Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 1 of 34 PowerPoint presentation Outcome 3: Know the principles of electrical science Unit 107: Electrical science and technology

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 2 of 34 Electron flow theory (3.1) The structure of matter Question: What are these things made of? Answer: Molecules

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 3 of 34 Electron flow theory (3.1) What are molecules made of? Atoms A nucleus containing protons and neutrons Electrons move around the nucleus Imagine the sun and orbiting planets!

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 4 of 34 Electron flow theory (3.1) Atoms? Different materials have different numbers of electrons If in steady state, protons = neutrons Electrons orbit paths known as shells Atoms in solids and liquids are tightly bound compared to gases

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 5 of 34 Electron flow theory (3.1) Current (I) is the flow of electrons: Electron flow can be measured If there is no pressure, electrons are random Electrons are negatively charged Protons are positively charged Opposites attract! Electrons like positive things and will move Add a battery and what happens? ‑ Closed circuit will conduct ‑ Electrons drift to +ve plate ‑ Electrons repelled by –ve plate ‑ Electron flow = electric current

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 6 of 34 Electron flow theory (3.1) What else do we know about electrons and current? Electrons flow from negative to positive Electrons are very small and difficult to count Electron flow is different to conventional current flow! Current (I) is measured in amperes (A) Electron flow: -ve to +ve Conventional current flow (I): +ve to -ve

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 7 of 34 Electron flow theory (3.1) What else do we know about electrons and current? If there are ‘free’ electrons in the outer shell = good conductor No free electrons = good insulator How good a conductor is depends on materials resistance (R), which is measured in ohms (Ω) Battery voltage source also known as potential difference (PD) or emf and is measured in volts R R Voltage source (V) (PD or emf) Conductor

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 8 of 34 Simple units of electrical measurement (3.2) What makes up a simple circuit? A conductor A load (such as a light) A switch A battery source (supply)

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 9 of 34 Simple units of electrical measurement (3.2) What makes up a simple circuit? Conductivity depends on material Each material has different resistance Resistance (R) measured in ohms (Ω) Current (I) measured in amperes (A) Charge (Q) is measured in coulombs (C) PD/emf, battery source (V) is measured in volts (v) Power, or heating effect, (P) is measured in watts (W)

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 10 of 34 Effects of electric current (3.3) What happens when a current passes through a conductor? Heats up – ‘thermal effect’ Chemical reaction – ‘chemical effect’ Magnetic field – ‘magnetic effect’

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 11 of 34 Simple electrical calculation (3.4) Electrons and coulombs Because electrons are difficult to count we use charge (Q) and charge is measured in coulombs (C) 1 Coulomb is a large group of electrons 6,700,000,000,000,000,000 electrons to be exact! 6.7 x A charge of 1 coulomb a second = 1 ampere Charge = Current x Time Q = I x t

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 12 of 34 Simple electrical calculations (3.4) Electrons and coulombs Question: What is the charge (Q) if a current of 2A flows for 2 seconds? Answer: Charge = Current x Time Q = I x t Q = 2 x 2 Q = 4 coulombs (or 4C)

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 13 of 34 Simple electrical calculations (3.4) Electrons and coulombs The equation for charge, current and time can be put into a triangle so it is easy to use: Cover what you want Say what you see! Above each other = ÷ Side by side = X Charge (Q) Current (I) Time (S)

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 14 of 34 Simple electrical calculations (3.4) Ohm’s law Voltage, current and resistance depend on each other in a circuit: Current, I (A) ‑ Current flow is like water ‑ Current flows through easiest path Voltage, PD (v) ‑ Pressure/ potential difference ‑ Voltage dropped across resistor Resistance, R (Ω) ‑ Makes current flow hard work ‑ Creates heat V I R

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 15 of 34 Simple electrical calculations (3.4) Ohm’s law The equation for Ohm’s law can be put into a triangle so it is easy to use: Cover what you want Say what you see! Above each other = ÷ Side by side = X Voltage (v) Current (I) Resistance (R)

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 16 of 34 Simple electrical calculations (3.4) Example: Simple circuit If resistor is 10Ω and voltage is 20v what is the current flowing? Use Ohm’s law triangle Cover up the current (I) Say what you see V = 20v I = ? R = 10Ω Voltage (v) Current (I) Resistance (R)

