it, the less resistance.

There is also a sliding resistance. This sliding resistance makes it possible to vary the resistance by turning a button. The sliding resistance can be used as a voltage divider.

## Circuits with resistors

Two types of circuits are possible with resistors. The series connection and the parallel connection. In a series connection, all resistors are placed one behind the other, so that the current 'encounters' all resistors one by one. The formula then applies: R total = R1 + R2 + R3 + ...
With a parallel connection, the resistors are positioned so that the current must 'choose' which resistor it passes through. The formula then applies: 1 / (R total) = 1 / R1 + 1 / R2 + 1 / R3 + ...
To measure the resistance, it is possible to connect the resistor to an ohmmeter. The resistance can also be determined with a volt or ammeter and the formula U = IxR.