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# P2.3 p2.4 lesson 4 resistance & ohm's law

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• 1. Content P2.3 - P2.4 Currents in electrical circuits Lesson 1 What is static electricity? Lesson 2 What is electrical current? Lesson 3 Series and parallel circuits Lesson 4 Resistance & Ohm’s Law Lesson 5 Investigating resistance Lesson 6 How LDR and thermistors work
• 2. Content P2.3 - P2.4 Currents in electrical circuits Lesson 7 Potential difference Lesson 8 Current in series & parallel circuits Lesson 9 The heating effect of current Lesson 10 Electrical power Lesson 11 Mains electricity Lesson 12 Fuses and circuit breakers
• 3. Lessons 4 Resistance Ohm’s Law Rich question: Can we control electricity? How? Agree learning outcomes D/E 1. State that the components (motors, bulbs, wires etc.) in a circuit resist the flow of charge. The bigger the resistance, the smaller the current. C 2. Describe how to find the resistance of a component by measuring the current through, and the potential difference across, the component 3. Calculate potential difference (V) using current (I) and resistance (R) B 4. Describe the relationship between the current through and potential difference across a resistor (at a constant temperature) as directly proportional (*also use a graph) A/A* 5. Use a model to explain how the resistance changes in terms of ions and electrons linking ideas to length of wire, thickness & temperature.
• 4. What is my obstacle to learning?
• 5. What does this word mean in Physics? Other words with similar meanings are… 11 December 2013 Why do we need mood lighting? What is the science behind this idea? [metal ions, electrons, collide]
• 6. Stage lighting engineer wanted Your mission – To design a lighting scheme for my new show coming to Stratford. Specification: 1. You must be able to set up and draw a simple circuit for making mood lighting. 2. Explain how it works using the idea of resistance. Use the words ‘metal ions and electrons’. 3. Use an analogy to explain to me so that I can understand why the bulb changes brightness.
• 7. Present new information Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire they collide with the metal ions. These collision make the atoms vibrate more…which makes the metal hotter.
• 8. Electron flow and resistance Resistance is a measure of how much a material tries to stop electricity passing through it. All wires and devices have some resistance, which is why electrical appliances always waste some energy as heat. 10 of 41 © Boardworks Ltd 2005
• 9. Investigating current and voltage 11 of 41 © Boardworks Ltd 2005
• 10. Construct meaning What is the explanation ? How does changing the length of wire in this variable resistor affects the bulb?
• 11. Apply to demonstrate
• 12. When the length of the wire increases… The effects: • The brightness of the bulb… • The size of current… • The wire feels… The cause: • This is because, the number of metal ions... • It is harder for the electrons to flow through the wire as there are more ……………… with the metal ions of the wire.
• 13. Factors that affect resistance The resistance of a wire depends on several factors:  Material – Different materials have different resistances because some materials are better conductors. Nichrome wire has a higher resistance than a copper wire of the same size.  Length – The longer a wire is the higher its resistance. When electrons travel down a long wire they can collide with more metal ions than in a short wire.  Thickness – A thin wire has a higher resistance than a thick wire.  Temperature – The higher the temperature of a wire the higher its resistance. Metal ions vibrate more at higher temperatures and so collisions with electrons are more likely to happen. 15 of 41 © Boardworks Ltd 2005
• 14. Review What is resistance?
