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Transcript
Physics Unit Chapter 8 – pages 268 – 303 Chapter 8 Section 8.3 Resistance and Ohm’s law Resistance and Ohm’s Law What is resistance? Resistance Is the property of any material that slows down the flow of electrons (i.e. a load) and converts electrical energy into other forms of energy (i.e. light or heat). Wires that connect the battery to a load have very little resistance; therefore, electrons can travel through the wire with almost no loss of electrical energy. Resistance and Current How does resistance relate to current? Voltage is directly proportional to current; therefore, as the voltage increases, so does the current (I). Now, if you use batteries with equal voltage: An electric circuit that has a large resistance will allow less current to flow through. An electric circuit that has lower resistance will allow more current to flow. Ohm’s Law R=V I R = resistance (ohms - Ω) V = voltage (volts – V) I = current (amperes – A) V R I Unit Conversions Divide by 1000 to convert mA (m = milli) A Times by 1000 to convert A mA Example 5 mA ? A 5 mA 1000 = 0.005 A 0.095 A ? mA 0.095 A x 1000 = 95 mA Unit Conversions Divide by 1000 to convert Ω k Ω (k = kilo) Times by 1000 to convert k Ω Ω Example 6000 Ω ? k Ω 6000 Ω 1000 =6kΩ 20 k Ω ? Ω 20 k Ω x 1000 20,000 Ω Unit Conversions Divide by 1,000,000 to convert V to MV (mega) Times by 1,000,000 to convert MV to V Example 25 MV ? V 25 MV x 1,000,000 = 25, 000,000V 4,000,000 V MV 4,000,000 V 1,000,000 = 4 MV Calculations Protocol Calculations using Ohm’s Law 1.Write formula 2.Insert numbers including proper units 3.Work vertically with the = signs lined up 4.Put a box around the final answer Sample Calculations Question 1. The current through a load in a circuit is 1.5 A. If the potential difference across the load is 12 V, what is the resistance of the load? R=V I R = 12 V 1.5A R = 8.0 Ω The resistance of the load is 8.0 Ω. Sample Calculations Question 2. The resistance of a car headlight is 15 Ω. If there is a current of 0.80 A through the headlight, what is the voltage across the headlight? V = IR V = (0.80 A)(15 Ω) V = 12 V The voltage across the headlight is 12 V. Sample Calculations Question 3. A 0.06 V potential difference is measured across a load that has a resistance of 15 Ω. What is the current through this load? I=V R I = .060 V 15 Ω I = 0.004 A The current through this load is 0.004 A. The Resistor What is a Resistor? Resistor – an electrical component that has a specific resistance. Units – ohms (Ω) Used to control the current or voltage in a circuit. When charge encounters resistance, it is transformed into other forms of energy (i.e. heat). Resistor Colour Code Table 8.2 Page 297 (Accuracy) (Accuracy) How To Read a Resistor First coloured band – first digit (#) Second coloured band – second digit (#) Third coloured band – number of zeros (multiplier) Fourth coloured band – accuracy/tolerance (+/-) Accuracy/Tolerance: Gold – 5% Silver – 10% No colour – 20% How To Read a Resistor Red, Black, Red, Gold Answer: 2000 Ω +/- 5% How To Read a Resistor Example: Red, blue, brown, gold 260 Ω +/- 5% Example: Green, orange, black, silver 53 Ω +/- 10%