CIE A Level Physics復習筆記9.3.1 Resistivity
Resistivity
- All materials have some?resistance?to the flow of charge
- As?free?electrons?move?through a metal wire, they collide with ions which get in their way
- As a result, they?transfer?some, or all, of their?kinetic?energy?on?collision, which causes electrical?heating
Free electrons collide with ions which resist their flow
- Since?current?is the?flow?of?charge, the ions resisting their flow causes?resistance
- Resistance depends on the?length?of the wire, the?cross-sectional area?through which the current is passing and the?resistivity?of the material
Electrical resistance equation
- The resistivity equation shows that:
- The?longer?the wire, the?greater?its resistance
- The?thicker?the wire, the?smaller?its resistance
The length and width of the wire affect its resistance
- Resistivity is a property that describes the extent to which a material opposes the flow of electric current through it
- It is a property of the material, and is dependent on temperature
- Resistivity is measured in?? m
Resistivity of some materials at room temperature
-
- The higher the resistivity of a material, the higher its resistance
- This is why copper, with its relatively low resistivity at room temperature, is used for electrical wires — current flows through it very easily
- Insulators have such a high resistivity that virtually no current will flow through them
Worked Example
Two electrically-conducting cylinders made from copper and aluminium respectively.
Their dimensions are shown below.
Copper resistivity = 1.7 × 10-8?Ω m
Aluminium resistivity = 2.6 × 10-8?Ω mWhich cylinder is the better conductor?
Exam Tip
- You won’t need to memorise the value of the resistivity of any material, these will be given in the exam question.
- Remember if the cross-sectional area is a circle e.g. in a wire, it is proportional to the diameter squared. This means if the diameter doubles, the area quadruples causing the resistance to drop by a quarter.
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