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Published byKaty Worsley Modified over 2 years ago

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Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire they collide with the metal atoms. These collision make the atoms vibrate more…which makes the metal hotter. Resistance is a measure of how much a material tries to stop electricity passing through it. 4.3 Resistance

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R (ohm) = V (volt) I (amp) Resistance (R) of a component = p.d across component (V) the current through it (I) Electrical resistance is the opposition to the flow of electrons. It is caused by the repeated collisions between charge carriers and fixed positive ions in a metal conductor

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Ohm’s law: The pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change Q. For the resistor opposite calculate: a) the resistance at this current b) the new pd when the current is 50 micro Amp 12V 2mA R ohm a) R = V = I 4.3 Resistance

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Ohm’s law: The pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change Q. For the resistor opposite calculate: a) the resistance at this current b) the new pd when the current is 50 micro Amp 12V 2mA R ohm a) R = V = 12 = 6000 Ohm I 2.0 x 10 -3 4.3 Resistance

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http://phet.colorado.edu/simulations/index.php?cat=Physics http://www.batesville.k12.in.us/physics/PhyNet/e&m/current/ECurrent_Notes.htm Ohm’s law: The pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change Q. For the resistor opposite calculate: a) the resistance at this current b) the new pd when the current is 50 micro Amp 12V 2mA R ohm a) R = V = 12 = 6000 Ohm I 2.0 x 10 -3 b) V = I R = 50x10 -6 x 6000 = 0.3 V 4.3 Resistance

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Measurement of resistance: A V Record the pd across R for increasing values of current. ( change r to change the circuit current () r R

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4.3 Resistance Measurement of resistance: A V Record the pd across R for increasing values of current. ( change r to change the circuit current () r The ammeter has a very low resistance ( 0.2 ohm) The voltmeter has a very high resistance ( 20,000 ohm) Why? R

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4.3 Resistance Measurement of resistance: A V Record the pd across R for increasing values of current. ( change r to change the circuit current () r The ammeter has a very low resistance ( 0.2 ohm) The voltmeter has a very high resistance ( 20,000 ohm) Why? Pd / V I / A Gradient = V I Gradient = resistance R

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4.1 Resistance Resistivity relates to the material rather than a specific component R L R 1 A Metal rod L A = Area of cross section (m 2 )

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4.1 Resistance Resistivity relates to the material rather than a specific component R L R 1 A R = L A Metal rod L A = Area of cross section (m 2 )

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4.1 Resistance Resistivity relates to the material rather than a specific component R L R 1 A Metal rod L A = Area of cross section (m 2 ) R = L A L /m R / ohm

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4.1 Resistance Resistivity relates to the material rather than a specific component R L R 1 A Metal rod L A = Area of cross section (m 2 ) R = L A L /m R / ohm Gradient = A

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4.1 Resistance Resistivity relates to the material rather than a specific component R L R 1 A Metal rod L A = Area of cross section (m 2 ) = R A L Ohm m 2 1 = ohm metre m Units: R = L A L /m R / ohm Gradient = A

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4.1 Resistance Superconductivity This is the property of a material which is at or below a critical temperature T c where it has zero resistivity. Implications: * Zero resistance * no pd exists across a superconductor with a current flowing * the current has no heating effects tempTcTc

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4.1 Resistance Superconductivity Implications: * Zero resistance * no pd exists across a superconductor with a current flowing * the current has no heating effects Applications: * high power electromagnets with strong magnetic fields * power cables can transfer energy without wasting energy This is the property of a material which is at or below a critical temperature T c where it has zero resistivity. tempTcTc

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Superconductivity tempTcTc Properties of a superconductor: * material losses the effect above the critical temperature Tc.

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Superconductivity tempTcTc Properties of a superconductor: * material losses the effect above the critical temperature Tc. * If Tc is above 77K ( -196 C) it’s a high temperature superconductor

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Superconductivity tempTcTc Properties of a superconductor: * material losses the effect above the critical temperature Tc. * If Tc is above 77K ( -196 C) it’s a high temperature superconductor current Tc max = 150 K - 123C

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