Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire...

Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire they collide with the metal atoms. These collision make the atoms vibrate more…which makes the metal hotter.

Resistance is a measure of how much a material tries to stop electricity passing through it.

4.3 Resistance

R (ohm) = V (volt) I (amp)

Resistance (R) of a component = p.d across component (V) the current through it (I)

Electrical resistance is the opposition to the flow of electrons.

It is caused by the repeated collisions between charge carriers andfixed positive ions in a metal conductor

Ohm’s law:The pd across a metallic conductor is proportional to the current through it,provided the physical conditions do not change

Q. For the resistor opposite calculate:a) the resistance at this currentb) the new pd when the current is 50 micro Amp

12V

2mAR ohm

a) R = V = I

4.3 Resistance



12V

2mAR ohm

a) R = V = 12 = 6000 Ohm I 2.0 x 10-3

4.3 Resistance

http://phet.colorado.edu/simulations/index.php?cat=Physics

http://www.batesville.k12.in.us/physics/PhyNet/e&m/current/ECurrent_Notes.htm



12V

2mAR ohm

a) R = V = 12 = 6000 Ohm I 2.0 x 10-3

b) V = I R = 50x10-6 x 6000 = 0.3 V

4.3 Resistance

4.3 Resistance

Measurement of resistance:

A

V

Record the pd across R for increasing values of current.

( change r to change the circuit current ()

r

R

4.3 Resistance


A

V



r

The ammeter has avery low resistance ( 0.2 ohm)

The voltmeter has avery high resistance ( 20,000 ohm)

Why?

R

4.3 Resistance


A

V



r

The ammeter has avery low resistance ( 0.2 ohm)

The voltmeter has avery high resistance ( 20,000 ohm)

Why?

Pd / V

I / A

Gradient = V

I

Gradient = resistance

R

4.1 Resistance

Resistivity relates to the material rather than a specific component

R L

R 1 A

Metal rod

L

A = Area of cross section (m2)

4.1 Resistance


R L

R 1 A

R = L A

Metal rod

L


4.1 Resistance


R L

R 1 A

Metal rod

L

A = Area of cross section (m2)R = L A

L /m

R /ohm

4.1 Resistance


R L

R 1 A

Metal rod

L

A = Area of cross section (m2)R = L A

L /m

R /ohm

Gradient = A

4.1 Resistance


R L

R 1 A

Metal rod

L


= R A L

Ohm m2 1 = ohm metre m

Units:

R = L A

L /m

R /ohm

Gradient = A

4.1 Resistance

Superconductivity

This is the property of a material which is at or below a critical temperature Tc where it has zero resistivity .

Implications:* Zero resistance* no pd exists across a superconductor with a current flowing * the current has no heating effects

tempTc

4.1 Resistance

Superconductivity

Implications:* Zero resistance* no pd exists across a superconductor with a current flowing * the current has no heating effects

Applications:* high power electromagnets with strong magnetic fields* power cables can transfer energy without wasting energy

This is the property of a material which is at or below a critical temperature Tc where it has zero resistivity .

tempTc

Superconductivity

tempTc

Properties of a superconductor:

* material losses the effect above the critical temperature Tc.

Superconductivity

tempTc



* If Tc is above 77K ( -196 C) it’s a high temperature superconductor

Superconductivity

tempTc



* If Tc is above 77K ( -196 C) it’s a high temperature superconductor

current Tc max = 150 K - 123C

Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire...

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