Electricity and Circuits: Grade 9 Physics

Electricity and Circuits  Electric Charges Three particles that make up an atom: – Protons – Neutrons IT IVE POS – Electrons AL NE U T R NE G AT...

Electricity and Circuits  Electric Charges Three particles that make up an atom: – Protons – Neutrons IT IVE POS – Electrons AL NE U T R NE G AT IVE  The Flow of Charges Which subatomic particle can move from one object to another? – Electrons! What does the addition of electrons do to the charge of an object? – The object will become NEGATIVELY charged. What does the removal of electrons do to the charge of an object? e- e- e- e- e- – The object will become POSITIVELY charged. How Charges Interact When two objects with the SAME charge come into contact, they _____________. When two objects with DIFFERENT charges come into contact, they _____________.  Electric Current What is an electric current? – The continuous flow of electric charges through a material. What is the unit of measurement for current? – amperes (A) or amps: the amount of charge flowing past a point in a certain amount of time. e-  CALCULATIONS OF ELECTRIC CURRENT  Potential Difference What’s potential energy (PE)? On a roller coaster, where does the car have more potential energy? It’s the same for electric Can the car move from an area of low PE to high PE without the help of a motor? current! – The charges in a current are like the roller coaster cars. – The track is like the wires. – The motor that gets the cars to the top of the first hill is like the battery.  Potential Difference Cont. So what is Potential Hig Difference? hP – The difference in potential energy En otent erg ial between two locations. y The difference between a rollercoaster and an electric current is that the potential difference is caused by the difference in electrical charges in two locations, not height. Lo wP ote nti al E ner gy  Potential Difference Cont. Electric charges will always flow from a region of ___________ potential energy to a region of ___________ potential energy. When the charge reaches the positive terminal and enters the battery, it gains electric potential energy as it travels through the battery to the negative terminal. e-  Voltage What is it? – Another term for potential difference. What unit is used to measure voltage? – Volts (V) So in a 9-volt battery, when an electric charge moves from the negative terminal, through the battery, to the positive terminal, how much potential energy does the charge gain? e- – 9 volts!  Basic Circuits A circuit is a path through which electricity can flow. Most circuits have three parts: – An energy source – One or more loads (run by electricity) – Conductors (wires) that connect the two  Create a basic circuit. Use only a battery, two wires, and a bulb. Draw your circuit on your notes page.  Parts of a Circuit Energy Source – A circuit needs an energy source to push a charge through the circuit. – Examples? Load – a load is a device in a circuit that operates using electrical energy. – Examples? Conductor – a conductor is a material that allows electrical energy to flow through it easily. – What makes a good conductor? – Examples?  Parts of a Circuit Cont. Resistor – a resistor is an object added to a circuit that restricts the flow of electrical energy. – Resistors inhibit the flow of electric current by producing a voltage drop when current passes through them. They limit current and cause some electric energy to be given off as heat. – Examples? Switch - a switch is a device that is used to control the flow of current through a circuit. – A switch works by separating (open) or bringing together (closed) two conductors attached to a circuit.  Circuit Switches Add a switch to your circuit. See what happens when you open and close the switch. On your notes, draw a circuit with an open switch and a closed switch. Indicate whether the light is on or off. When an electric current reaches an open switch, the current _________. – This creates an __________ circuit. When an electric current reaches a closed switch, the current _________. – This creates a __________ circuit. Why would you want a switch in a circuit?  Types of Circuits Two types of circuits: – Series Circuit Provides only one possible path for the flow of current. – Parallel Circuit Offers more than one path for the flow of electricity. Series Circuits In a series circuit, the loads are set up in a series, or line, that requires the current to flow through one load before passing through the next. Draw your circuit on your notes sheet. Use arrows to indicate the direction the electric current is traveling. Advantages and disadvantages of a series circuit. On your notes, list the disadvantages of using a series circuit. Parallel Circuits In a parallel circuit, each load has its own path for electricity. Draw your circuit on your notes sheet. Use arrows to indicate the directions the electric current is traveling.  Summary On your notes, list the advantages of using a series circuit and the disadvantages of using a parallel circuit. Circuit symbols  Electric circuit symbols On whiteboards – how 1 2 3 many can you remember? 