1 / 12

Resistivity

Resistivity. Electricity Lesson 5. Learning Objectives. To define resistivity . To know what causes resistance. To know how to measure resistance. Practice Conversion. If a wire has a cross sectional area of 1.23 square millimetres What is its area in square metres.

Download Presentation

Resistivity

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Resistivity Electricity Lesson 5

  2. Learning Objectives To define resistivity. To know what causes resistance. To know how to measure resistance.

  3. Practice Conversion If a wire has a cross sectional area of 1.23 square millimetres What is its area in square metres.

  4. What does resistance depend on? The resistance of a wire depends on three factors: the length; double the length, the resistance doubles. the area; double the area, the resistance halves. the material that the wire is made of.

  5. Resistivity For a conductor of length L and uniform cross-sectional area, A, its resistance R is proportional to L but inversely proportional to A. So the resistance is given by:- Where ρ is the resistivity of the conductor.

  6. Resistivity Rearranging gives an equation for resistivity:- The unit of resistivity is the ohm metre (Ωm) In words:-

  7. Resistivity Resistivity is a property of the material.  It is defined as the resistance of a wire of the material of unit area and unit length. It has the symbol ρ, don’t mix this up with density!

  8. Cross Sectional Area For a circular conductor with a radius r, diameter d, the cross sectional area is given by:-

  9. Question Constantan has a resistivity of 47 × 10-8Ωm.  How much of this wire is needed to make a 10 ohm resistor, if the diameter is 0.5 mm? 

  10. Answer  Work out the radius in metres:  r  = 0.25 × 10-3 m  Now work out the area:  A = πr2 = π × (0.25 × 10-3)2 =  π × 6.25 × 10-8 m2= 1.96 × 10-7 m2  Now work out R: R = ρL/A . 10 = (47 × 10-8 Wm × L ) / 1.96 × 10-7 m2   L = 10 × 1.96 × 10-7 m2 ¸ 47 × 10-8 =  4.17 m 

  11. Superconductivity • Normally materials have some resistance. • When current flows through them they heat up. • But superconductors can be cooled below a transition temperature below which the resistivity disappears. • This means no energy is lost as heat energy. • For metals the transition temp is about 10 K • (-263 °C)

  12. Uses of superconductors... Power cables that can transmit electricity with no loss of energy. Really strong electromagnets that don’t need a constant power source (e.g. Maglev trains). Electronic circuits that work really fast.

More Related