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Basic Electronics Electricity is the flow of electrons through a conductive material. This is mostly metals. Electrons have a negative charge and will normally sit around the positive protons of their own atom, but if a neighbouring atom has a positive charge (due to a missing electron) then the electron will ‘jump' across. When the electron jumps it leaves behind a gap (a positive charge in that atom), this leaves room for another electron to fill that gap and so on. This gives the effect of the gaps (positive charge) moving in one direction, and the electrons moving in another.
Conventional current flows to the right, while the electrons flow to the left. This kind of current is called direct current or DC, which means the current only goes in one direction. There is another form of current called alternating current, AC, which will be covered in a later tutorial.
S.I Units Electronics has many different units, there are 4 primary units: Q - Charge, measured in coulomb (C) J - Joules, (1J is the amount of energy needed to raise one gram of water by 1 degree Celsius) t - time, measured in seconds
From these we can derive the following units: V - Voltage, also known as potential difference, measured in volts, which is equal to Jules per coulomb (J/c) I - Current, measured in amps (A), which is equal to coulomb per second (C/s) R - Resistance, measured in ohms () P - Power, measured in watts (W), which is equal to Jules per second (J/s)
In addition to these all the metric prefixes apply, for example 1kV = 1,000V and 1mA = 0.001A.
Formulas There are 2 basic equations used in electronics V=IR P=IV These 2 equations can be transformed to work out various things that we need solutions to. For Example:
P=v^2/R P=I^2R R=P/I etc.
For resistors in parallel we have:
Rt = 1/R 1 + 1/R 2
Circuits There are many different electrical components, all with their own different functions. So that electronics can be recorded onto paper or computer we have to have symbols to represent all the components. Some basic components and their respective symbols are:
Resistor -Dissipates electrical energy as heat.
Power supply -Supplies a current and a volta ge to the circuit, for example a battery.
Light bulb -Converts electrical energy to light and heat.
Ammeter - An ammeter measures current in wire.
Voltmeter - Measures the potential difference in voltage across two points.
All the components are connected via wires, which is just a line.
Here is an example of a basic circuit:
Rt = R1 The electrical resistance of this circuit is equal to the resistance of the single component in it, in this case the resistor R.
If two resistors are placed in a circuit like so:
Rt = R1 + R2 Then the resistors are said to be in series, and the total resistance of the circuit is equal to the sum of the resistors.
There is another way to place the resistors in this circuit, like so:
Rt = 1/R1 + 1/R2 These resistors are said to be in parallel, now the total resistance of the circuit is summarised by the above equation.
Example 1
Find the total resistance of the following circuit:
First simplify the 10 and 20 resistors by considering them as one 30 resistor: Now apply the equation: 1/Rt = 1/R1 + 1/R2 1/Rt = 1/20 + 1/30 :: substitute in values 1/Rt = 5/60 :: add the two fractions Rt = 12 :: flip both sides of the equations over
This circuit has a total resistance of 12 Ohms.
Ammeters, Voltmeters & V=IR Ammeters measure current, voltmeters measure volta ge. There is also such a device that can measure both (not simultaneously) called a multimeter. To work out the volta ge on a component the voltmeter must be parallel to the component, while if you want to measure the current through it you must place it in series.
The circuit shows a 100 Ohm resistor with 9 Volts potential across it. These nine volts are shown on the voltmeter. The ammeter shows that the resistor has 90mA running through it.
The relationship between these variable is summarised with the equation: V=IR
Now if you are given any two of these quantities you can work out the third.
Example 2 Given the values for the current and voltage, what should the resistance of the resistor in this circuit be?
9=0.036R Remember to convert mA to A R=9/0.036 Divide both sides by .036 R=250 Ohms
Example 3
What does the Ammeter read?
First find the total resistance of the circuit (ammeter has negligible resistance) Rt = 250 + 100 = 350 Ohms
V=IR 9=350I sub in values I=9/350 I=0.0257A=25.7mA
Challenge Question
Find the current running through the ammeter and 250 Ohm resistor. Answer: 36mA (highlight this line)
Power Power is the amount of energy dissipated in a component per second it is measured in Watts (with units J/s). If a component uses 100J in 5 seconds it is running at 20W. P=IV, P=V 2 /R also P=I 2 R In a resistor this energy is dissipated as heat, if to much energy is put through a resistor then it will be fried, just like a light bulb burning out. Example 4 A resistor with potential difference 2V across it and a current of 1 amp will have what power dissipated in it? P=IV P=1x2 P=2Watts
Example 5 What is the power dissipated in the 100 Ohm resistor? Long Way First find the total current flowing from the 12V power supply. 1/Rt = 1/100 + 1/200 1/Rt = 3/200 Rt = 66.7 Ohms is the total resistance of the circuit. V = IR 12 = I*66.7 I = 12/66.7 I = 0.18 A ps We now know the total current is 0.18 Amps, we also know that there is .06 Amps flowing through the ammeter and the 200 Ohm resistor, this leaves 0.12 Am ps to flow through the 100 Ohm resistor. P = IV P = 0.12*12 P = 1.44 Watts Short Way Using one of the above equations: P = V 2 /R P = 12 2 /100 P = 1.44 Watts Now that one was much better wasn't it! But there are some questions that will be designed to make this approach impossible, so it is useful to understand both methods.
That's it for Basic Electronics! Thanks for reading. Jayden Newstead
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