Announcements

Announcements Homework for tomorrow…

Ch. 30: CQ 11, Probs. 20, 22, & 32CQ3: The answer lies in your (recently used) garden hose (look inside while turning it on).CQ4: No30.4: 9.3 x 10-4 m30.6: a) 4.6 x 1021b) 4.3 x 10-12 m

Office hours…MW 12:30-1:30 pm TR 9-10 am F 1-2 pm

Tutorial Learning Center (TLC) hours:MW 8-10 am, 12-6 pm R 8-12 pm, 2-6 pm T 8-1 pm, 2-6 pm F 8-10 am, 2-5 pm

Chapter 30

Current & Resistance(Conductivity and Resistivity &

Resistance and Ohm’s Law)

Current..

Current Density..

Kirchoff’s Junction Rule..

Current Density related to the E-field..

Review…

The current density in this wire is

1. 4 106 A/m2.

2. 2 106 A/m2.

3. 4 103 A/m2.

4. 2 103 A/m2.

5. Some other value.

Quiz Question 1

How the current density, J, is related to the E-field driving the current:

where

SI Units?

30.4:Conductivity and Resistivity

How the current density, J, is related to the E-field driving the current:

Where

SI Units?

30.4:Conductivity and Resistivity

Both segments of the wire are made of the same metal. Current I1 flows into segment 1 from the left.

How does current I1 in segment 1 compare to current I2 in segment 2?

1. I1 > I2.

2. I1 = I2.

3. I1 < I2.

4. There’s not enough information to compare them.

Quiz Question 2

Both segments of the wire are made of the same metal. Current I1 flows into segment 1 from the left.

How does current density J1 in segment 1 compare to current density J2 in segment 2?

1. J1 J2.

2. J1 J2.

3. J1 J2.

4. There’s not enough information to compare them.

Quiz Question 3

Both segments of the wire are made of the same metal. Current I1 flows into segment 1 from the left.

How does the electric field E1 in segment 1 compare to the electric field E2 in segment 2?

1. E1 > E2.

2. E1 = E2 but not zero.

3. E1 < E2.

4. Both are zero because metal is a conductor.

5. There’s not enough information to compare them.

Quiz Question 4

Define the resistivity…

SI Units?

30.4:Conductivity and Resistivity

Define the resistivity…

SI Units?

30.4:Conductivity and Resistivity

Define the resistivity…

SI Units?

30.4:Conductivity and Resistivity

A 2.0 mm diameter aluminum wire carries a current of 800 mA. What is the E-field strength inside the wire?

i.e. 30.6:The E-field in a wire

How is the current, I, related to the potential difference, ΔV in a wire?

30.5:Resistance and Ohm’s Law

How is the current, I, related to the potential difference, ΔV in a wire?

SI Units of R?

30.5:Resistance and Ohm’s Law

- Ohm’s Law

How is the current, I, related to the potential difference, ΔV in a wire?

SI Units of R?

30.5:Resistance and Ohm’s Law

- Ohm’s Law

Notice: Resistivity, ρ, describes just the material. Resistance, R, characterizes a specific piece of the

conductor with a specific geometry.

30.5:Resistance and Ohm’s Law

Resistance of a wire

Wire 2 has twice the length and twice the diameter of wire 1. What is the ratio R2/R1 of their resistances?

1. 1/4.

2. 1/2.

3. 1.

4. 2.

5. 4.

Quiz Question 5

A battery is a source of potential difference Vbat.

The battery creates a potential difference Vwire = Vbat between the ends of the wire.

The potential difference in the wire Vwire generates an E-field in the wire.

The E-field establishes a current I = JA = σAE in the wire.

The current in the wire is determined jointly by the battery and the wire’s resistance, R to be:

I Vwire/R

Resistance and Ohm’s Law