4 5 6 7 8 9 10 11 12 Calculations in electric circuits  CHARGE We work out charge, Q, in coulombs using the formula: Charge, Q = Current, I x time, t Where: - Charge, Q is measured in coulombs (C) - Current, I is measured in amperes (A) - time, t is measured in seconds (s) There is more charge when a larger current flows. There is more charge when a current flows for a longer time.  EXAMPLES 1. Calculate the charge when a current of 3A flows for 10 seconds. 2. Calculate the charge when a current of 0.75A flows for 5 minutes. 3. Calculate the time it takes for a current of 2A to move 16C of charge. 4. Calculate the current if 30C of charge flows in 0.5 seconds. RESISTANCE Resistance is a property of a material or component that makes it difficult for charge to pass through. Resistance is caused by free electrons colliding with ions in a fixed position. Resistance is measured in ohms, Ω. Resistance is effected by: The material – different materials have different resistances. The length - a longer wire will have more resistance than a shorter wire. The thickness – a thicker wire will have less resistance than a thinner wire. A hotter wire will have more resistance than a cooler wire. Ohm’s law  RELATIONSHIP OF VOLTAGE, CURRENT AND RESISTANCE We can then use the formula below to calculate resistance. Resistance = Potential difference ÷ current Resistance, R is measured in ohms, (Ω) Potential difference, V is measured in volts (V) Current, I is measured in amperes (A)  EXAMPLES 1. Calculate the resistance if a 10A flows with a potential difference of 1.2V 2. Calculate the current if a voltmeter measured 12V across a 7 Ω resistor. Give your answer to 2 s.f. 3. Calculate the potential difference if a voltmeter measured 6A flows through an 9Ω resistor.  Electric Circuits Task 4: Complete task 4. A student needs to find the resistance of a lamp. a) Add a voltmeter to the circuit below that could be used to find the potential difference across the lamp. V b) Calculate the resistance of the lamp when the ammeter read 1.3A and the voltmeterRreads = V / I =4.2V. 4.2 / 1.3 = 3.2Ω c) What would happen to the resistance of the bulb if it got hotter? The resistance would increase. Series and Parallel circuits Current in series circuits is the same everywhere. Potential difference is shared by components in series circuits. Current in parallel circuits splits between each branch. Potential difference is the same in each branch of a parallel circuit. You will be investigating the rules of series and parallel circuits. TOTAL RESISTANCE IN SERIES Adding components in series causes the total resistance of the circuit to increase. The total resistance of a series circuit RT is the sum of the individual resistances added together. We can say: RT = R1 + R2 + R3... TOTAL RESISTANCE IN PARALLEL AS COMPARED TO SERIES Adding components in parallel causes the total resistance of the circuit to decrease. This is because the charge has more paths to travel through. CLASS ACTIVITY All the resistors are 6Ω. 1.Which circuit has the greatest resistance? The series circuit has higher 2.What resistance. would happen if you added another resistor in parallel? The total resistance would be less. 3.What would happen if you added another resistor in series? The total resistance would increase. 4.What is the total resistance of the series circuit? RT = 6 + 6 + 6 = 18Ω 5.R3 in the series circuit is replaced. The total resistance of the circuit is 20Ω, what is the resistance of R3? 20 = 6 + 6 + R , R 3 3  HOMEWORK Task 6: Complete the table below to describe how current, resistance and voltage behave in series and parallel. Current Potential Resistance. difference Series The same everywhere. Shared between Total resistance = components. Sum of sum of individual pd across resistance. components = pd Adding resistors supplied by cell. in series increases overall resistance. Parallel Shared between branches. The same through Adding resistors in Current before branch – sum each loop. parallel reduces of currents through each total resistance of branch. Current joins up after the circuit. branches. TYPICAL EXAM QUESTIONS Complete the exam questions. X X  Complete the exam questions. Series circuits have only one loop. Parallel circuits have more than one loop.  Complete the exam questions. X X  Complete the exam questions. RT = 10 + 15 = 25Ω Series circuit – PD is shared. PD = 6V / 3 = 2V  Complete the exam questions.  EXAM QUESTIONS The current in I2 is equal to current I3. Current I1 is equal to the sum of I2 + I3. QUIZ. What does the picture below show? Give as much detail as you can.  COST OF ELECTRICITY An electric hairdryer is labelled as: 240 V, 2000 W. Assume the cost of electricity per unit is R1,53 per unit. How much would it cost you at the end of the month(January) if you used the hairdryer every morning for 15 minutes? SOLUTION: Power = 2000 W = 2 kW Time = 31 x 0,25 = 7,75 hours Cost = 2 kWh x 7, 75h x R1,53 = R23,72  Question 2 A 1800 W geyser switches on at 15:00 and off again at every afternoon. The municipality tariff for 2019 was R2,20 per kWh. Calculate the cost of using the geyser during February 2019.  MAY THE ELECTRIC CURRENT BE WITH YOU! THE END