__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*

*Basic Electricity*

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

- Current
- Potential Difference (Voltage)
- Power
- Resistance

__Current__

__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.

*P**otential 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__

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 × 10
^{6}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__

is the opposition to the flow of an electric current.

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__

__The Heating Effect of a Current__

* P = I ^{2}R* or

*P = V*

^{2}/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} = R_{1} + R_{2} + R_{3} + … + 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} = R_{1} + R_{2} + R_{3} *

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 } R_{1} R_{2} R_{3}*

Now try Question 5

The symbol of a **variable resistor** is shown below:

__The E24 Series of Resistors__

E24 series

10 | 11 | 12 | 13 | 15 | 16 | 18 | 20 | 22 | 24 | 27 | 30 |

33 | 36 | 39 | 43 | 47 | 51 | 56 | 62 | 68 | 75 | 82 | 91 |

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

Power of 10 |
0 | 1 | 2 | 3 | 4 | 5 | 6 |

× | 1 | 10 | 100 | 1000 | 10000 | 100000 | 1000000 |

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:

Colour | Black | Brown | Red | Orange | Yellow | Green | Blue | Violet | Grey | White | Gold | Silver |

Value | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||

Multiplier | 10^{0} |
10^{1} |
10^{2} |
10^{3} |
10^{4} |
10^{5} |
10^{6} |
10^{7} |
10^{8} |
10^{9} |
10^{-1} |
10^{-2} |

Tolerance
(%) |
± 1 | ± 2 | ± 0.50 | ± 0.25 | ± 0.10 | ± 0.05 | ± 5 | ± 10 |

- 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 10^{4} (= 10000), tolerance ± 5%

23 × 10000 = 23 × 10^{4} 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 × 10
^{6}W

There are other suffix letters that denote the tolerance: