Topic 3 Basic Electricity

Grounding

In many circuits, a zero reference point is shown connected to earth, or grounded.

In this circuit, the point Y has been connected to ground, and is at 0 V.  Therefore point X could be considered to be at +9 V relative to Y and Z is –9 V relative to Y.

  In many devices the 0 V point is connected to the metal frame or chassis.

Basic Electricity

There are four basic electrical quantities that we need to know:

• Current

• Potential Difference (Voltage)

• Power

• Resistance

Current

Current is a flow of charge.   Each electron carries a charge of 1.6 × 10^-19 coulombs.  This is far too small to be any use, so we consider electricity to flow in packets called coulombs.  When there is a flow of 1 coulomb per second, a current of 1 amp is flowing.   Current is measured in ampères, or amps (A). 

There are some important multipliers for current:

• 1 mA = 1 × 10^-6 A
• 1 mA = 1 × 10^-3 A

  These are useful when we are dealing with small currents, especially in electronic circuits.  However we must remember to convert to amps for doing calculations.

Potential Difference

Potential difference is often referred to as voltage.  There are several ways of defining voltage;  the correct physics definition is energy per unit charge, in other words, how big a job of work each lump of charge can do.

  In electronic circuits you will come across voltages measured in:

• microvolts: 1 mV = 1 × 10^-6 V
• millivolts : 1 mV = 1 × 10^-3 V.

Power in a Circuit

Power in a circuit can be worked out using the simple relationship:

            Power (W) = Voltage (V) × Current (A)

  In physics code, this is written:

P = IV      The physics code for current is I which stands for "intensité", the French word for intensity.       

  Power is measured in watts (W).  Remember:

• 1 kW = 1000W

• 1MW = 1 × 10⁶ W

In electronic circuits the power may be low, say ½ watt.  However if the resistors are rated at ¼ watt, they will start to get hot.  Now try Question 1.

Resistance

Resistance (W) =Potential difference(V)  

                                     Current (A)

  In Physics code:

R = V/I         

The unit for resistance is ohm (W).  (The curious symbol ‘W’ is Omega, a Greek capital letter long Ō.)  You will also come across kilohms (kW) and megohms (MW).  Be sure you know what these are. Now try Question 2

The Heating Effect of a Current

                        P = I²R or P = V²/R

  Power is in Watts (W) or kilowatts (kW) or milliwatts (mW)

Now try Question 3

Series and Parallel Resistors

Series Circuits

The resistances of the components add up to a total resistance.

                        R_tot = R₁ + R₂ + R₃ + … + R_x

•  All the voltages add up to the voltage given out by the battery.

• The current is the same all the way round.

R_tot = R₁ + R₂ + R₃  

Now try Question 4

Parallel Resistors

  The current will split into three.

• The voltage across each branch is the same

• The currents in each branch add up to the total current.

     1   =    1  +   1  +   1

    R_tot      R₁     R₂     R₃

Now try Question 5

The symbol of a variable resistor is shown below:

The E24 Series of Resistors

E24 series

  This set of values is marketed in decades, or powers of 10:

So we can have values such as:

            47 W, 1500 W, 120 W, 68000 W, 5600000 W, etc.

Identification of Resistors with the Colour Code and BS 1852 Code

The values of the bands are shown in the table below:

• The first and second band give the number of significant figures.  (In five band coding, it is the first second and third bands.)

• The third band gives the multiplier

• The fourth band gives the tolerance.

For example a resistor marked red orange yellow gold would give:

            23                                 Multiplier 10⁴ (= 10000),                           tolerance ± 5%

            23 × 10000 = 23 × 10⁴ W = 230 kW

  A tolerance of ± 5 % means that the resistor would have a value between 220 kW and 240 kW.

Now try Question 6.

Some resistors have their values marked not in colours but in numbers and letters.  They use the BS1852 resistance code.

Instead of the W the resistors are marked with the letters R, K or M.  The R, K or M denotes the decimal point:

·        R47 = 0.47 W

·        4R7 = 4.7 W

·        47R = 47 W

·        470 R = 470 W

·        1K = 1000 W

·        1K5 = 1500 W

·        10K= 10000 W

·        4M7 = 4.7 × 10⁶ W

  There are other suffix letters that denote the tolerance:

Be careful not to confuse the letter K with the K for kilohms. 

Now try Question 7

68KK = 68 kW ± 10%

Now try the Quiz

Useful Websites

http://www.doctronics.co.uk/circuits.htm http://www.doctronics.co.uk/resistor.htm

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/passive/resistor/resistor.htm

Presentations

Basic Electricity