# 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.

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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

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

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

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.

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?

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

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

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

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

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

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

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

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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