1 Chapter 11- Electricity
2 Course Content Definition of Electricity Circuit Diagrams Series and Parallel Circuits Calculating total resistances Measurement of Electricity Ammeters and Voltmeters Ohm s Law Safety Devices and Hazards Wiring the Plug
3 What is Electricity An atom is a fundamental unit of matter made up of protons (with a positive charge) neutrons (neutral no charge) electrons (with a negative charge)
4 What is electricity? Everything is made of atoms which contain POSITIVE particles called PROTONS and NEGATIVE particles called ELECTRONS. Electron (-) Proton (+) Neutron
5 What is Electricity? Electricity the flow of electrons through a circuit i.e. the flow of electrical charges The path along which the electric current moves is called the ELECTRIC CIRCUIT The device which causes the flow of electrons through a circuit is called a CELL or POWER SOURCE (Battery)
6 What is Electricity? Electrons move round a circuit from negative terminal to the positive terminal of the cell, producing an electric current. (note: like charges repel, opposites attract) The rate of flow of electric charges is called ELECTRIC CURRENT electric cell connecting wire filament flow of electrons
7 Electric Current Conventional Current flow electric current flows from positive terminal to negative terminal Alternating Current (AC) electric current flows in a clockwise direction and then changes direction to flow anti-clockwise Direct Current electric current flows only in one direction
8 Electric Current Electric current is given the symbol, I Can also be defined as - the amount of charge flowing per second Electric Current is measured in Amps (A) Thus, Current in Amps (A) = Current in Amps (A) I = Q t Charge transferred in coulombs (C) time in seconds (s)
9 Electric Current This can be rearranged as Q = It or t = Q I
10 Electric Current The SI Unit for Electric Current is Ampere (A) Electric Current is measured using an Ammeter
11 Ammeter It must be connected in series in the circuit. Positive side of ammeter must be connected nearest to the positive terminal of the battery (electric cell), and vice versa.
12 Electric Circuit Electric circuit :- is a continuous and closed path of an electric current. A schematic diagram of an electric circuit comprising of a cell, electric bulb, ammeter and plug key. - cell + bulb A + - ammeter plug key
13 Electric Circuits Electric circuits are made up of electrical components connecting wires electric cell These components must be joined together without any gap in between to form a closed circuit. light bulb circuit board Note: Components refer to the light bulb, wires, battery
14 Electric Circuits Incomplete circuits are called open circuits. connecting wire is missing no source of electrical energy Both the circuits in the diagram are incomplete, hence they are known as open circuits.
15 An electric current flows only when there is: a source of electrical energy and a closed circuit connecting wires electric cell light bulb circuit board
16 How to draw Circuit Diagrams Component Symbol Component Symbol + + An electric cell Battery Switch (open) Light bulb (lamp) Switch Switch (closed) Connecting wires (not joined) Connecting wires (joined) Symbols are used to represent the various electrical components in circuits.
19 Summary: Electrical Charge Electric charge is given the symbol Q Electrons are the charge carriers that flow in an electrical circuit from the negative to positive terminals.
20 Summary: Electrical Charge Charge is measured in Coulombs which is given the symbol C
21 Summary: Electrical Charge The charge on a proton is 1.6 x C which is the same size as the charge on an electron.
22 Summary: What is electricity? So electricity is movement of electric charge round a circuit. We call this electric current.
23 Summary: Charge, Current & Time Electric current is given the symbol Electric current is the movement of I negative charges (electrons) in a circuit
24 Summary: Charge, Current & Time Electric Current is the amount of charge flowing per second and is given the unit Amps (A)
25 Summary: Charge, Current & Time If so a current is of charge 1 A is flowing 1 C of charge per second transferred then in 1 s. I = Q t Charge transferred in coulombs (C) Current in Amps (A) time in seconds (s)
26 Summary: Circuit Diagrams Component Symbol Component Symbol + + An electric cell Battery Switch (open) Light bulb (lamp) Switch Switch (closed) Connecting wires (not joined) Connecting wires (joined) Symbols are used to represent the various electrical components in circuits.
27 Summary: Circuit Diagrams
28 Different types of circuit There are different ways in which you can connect cells and electrical components (such as lamps) to create a circuit: series parallel a mixture of both
29 Series Circuit A series circuit connects the components one after the other A single loop is formed A break in any part of a series circuit stops the flow of current in the whole circuit.
30 Parallel Circuit A parallel circuit divides into two or more branches. The current divides and flows through each parallel branch. If a component breaks or is removed, the other components remain on.
31 Voltage An electric cell gives energy to the electrons and pushes them round a circuit. Voltage is a measure of how much energy the electrons receive. Different voltages are supplied by different cells and batteries. 1.5 V Dry Cell 12 V Car Battery 9 V Dry Cell
32 How to Measure Voltage? The SI unit for voltage is volt (V). A voltmeter is an instrument used for measuring voltages.
33 Voltmeter Voltmeters must be connected in parallel to the circuit. The positive side of voltmeter is connected to the positive terminal of the cell, and vice versa.
34 Recall: Ammeter!!! It must be connected in series in the circuit. Positive side of ammeter must be connected nearest to the positive terminal of the battery (electric cell), and vice versa.
