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If you've ever watched a thunderstorm at night, it can be quite dramatic to view lightning streaks across the sky, turning night into day temporarily. A bolt of lightning is a flow of electric charge from clouds to the ground. Recall that the flow of charge is defined as an electric current. We know that lightning bolts carry tremendous amounts of energy and so watching these thunderstorms was our first clue as to how energy can be transferred by electric currents.
The electric currents we use in everyday life come from power stations where the electricity is generated to substations and finally into our homes. All of our appliances have electric circuits that control how they operate and use electrical energy. Each appliance has a circuit diagram which is a picture representing the connections between components that are required in the electric circuit. It gives engineers and electricians an idea of how to build or repair a certain device or appliance. Each electrical component is given a symbol to differentiate it from other components. These are known as circuit symbols. The figure below is an example of a simple electric circuit with symbols for different components shown.
We know that a circuit can only contain an electric current if it contains some source of energy. This is provided by a power supply which is usually a cell or battery. A cell converts chemical potential energy into electric potential energy allowing a current to be passed through the circuit. The circuit symbol for a cell consists of two parallel lines, one longer than the other. The longer line indicates the positive terminal of the cell and the shorter line represents the negative terminal. The cell circuit symbol is shown in the following figure.
Note that the positive terminal is generally marked by a + symbol, but if it isn't, it is assumed that the person reading the diagram knows that the longer line represents the positive terminal.
A battery consists of two or more cells that are connected in series in a circuit. The combination of cells can provide greater electrical energy than a single one of the cells. The symbol for a battery is simply the symbol for a cell repeated, as shown in the figure below.
A switch is used in an electric circuit to break the wires in a circuit or join them. An open switch does not allow current to flow and when the switch is closed a current can flow again. The symbols for an open switch and closed switch are shown in the figure below.
A lamp, also called a bulb, is a device that emits light when an electric current passes through it. It can be used as a simple test to tell if a current is passing through part of a circuit, or for illumination. The circuit symbol for a lamp is a cross within a circle as provided in the figure below.
A resistor is a device that resists the flow of current. A fixed resistor has a value of resistance (measured in ohms) that does not change and will provide constant resistance to the flow of current. The symbol for a fixed resistor is a rectangle as shown in the following figure.
An ammeter is an instrument used to measure the current in an electric circuit. It is always connected in series with the device through which the current is to be measured. The circuit symbol for an ammeter is represented by the letter A inside a circle, as in the following figure.
A voltmeter is an instrument used to measure the potential difference between two points in an electric circuit. As such, it is always connected in parallel between the two points of interest. The circuit symbol for a voltmeter is the letter V contained within a circle as shown in the figure below.
A variable resistor is a type of resistor whose resistance can be varied. It can therefore be used to vary the current in a circuit. The symbol for a variable resistor is similar to that for a resistor but an arrow passes through the rectangle as is shown in the following figure.
A fuse is a device that will melt when the current passes a certain value. The circuit will then become open and no current can flow through it. Different fuses have different current ratings. They prevent the current from surpassing a value (their rating) that would damage other circuit components. The symbol for a fuse is similar to that for a resistor with a horizontal line passing through the rectangle. The circuit symbol is shown in the figure below.
A diode is a component that only allows current to flow in one direction in a circuit. If the current flows in the opposite direction, it cannot flow past the diode. The diode circuit symbol is an arrow that points in the direction in which current is allowed to flow. This is shown in the figure below.
A light-emitting diode (LED) is a type of diode that allows current to only pass through it in one direction and when a current passes through it, it will emit light. Many LEDs can be used for illumination whereas a single LED can be used to indicate a current passing through an electrical component. The symbol is similar to the diode symbol but with outward-pointing arrows to indicate that light is emitted, as seen in the next figure.
A light-dependent resistor (LDR) is a variable resistor whose resistance varies with the intensity of light that is incident on it. LDRs have a higher resistance in low-intensity light conditions (dim light) and have lower resistance in high-intensity light (bright light). Its symbol is similar to the resistor, but the rectangle is surrounded by a circle and inward-pointing arrows indicate the incoming light, as in the below figure.
A thermistor is another type of variable resistor whose resistance changes with temperature. The most common type of thermistor is the negative temperature coefficient thermistor, whose resistance decreases when the temperature increases and its resistance increases when the temperature decreases. It has a similar symbol to a resistor but with a line through the rectangle as shown in the following figure.
The following example will test your knowledge of the circuit symbols that were discussed in this article.
Label all of the circuit components (A, B, C, D and E) for the electric circuit that is shown in the figure below.
The correct labels are as follows:
They are the symbols given to electrical components in order to differentiate them from each other in a circuit diagram.
The most common circuit symbols are those used for the cell, battery, switch, resistor, ammeter, voltmeter, lamp, fuse, diode and LED.
The 5 parts of a circuit are the power source, conductor, switch, load and any other component (capacitor, transistor, etc.).
We use circuit symbols to differentiate circuit components from each other in a circuit diagram.
The name we give to the symbols used to represent circuit components is 'circuit symbols'.
What are the symbols in an electric circuit diagram known as?
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