• 15. How are current and voltage related for a resistor? Set up this circuit with a resistor and a variable resistor. voltage (V) 0 0.5 1.0 A V Slowly move the variable resistor so that the voltage increases by 0.5 V and record the current for each setting. current (A) 0 0.6 1.1 1.5 1.8 2.5 3.0 2.0 2.5 3.0 3.5 3.5 4.2 Plot a current-voltage graph of the results. 17 of 41 © Boardworks Ltd 2005
• 16. The graph is a straight line so the two quantities current and voltage are proportional. So if voltage doubles then the current doubles and so on. x x x x x Plot the current and voltage readings for a resistor on a graph and draw a line of best fit. Current / A Current-voltage graph for a resistor x x x Potential difference / V This is called Ohm’s Law after the scientist Georg Ohm. Ohm’s Law The current flowing through a wire is proportional to the potential difference (voltage) across it provided the temperature remains constant. 18 of 41 © Boardworks Ltd 2005
• 17. Current-voltage graphs for different wires The copper wire has a steeper gradient and so has a lower resistance than the nichrome. x Current / A The points produce straight lines with different gradients. copper x x x x Plot the current and voltage readings for nichrome and copper wires of the same size. x x x nichrome x x x x Potential difference / V At the same potential difference, a copper wire lets a larger current flow than a nichrome wire of the same length and thickness. The steeper the gradient of a current-voltage graph, the lower the resistance of the wire. 19 of 41 © Boardworks Ltd 2005
• 18. Current-voltage graphs for a bulb Increasing the voltage across the filament in the bulb causes this wire to get very hot and give out light. x Current / A The graph produced is not a straight line but a curved line. x x x Plot the current and voltage readings for a filament bulb. x x x x Potential difference / V As the wire gets hotter, its resistance gets higher, which means the current flow is less. So as the temperature rises the current is not proportional to the voltage. The higher the temperature of a wire, the higher its resistance. 20 of 41 © Boardworks Ltd 2005
• 19. Current-voltage graphs and Ohm’s Law Which of the components obeys Ohm’s Law? I 1 I 2 I 3 V V V 1. A wire or resistor 2. A filament lamp 3. Wires of different materials  21 of 41   © Boardworks Ltd 2005
• 20. Resistance – true or false? 22 of 41 © Boardworks Ltd 2005
• 21. P2 REVISION – CHAPTER 4 – Current Electricity Electrical Charges If you rub two electrically insulating materials are rubbed together, __________________ are rubbed off one material and deposited on the other. Objects that have opposite electric charges ______________ each other, if they have the same electric charge they___________. Electric Circuits Electric symbols I is the current in amperes, A Q is the charge in coulombs, C t is the time in seconds, s. Every component has an agreed circuit symbol. Make sure you can recognise and draw them! Resistance Current is measured with an ammeter. Where are ammeters placed in relation to the component? What is the unit of current? The potential difference(pd) across a component is measured with a voltmeter. These are always placed in parallel with the component. What is the unit of potential difference? V is ? W is ? Q is ? Don’t forget units! R is ? V is ? I is ? Don’t forget units! Ohm’s law: states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor. KEY WORDS: Insulating Electron Attract Repel Resistance Series ASSESSMENT:
• 22. P2 REVISION – CHAPTER 4 – Current Electricity Cont. Series Circuits Current-Potential Difference Graphs A current-potential difference graph for a resistor. How are the components connected in a series circuit? What happens if there is a break in the circuit? Is the current the same or different through each component? If you add together the potential difference what does it give you? The resistance of the individual components add up to give the total resistance of the circuit. A current-potential difference graph for a filament bulb, line is a curve so the current is not directly proportional to the __________________ ___________________. The current in a diode flows in one direction only, in the reverse direction the diode has a very high resistance so the current will be what? Parallel Circuits How are the components connected in a series circuit? What happens if there is a break in the circuit? Is the pd across each component the same or different? The bigger the resistance of a component, the ______________ the current through it. Use this equation to work out the current through a component in a parallel circuit. Thermistor: Resistance _______________ if its temperature increases. LDR: resistance decreases if the light intensity on it ___________. KEY WORDS: Diode Filament bulb Resistor Thermistor ASSESSMENT:
• 23. P2 REVISION – CHAPTER 5 – Mains Electricity Alternating Current Direct current is supplied by cells and batteries and passes round the circuit in one direction. Alternating current is from the mains, how does it travel? Frequency of am ac supply can be worked out from an oscilloscope trace using the equation: Cables & Plugs Fuses How does a fuse work? Where is a fuse fitted in a circuit? Electrical Energy & Charge An electric current is the flow of what? What is the equation that relates to charge, current and time? When charge flows through a resistor, what makes it hot? How does a circuit breaker work? What can you use this equation for? Electrical Power & Potential Difference Electrical Issues Electrical faults are dangerous what two things can they cause? Power can be calculated using the above equation. Why are the pins of a plug made of brass? What does the earth wire earth? Why are some cables thicker than others? Using the current and the pd and the equation above enables us to calculate the power of an appliance. To work out the correct rating in amperes for a fuse rearrange the above equation, what would the equation now be? Why must you check cables, plugs and sockets for damage regularly? What must you not touch with wet hands? Why are filament bulbs very inefficient? KEY WORDS: Current Frequency Oscilloscope Circuit breaker ASSESSMENT: