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Voltage and Current
Voltage is the Cause, Current is the EffectVoltage attempts to make a current flow, and current will flow if the circuit is complete. Voltage is sometimes described as the 'push' or 'force' of the electricity, it isn't really a force but this may help you to imagine what is happening. It is possible to have voltage without current, but current cannot flow without voltage.
Voltage at a point and 0V (zero volts)Voltage is a difference between two points, but in electronics we often refer to voltage at a point meaning the voltage difference between that point and a reference point of 0V (zero volts).
Zero volts could be any point in the circuit, but to be consistent it is normally the negative terminal of the battery or power supply. You will often see circuit diagrams labelled with 0V as a reminder.
You may find it helpful to think of voltage like height in geography. The reference point
of zero height is the mean (average) sea level and all heights are measured from that point.
The zero volts in an electronic circuit is like the mean sea level in geography.
Zero volts for circuits with a dual supplySome circuits require a dual supply with three supply connections as shown in the diagram. For these circuits the zero volts reference point is the middle terminal between the two parts of the supply.
On complex circuit diagrams using a dual supply the earth symbol is often used to indicate a connection to 0V, this helps to reduce the number of wires drawn on the diagram.
The diagram shows a ±9V dual supply, the positive
terminal is +9V, the negative terminal is -9V and the middle terminal is 0V.
1mA = 0.001A, or 1000mA = 1A
The need to break the circuit to connect in series means that ammeters are difficult
to use on soldered circuits. Most testing in electronics is done with voltmeters which can
be easily connected without disturbing circuits.
Voltage and Current for components in SeriesVoltages add up for components connected in series.
Currents are the same through all components connected in series.
In this circuit the 4V across the resistor and the 2V across the LED add up to the battery voltage: 2V + 4V = 6V.
The current through all parts (battery, resistor and LED) is 20mA.
Voltage and Current for components in ParallelVoltages are the same across all components connected in parallel.
Currents add up for components connected in parallel.
In this circuit the battery, resistor and lamp all have 6V across them.
The 30mA current through the resistor and the 60mA current through the lamp add up
to the 90mA current through the battery.
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