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4.3 Resistance 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.

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**R (ohm) = V (volt) I (amp)**

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 Resistance (R) of a component = p.d across component (V) the current through it (I) R (ohm) = V (volt) I (amp)

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

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

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4.3 Resistance Ohm’s law: The pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change 12V 2mA R ohm Q. For the resistor opposite calculate: a) the resistance at this current b) the new pd when the current is 50 micro Amp a) R = V = = Ohm I x 10-3 b) V = I R = x10-6 x = 0.3 V

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

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

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

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**A = Area of cross section (m2)**

4.1 Resistance Resistivity relates to the material rather than a specific component L R L Metal rod R A A = Area of cross section (m2)

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**A = Area of cross section (m2)**

4.1 Resistance Resistivity relates to the material rather than a specific component L R L Metal rod R A A = Area of cross section (m2) R = L A

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**A = Area of cross section (m2)**

4.1 Resistance Resistivity relates to the material rather than a specific component L R L Metal rod R A A = Area of cross section (m2) R = L A R / ohm L /m

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**A = Area of cross section (m2)**

4.1 Resistance Resistivity relates to the material rather than a specific component L R L Metal rod R A A = Area of cross section (m2) R = L A R / ohm Gradient = A L /m

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**A = Area of cross section (m2)**

4.1 Resistance Resistivity relates to the material rather than a specific component L R L Metal rod R A A = Area of cross section (m2) R = L A R / ohm = R A L Gradient = A Ohm m = ohm metre m Units: L /m

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

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4.1 Resistance Superconductivity Tc temp This is the property of a material which is at or below a critical temperature Tc where it has zero resistivity . 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

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

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Superconductivity Tc temp 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 Tc temp 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 C

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Current and Resistance

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