35 measuring current Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A
36 measuring current This is how we draw an ammeter in a circuit. A A SERIES CIRCUIT PARALLEL CIRCUIT
37 measuring voltage The electrical push which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter V
38 measuring voltage This is how we draw a voltmeter in a circuit. V SERIES CIRCUIT V PARALLEL CIRCUIT
39 measuring current SERIES CIRCUIT current is the same 2A 2A at all points in the circuit. 2A PARALLEL CIRCUIT current is shared between the components 2A 1A 1A 2A
40 fill in the missing ammeter readings. 4A? 3A? 3A 4A? 1A? 4A 1A 1A?
41 measuring voltage V V V V
42 series circuit voltage is shared between the components 3V 1.5V 1.5V
43 parallel circuit voltage is the same in all parts of the circuit. 3V 3V 3V
44 measuring current & voltage complete the missing current and voltage readings. a) 4A 6V A V V A
45 measuring current & voltage b) 4A 6V V A A V A
46 answers a) b) 6V 4A 6V 4A 4A 4A 6V 3V 3V 2A 4A 6V 2A
47 The circuit is no longer complete, therefore current can not flow The voltage decreases because the resistance increases. The current flow in each bulb is the same.
49 Resistance When an electric current flows through a circuit, there will be some resistance that opposes it. (similar to friction) It can be measured by dividing voltage by the current. R = V I R = Resistance V = Voltage I = Current
50 Ohm s Law Ohm s law states that : The current flowing through a conductor is directly proportional to the potential difference between its ends provided its temperature remains constant. I = V / R I V R = Current (Amperes) (amps) = Voltage (Volts) = Resistance (ohms)
51 Resistance Good conductors of electricity have LOW RESISTANCE. (Eg. Metal objects) Electricity is able to flow through them very easily Poor conductors of electricity have HIGH RESISTANCE. (Eg. Wood, cloth) Electricity is not able to flow through them easily The SI unit for resistance is ohm (Ω)
52 Example 1 An electric rice cooker operates at 240 V and uses a current of 8 A. What is the resistance of the rice cooker? Voltage (V) = 240 V Current (I) = 8 A R = R = V I = 30 Ω
53 Resistors in Series When resistors are connected in series, the resistance will add up and increase. R = R 1 + R 2 + R 3 2 Ω 3 Ω The total resistance for this circuit is: 2 Ω + 3 Ω = 5 Ω
54 Resistors in Parallel When resistors are connected in parallel, the resistance will decrease This is because they will provide alternate routes for the current to flow.
55 Resistors in Parallel 1 R = R + R + R What is the resistance of the circuit? 1 R = R = = R = 3Ω 3 6 Ω 6 Ω The final resistance is smaller
56 Summary: Series and Parallel Circuits Voltage, Current and Resistance V s - + I 1 I 3 V 1 V 2 V 3 R 1 R 2 R 3 What type of circuit is this? I 2
57 - V s + I 1 I 3 V 1 V 2 V 3 R 1 R 2 R 3 I 2 One electrical path from negative to positive therefore series.
58 - V s + I 1 I 3 V 1 V 2 V 3 R 1 R 2 R 3 I 2 What is the relationship between the three currents? The current is the same at each point. I = = I I 1 2 3
59 - V s + I 1 I 3 V 1 V 2 V 3 R 1 R 2 R 3 I 2 What is the relationship between the four voltages? They add to equal the supply voltage. V = V + s + V V 1 2 3
60 - V s + I 1 I 3 V 1 V 2 V 3 R 1 R 2 R 3 How do you find total resistance in series? Add each resistance together. I 2 R = R + R + R total 1 2 3
61 Disadvantages of Series Circuits? When one component fails the whole circuit fails. The current is the same at all points and the voltage is divided between the bulbs. The more bulbs added the dimmer each one is.
62 - V s + I T V 1 I T R 1 I 1 V 2 R 2 I 2 V 3 R 3 I 3 What type of circuit is this?
63 - V s + I T V 1 I T R 1 I 1 V 2 R 2 I 2 V 3 R 3 I 3 More than one electrical path components connected on different branches therefore parallel.
64 I T - V s V 1 R 1 + I 1 I T What is the relationship between the four currents? V 2 R 2 I 2 The four currents add to give the total current. V 3 R 3 I 3 I = I + T + I I 1 2 3
65 I T - V s V 1 R 1 + I 1 I T What is the relationship between the four voltages? V 2 R 2 I 2 Each voltage is equal to the supply voltage. V 3 R 3 I 3 V = V = S = V V 1 2 3
66 - V s + I T V 1 R 1 I 1 I T The V 2 R 2 I 2 resistance in parallel? V 3 R 3 I = + + R R R T R 3
67 If more resistors are connected in parallel the total resistance will always decrease This is because there are more branches through which the electricity can flow.
68 Advantages of the Parallel Circuit? When one bulb fails the rest of the circuit continues to work. The more components, the lower the resistance. The total current drawn increases. Voltage in each branch is the same as the supply voltage therefore bulbs in parallel will each be as bright as a single bulb.
69 NOTE!! Hazards of electricity + Safety Devices + Wiring of an Electric Plug = Self Study