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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