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From last lesson we found… VOL = distance x Area = speed x time x Area # ELECTRONS = charge/charge of 1 electron = current x time / e NUMBER DENSITY, n = # electrons/vol n = (I x t)n = I/evA e x v x t x A nAve = I

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The big question: What did Kirchoff say? Why are we learning this? Supermax (super maximum security) prisons e.g. Alcatraz

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Learning Objective: To evaluate Kirchoff’s first law The big question: What does Kirchoff say? A – B – C – D -

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Calculate velocity A copper wire has cross-sectional area 3.0 mm 2 and current 5.0A. Copper’s number density is 6.0x10 28 m -3. A brass wire has cross-sectional area 6.0mm 2 and current 9.0A. Assume Brass’s number density is 9.0x10 28 m -3. A brass wire has radius ½ that of the previous question, with the same current.

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On a whiteboard: (b) A silver wire has cross-sectional area 3.0 mm 2 and current 2.0A. Copper’s number density is 4.0x10 28 m -3. (A) If the radius was ¼ as small, then the number of electrons will be what factor smaller?

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Learning Objective: To combine knowledge on density and electricity to calculate velocity The big question: How do we combine density with velocity? A – Predict insulator number densities B – Analyse how number density changes drift velocity C – Create a formula for drift velocity and Current D – Calculate drift velocity

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Calculate mean drift velocity: A copper wire has cross-sectional area 4.0 mm 2 and current 5.0A. Copper’s number density is 8.0x10 28 m -3. A brass wire has cross-sectional area 6.0mm 2 and current 15.0A. Assume Brass’s number density is 9.0x10 28 m -3. (C) Using letters n for number density, I for current, e for charge of 1 electron and A for cross-sectional area, write an expression for v

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What do they do? A – Predict insulator number densities, using your expression from C and B B – Analyse how number density (n) changes drift velocity. Keep all other numbers the same and change n up and down, to see the effect on velocity C – Create a formula for drift velocity, v and rearrange it for current, I

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Current, I = nA/veve/nA n/vAenAve

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If the wire had a high number density V is velocity v would increasev would stay the same v would decrease

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If the wire was an insulator number density number density would increasewould decrease number density would stay same

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Learning Objective: To combine knowledge on density and electricity to calculate velocity The big question: How do we combine density with velocity? A – Predict insulator number densities B – Analyse how number density changes drift velocity C – Create a formula for drift velocity and Current D – Calculate drift velocity

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Learning Objective: To combine knowledge on density and electricity to calculate velocity The big question: How do we combine density with velocity? A – Predict insulator number densities B – Analyse how number density changes drift velocity C – Create a formula for drift velocity and Current D – Calculate drift velocity

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A. their drift speed increases. B. their drift speed decreases. C. their drift speed stays the same. D. not enough information given to decide Q25.1 Two.

A. their drift speed increases. B. their drift speed decreases. C. their drift speed stays the same. D. not enough information given to decide Q25.1 Two.

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