All
matter consists of atoms which consist of protons and neutrons making
up the nucleus and electrons in orbit around the nucleus. The
electrons can break loose from some atoms and are free to move.
Electronic circuits depend on these free electrons moving around a circuit. The way they move depends on the type of material.
In CONDUCTORS electrons move easily - for example metals and carbon
In INSULATORS electrons cannot move easily or at all - for example mica, glass, polythene
In SEMICONDUCTORS the movement of electrons depends on the voltage applied - for example germanium and silicon
3b.1 Understand the relationship between power, potential difference and current.
Three important terms are POWER, POTENTIAL DIFFERENCE AND CURRENT.
Current (I) is a measure of the number of electrons flowing in a circuit at a particular point and is measured in AMPS
Potential
Difference is the difference between the number of electrons at two
points in a circuit and is measure in VOLTS (V). Sometimes given the
symbol E.
Power (P) is the rate of doing work, measured in WATTS. For the
purposes of radio two examples of power would be the work done in
producing radio waves or the power of audio from a speaker
Manipulate the equation P=V×I to find the unknown quantity given the other two. The prefixes milli and kilo may be used.
These three terms are related by the equations:
So, if we know 2 of these we can work out the third. Try the problems opposite
In this circuit the electrical
energy in the power source will be changed to heat energy in the
resistor. So, we can detect the power in the resistor by the fact that
it has been turned to heat.
Example 1
Example 2
Units
- in the above equation if V is volts and I is in amps then power will
be in Watts. In high power transmitters we often use larger units.
For example:
Instead of volts we use kilo-volts. A kilo-volt is one thousand volts. or 1000V = 1kV
Instead of amps we use kilo-amps. A kilo-amp is one thousand amps or 1000A = 1kA
Instead of using watts we use kilo-watts. A kilo-watt is one thousand watts or 1000W = 1kW
So we can use the equation with kilo-volts, kilo-amps and kilo-watts
In low power equipment we can work in units 1000 times less than our standard watt, amp and volt.
Instead of volts we use millivolts. A millivolt is one thousandth of a volt. or 1000mV = 1V
Instead of amps we use milliamps. A milliamp is one thousandth of an amp or 10000mA = 1A
Instead of using watts we use milliwatts. A milliwatt is one thousandth of a watt or 1000mW = 1W
So we can use the equation with mega-watts, mega-amps and mega-volts
Be careful not to mix your units. You can't use amps, milli watts and
mega-volts in the equation. You would have to change them all to the base unit (i.e. Volts, Amps and Watts)
Example 3
Calculate the effective resistance of two or three equal resistors in parallel. The formula for calculating the combined value of 3 resistors in parallel is:
For 2 resistors we would not have an R3
In the image opposite:
So, 1/RT = 1/5+1/10+1/20
So, 1/RT =0.2+0.1+0.05 = 0.35
So RT = 1/0.35 = 2.86 Ohms (Ω)
Calculate the combined resistance of two or three resistors in series. Understand circuits comprising series and parallel connections of resistors and cells. Calculate currents and potential differences in such circuits.
The formula for calculating the total resistance of resistors in series is:
The total resistance in the series circuit shown opposite = 5+10+20=35ohms
Calculate currents and potential differences in such circuits.
The voltage from the two cells = 2.4 volts. So, the total current drawn can be calculated from:
2.4=Ix35
I=2.4/35
I=0.0686 A or 68.6mA
The voltage drop across each resistor can then be calculated.
Voltage across the 5 ohm resistor
V=IR
V=0.0686x 5
V=0.343 across the 5 ohm resistor
Voltage across the 10ohm resistor
V=0.0686x10
V=0.686
Voltage across the 20ohm resistor
V=0.0686x20
V= 1.372
As a check, the sum of the voltage across each component should equal that from the cells.
Check sum 0.343+0.686+1.372 = 2.401V which t0 1 decimal place = 2.4V
NB in a series circuit such as this, the same current flows through each component - in this case 0.0686 Amps.
Primary and Secondary Cells
3c.1 Understand that cells store energy in chemical form. Recall that a
primary cell, once discharged, must be properly disposed of. Understand
that a secondary cell is rechargeable as the chemical discharge process
within it is reversible.
There are two basic type of cell used by radio amateurs.
Primary Cells can only be used
once. They are not rechargeable. Once they stop delivering current they
need to be disposed of safely. Many shops have a box where you can
dispose of used cells (old batteries). Remember that a battery is made
up of several cells in series. So, a 9v battery will have about 8x1.2
cells in it. 8 x 1.2 = 9.6v.
Secondary cells are re-chargeable. For example a car battery, Ni-CAD batteries and Ni-MH.
Both primary and secondary
cells store energy as chemical energy. When they are connected to a
circuit the chemical energy is converted to electrical energy. In the
case of secondary cells the process can be reversed i.e. electrical energy
can be converted back to chemical energy. This cannot be done with primary cells.
