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) prisonse.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 mm2 and current 5.0A. Copper’s number density is 6.0x1028 m-3.

A brass wire has cross-sectional area 6.0mm2 and current 9.0A. Assume Brass’s number density is 9.0x1028 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 mm2 and current 2.0A. Copper’s number density is 4.0x1028 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 densitiesB – Analyse how number density changes drift velocityC – Create a formula for drift velocity and CurrentD – Calculate drift velocity

Calculate mean drift velocity:

A copper wire has cross-sectional area 4.0 mm2 and current 5.0A. Copper’s number density is 8.0x1028 m-3.

A brass wire has cross-sectional area 6.0mm2 and current 15.0A. Assume Brass’s number density is 9.0x1028 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/ve ve/nA

n/vAe nAve

If the wire had a high number densityV is velocity

v would increase v would stay thesame

v would decrease

If the wire was an insulator

number density number density would increase would 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 densitiesB – Analyse how number density changes drift velocityC – Create a formula for drift velocity and CurrentD – 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 densitiesB – Analyse how number density changes drift velocityC – Create a formula for drift velocity and CurrentD – Calculate drift velocity