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 17 of 34 Simple electrical calculations (3.4) Example: Simple circuit If resistor is 10Ω and voltage is 20v what is the current flowing? I = V ÷ R I = 20 ÷ 10 I = 2A V = 20v I = ? R = 10Ω Voltage (v) Current (I) Resistance (R)

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 18 of 34 Simple electrical calculations (3.4) Simple circuit: Power Power is the heating effect in a circuit As current passes through R it heats up! Power is measured in watts Power = Voltage x Current P = V x I P = 20 x 2 P = 40W V = 20v I = 2A R = 10Ω

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 19 of 34 Simple electrical calculations (3.4) Series circuits: If resistors are connected end to end Said to be a ‘series circuit’ Remember current is like water! Only one path for current Current flows through R 1 & R 2 Resistance total = R 1 + R 2 Current = Voltage ÷ Resistance V I R1R1 R2R2

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 20 of 34 Simple electrical calculations (3.4) Example: Series circuits What is the current flowing in this series circuit? First have to find the total resistance Add the two resistors Resistance total = R 1 + R 2 Resistance total = = 15Ω Current is voltage ÷ resistance I = V ÷ R I = 30 ÷ 15 I = 2A V = 30v I = ? R 1 =5ΩR 2 =10Ω

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 21 of 34 Simple electrical calculations (3.4) Parallel circuits Current has two paths now Current is like water Current will split up Voltage is dropped across both resistors, R 1 & R 2 Voltage across both resistors is same You can still use Ohm’s law on each part of the circuit! V I R1R1 R2R2 V 1 = V V 2 = V

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 22 of 34 Simple electrical calculations (3.4) Parallel circuits Resistors are now in parallel Overall resistance is less as there are two paths for current to flow Total resistance found using fractions: This can be simplified: V I R1R1 R2R2 1 = RTRT R1R1 R2R2 RTRT = R 1 x R 2 R 1 + R 2

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 23 of 34 Simple electrical calculations (3.4) Example: Parallel circuits Use the parallel resistor formula and put in the values: V I R 1 = 3Ω R 2 = 6Ω RTRT = R 1 x R 2 R 1 + R 2 RTRT = 3 x RTRT = 18 9 RTRT = 2Ω

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 24 of 34 Simple electrical calculations (3.4) Example: Parallel circuits What is the current drawn by the circuit? Use Ohm’s law again! I = V ÷ R The parallel resistance is 2Ω Put the values into Ohm’s law: I = 30 ÷ 2 I = 15A V = 30v I R 1 = 3Ω R 2 = 6Ω

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 25 of 34 Simple electrical calculations (3.4) Example: Parallel circuits If there are more than two resistors in parallel you can use fractions or the button Try this on your calculator: = = 2 V = 30v I R 1 = 6Ω R 2 = 6Ω x X -1 R 3 = 6Ω 1 = RTRT R1R1 R2R2 R3R3 x X -1

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 26 of 34 Simple electrical calculations (3.4) Electrical energy Energy is ability to do work Measured in Joules Energy = Power x Time For houses we measure energy in kilowatt hours Energy = kW x hours (kWh) 1 kWh = 1 unit

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 27 of 34 Simple electrical calculations (3.4) Example: Electrical energy A house has a 3kW heater and it is on for 2 hours. How much energy has been used? Energy = kW x hours (kWh) Energy = 3 kW x 2 hours = 6 kWh

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 28 of 34 AC and DC supplies (3.5) Direct current (DC) supplies Current flows in one direction only It can be +ve or –ve but not both! Battery DC generator Dynamo PV cells

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 29 of 34 AC and DC supplies (3.5) Alternating current (AC) supplies Current flows in one direction, changes and flows back the other way (ie, it ‘alternates’) AC generator Alternator Transformer

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 30 of 34 Connecting measuring instruments (3.6) Ammeter Current flows through Connected in series Break the circuit to connect up Take great care! Large currents measured using clamp meter

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 31 of 34 Connecting measuring instruments (3.6) Voltmeter Measures voltage drop across load Connected in parallel to load

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 32 of 34 Connecting measuring instruments (3.6) Ohmmeter Only used if not energised (must not be powered up!) Small internal current & voltage Connected in parallel

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 33 of 34 Connecting measuring instruments (3.6) Wattmeter Measure power Combination of voltage and current Connected in parallel and series

Level 1 Diploma in Electrical Installation © 2013 City and Guilds of London Institute. All rights reserved. 34 of 34 Any questions?