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Circuits and current

An electrical circuit is a complete sequence of electrical devices connected by wires, which returns to the same point. Circuits need a source of electrical energy, such as a battery. When electrons enter the circuit from the negative electrode of a battery, they travel around the circuit and return to the positive electrode of the battery.

An electrical circuit needs two things:

  1. A continuous electrical conducting path
  2. A source of energy

Conducting Path

Circuit
Circuits have a power source, a continuous conductive path. The electrons travel in the opposite direction to the current.

An electrical circuit may be very short, such as when a battery is 'shorted' by a wire connecting the two electrodes directly, or very long, such as when power is generated in a power station and transported hundreds of kilometers to a household.

An example is a battery with wires passing to a lamp. Electrons pass from the negative electrode of the battery along a wire to one pole of the lamp. The electrons then enter the lamp, pass through the filament, exciting it to emit light and heat, then pass out of the lamp via the other pole.

The electrons then continue back to the battery, entering it at the positive electrode. Any break in the wire will stop the flow of electrons at all points in the circuit.

Source of Energy

Along with a continuous conducting path (wires), a circuit must have a source of energy. This could be AC (alternating current) or DC (direct current). The mains electricity, which comes out of a socket in the wall, is AC, and batteries supply DC energy.

Even though a laptop computer is plugged into the wall socket, it has a transformer. A transformer is a device which converts electricity from one form to another. In the case of a computer, the mains power is converted from 240V AC to about 20V DC.

Batteries

Energy in a battery is stored as chemical potential energy. This energy is supplied as DC, direct current.

Positive ions (cations) and negative ions (anions) are in a fluid (electrolyte). In most small batteries, the electrolyte is a thick jell. When a circuit is completed between the two electrodes, the ions are drawn through the electrolyte to the electrode which is opposite their charge. Here they release or gain electrons, providing a transfer of electrons through the circuit.

If there is nothing but wire between the two electrodes of a battery, there is little to stop the electrons racing around very quickly, and the battery would lose power rapidly. This is called shorting, because the route of the electrons is too short.

To recharge the battery, the current is reversed, and electrons are 'pumped' back into the electrolyte.

Electrical Circuit Diagrams

Circuits can become very complicated. To avoid confusion, engineers use special symbols so every other engineer in the world will understand what they mean.

Circuit components
Circuit components have special symbols recognised all around the world

Even though wires are usually bundled together, a circuit is drawn as a series of rectangles. The circuit does not really look like this, it makes it much easier to see what connects to what.

Current

The current is a measure of how many electrons are passing through a circuit per second. Increasing the voltage across the same load will increase the current flowing. The unit of current is the ampere (A), or amps. The symbol in equations for current is I.

Current is measured with an ammeter. Differently to the voltmeter, an ammeter takes part in the circuit. Its electrodes are positioned so that all the electrons of the circuit pass through the ammeter. This means it is in series with the load, or resistor.

An ampere is one coulomb per second passing through a conductor. A coulomb is $6.24 x 10^18$ electrons. The reason for this strange number is to equate electrical current to power (watts) and energy (joules).

Voltage

Ammeter and voltmeter in a circuit
Ammeter and voltmeter in a circuit

The voltage is a measure of how much push the battery can give to the electrons. The higher the voltage, the more power the electrons have as they pass through the wires and loads in a circuit. Its unit is volt (V).

Voltage is measured with a voltmeter. A voltmeter does not take part directly in the circuit, but rather its electrodes are placed either side of a circuit component, such as a resistor. The meter gives the voltage difference across the component.

Resistance

Resistance is a measure of how much the electrons in a circuit are obstructed from travelling around the circuit.

Resistance is used to control the voltage and current in different parts of a circuit.

The symbol for resistance is R, and the unit is ohm (Ω), named after the German physicist Georg Ohm.

Ohm's Law

Ohm's Law relates the voltage V, current I, and resistance R, of a circuit.

$$V = IR$$

Resistance in wires is measured in ohms Ω. Georg Ohm, 1789 - 1854, was a German physicist and pioneer of electricity.

A circuit has a power supply and a load. The load could be a lamp, an LED, a buzzer, a dishwasher, .... in fact anything that requires an electrical current over a voltage difference to operate. All of these loads have one thing in common: resistance.

Ohmeter
Multimeters include ohmeters, to measure resistance in a section of circuit

Without resistance, connecting the poles of a battery would cause all the electrons to flow very quickly without hindrance from the negative electrode to the positive electrode, and the battery would lose its power and go flat. A resistance is just that - a resistance to the movement of the electrons.

It works like a hose with water. A very wide hose would allow all the water to race through at the pressure the tap is providing. To slow the water down, we could use a narrower hose - in other words, make it harder for the water to pass through. We could say the narrower tube creates a resistance to the flow. An electrical resistor does exactly the same in an electrical circuit.

Resistance regulates the flow of current in a circuit. As the resistance increases, the current decreases. Resistance has no effect on the total voltage in a circuit, because that is supplied by the power source.

Resistors
Resistors have coloured bands to indicate their value

Load

The load is a generic term for all of the components in an electrical circuit which require power to allow electrons to pass around the circuit.

If a wire with low resistance is allowed to touch both electrodes of a battery, the battery finds that there is little or no resistance to its electrons. The battery in this condition will 'short', causing the battery's charge to be lost in a very short time, and probably damaging the battery in the process.

To prevent this happening, a load is always used in a circuit to control the flow of electrons in a safe way. We say that the load offers a resistance to the flow of electrons. They have to be pushed through a resistor. This 'push' is known as the 'electromotive force (EMF)' of the battery.

The unit of resistance is the ohm, and is measured therefore by an ohmeter. The symbol for ohms is the Greek letter upper case omega (Ω).

Content © Andrew Bone. All rights reserved. Created : April 10, 2014

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