Electrical Circuit Theory

What ever the source of the power A.C / D.C. the electricity flows the circuit due to the movement of electrons. If the circuit is not complete e.g. an open switch or break in the conductor; the electrons cannot flow.

- + Electro Motive Force EMF Voltage Electrons have a negative charge. The current (flow of electrons) flows from the negative terminal towards the positive terminal.

In the simple circuit the electricity is stored chemically in the battery; but does not flow until the switch is closed. Note the flow of electrons is negative to positive. An electron is attracted to the positive pole of the battery; the electron will always remain negative.

In the next circuit we have a lamp added. Note the lamp only lights up when the circuit is completed.

Series Circuit Neutral Live

Series Circuit In the series circuit once the switch is closed the electrons / current flows and the lamps light up. If one of the bulbs fails the power to the whole circuit stops.

230 Volt power supply Neutral Live Parallel Circuit

In a parallel circuit each lamp is controlled independently by its own switch. Only the lamp connected to the switch will light up when the switch is closed. If one of the lamps fail the others will continue to work.

Ammeter An ammeter measures the amount of current that is passing through the circuit / component. Ammeters need to be connected in series to the circuit; and the circuit needs to be powered. A

Voltmeter A voltmeter measures the voltage potential across the circuit or load. Voltmeters need to be connected across the load in parallel. V

Ohmmeter An ohmmeter measures the amount of resistance in a circuit or component. Ohmmeters need to be connected in parallel and the circuit needs to be unpowered. Ohmmeters can also be used as a continuity tester; to check if there is a brake in the circuit.

Example 1. Using ohms law we can use the given variables to calculate the current in the circuit. V ÷ R(Ω) = I 12 ÷ 4 = 3A 12V 4Ω4Ω

Example 2. The first thing to do is to calculate the total circuit resistance. 6Ω + 3Ω = 9Ω Then calculate the current flow in the circuit. V ÷ R(Ω) = I 18 ÷ 9 = 2 A 18V 3Ω3Ω 6Ω6Ω

To calculate the voltage drop across the resistors we need to use Ohms law in a different configuration. I x R = V Resistor 1. Resistor 2. 2A x 6Ω = 12Volts2A x 3Ω = 6Volts As you can see the total voltage has not changed ( = 18) and the distribution of voltage is proportional to the resistance.

Example 3. The first thing to do is to calculate the total circuit resistance. 3Ω + 10Ω + 5Ω = 18Ω Then calculate the current flow in the circuit. V ÷ R(Ω) = I 24 ÷ 18 = A 3Ω3Ω 10Ω 5Ω5Ω 24V

To calculate the voltage drop across the resistors we need to use Ohms law in a different configuration. I x R = V Resistor A x 3Ω = Volts Resistor A x 10Ω = Volts Resistor A x 5Ω = Volts As you can see the total voltage has not changed ( = 24) and the distribution of voltage is proportional to the resistance.

Parallel Circuit In a parallel circuit the voltage remains the same but the current draw increases with each new load that is added. It is important to ensure that the supply wire from the power source is capable of handling the current if all of the loads are applied to the circuit.

Example 4. Calculate the current drawn through each resistor. R1. 230V ÷ 529.9Ω = 0.434A R2. 230V ÷ 884.6Ω = 0.260A 230V 884.6Ω 529.9Ω

Example 4. Now we know the voltage, resistance and current in the circuit we can now look at the power in the circuit. Power is measured in watts; we use the following triangle to help remember the ratio between the voltage & amps W VI

Example 4. R1. 230V x 0.434A = Watts R2. 230V x 0.260A = 59.8 Watts If we round up R1. will become 100Watts and R2. will become 60Watts. Can you guess what R1. and R2. are?

Exercise 1. If the load in the above circuit is drawing 1.5 Amps what is the voltage? 10Ω V ?

Exercise 2. Calculate;- –The total circuit resistance. –The voltage drop across each load. –The current in each load. 24V 6Ω6Ω 8Ω8Ω

Exercise 3. Calculate;- –The total circuit resistance. –The current in each load. –The voltage drop across each load. –The wattage of each load. 230V 884.6Ω Ω

Exercise 4. Calculate;- –The total circuit resistance. –The current in each load. –The voltage drop across each load. 10Ω 20Ω 110V

Exercise 5. Calculate;- –The current in each load. 12V 15Ω 20Ω

Exercise 6. Calculate;- –The current in each load. –The voltage drop across each load. –The wattage of each load. –The current at point if all of the loads are powered. 230V 150Ω 200Ω 150Ω a a

When plumbing there are occasions when we need to work with electricity. On most occasions plumbers will not be allowed to work on any electrical work due to part P building regulations, these classify areas where a plumber would work (kitchen, bathroom, garden, etc.) as special areas and only a competent and trained person can undertake any electrical work in these areas