5.4 - resistance and conductivity
TRANSCRIPT
Syllabus points Resistance depends upon the nature and dimensions of a conductor
Resistance for ohmic and non-ohmic components is defined as the ratio of potential difference across the component to the current in the component
this includes applying the relationship:
𝑅 =𝑉
𝐼
Learning GoalsDefine:
Ohmic conductor Non-ohmic conductor Resistance Resistivity
Explain and apply Ohm’s Law (R=V/I)Explain how the resistivity of a conductor effects its resistanceExplain how the dimensions of a conductor effect its resistanceRecognise and draw an I-V graph for an ohmic conductor Recognise and draw an I-V graph for a non-ohmic conductorEvaluate the resistance of any piece of conductive material Identify that once established, the current is the same in every
part of the circuit
Electrical Resistance Similar to friction or air resistance
As charge moves through a wire it zig zags as it collides with atoms
In these collisions charges transfer some of their energy to the atoms
Units Ohms, Ω
1 Ω = 1 V/A
• Georg Ohm found that current flowing through a substance is directly
proportional to the voltage
𝐼 ∝ 𝑉
• Resistance affects current and voltage
Ohm’s law
𝑅 =𝑉
𝐼Resistance (Ω)
Potential Difference (V)
Current (A)
Example 1When a potential difference of 16 V is applied across the ends of a wire, the current flowing in the wire is 2.4 A.
a) What is the resistance of the wire?
b) What potential difference is needed to make a current of 3.0 A flow through the wire?
Example 1When a potential difference of 16 V is applied across the ends of a wire, the current flowing in the wire is 2.4 A.
a) What is the resistance of the wire?
Example 1When a potential difference of 16 V is applied across the ends of a wire, the current flowing in the wire is 2.4 A.
b) What potential difference is needed to make a current of 3.0 A flow through the wire?
Example 2If 750 μA is flowing through 11 kΩ of resistance, what is the potential difference across the resistor?
Example 2
If 750 μA is flowing through 11 kΩ of resistance, what is the potential difference across the resistor?
Factors affecting resistance Resistivity
Some materials resist the movement of charge more than others
Resistivity = how much a material opposes the flow of charge, innate property of that material
Temperature
Increase in temperature, increase in resistance
Atoms vibrating more rapidly, charge moving through conductor experience more collisions
Resistivity of common materials
Material Resistivity - ρ (Ωm)
Copper 1.7 × 10-8
Silver 1.6 × 10-8
Gold 2.4 × 10-8
Aluminium 2.8 × 10-8
Nichrome (Ni, Fe, Cr alloy) 100 × 10-8
Pure water 5 × 103
Wood 108 1011
Glass 1010 1014
These values only apply at 20 °C
Factors affecting resistance Length of wire
Longer the wire, increase in resistance
Charges will experience more collisions before reaching end of wire
Area of wire
Increase the area, decrease in resistance
More area for charges to move, less collisions
𝑅 = 𝜌𝑙
𝐴Resistance (Ω)
Length of the wire (m)
Cross-sectional area of the wire (m2)
Resistivity (Ωm)
Note: This equation is not in the syllabus and is not on the formula sheet. However, you still need to understand the concepts behind it (resistance and the factors which can impact resistance).
This equation can appear in assessments, however the equation and what the symbols stand for would be provided for you.
Example 3What is the resistance of a 20.0 m long piece of 12-gauge copper wire having a 2.053 mm diameter?
Example 3What is the resistance of a 20.0 m long piece of 12-gauge copper wire having a 2.053 mm diameter?
Example 4Does the resistance of an object depend on the path current takes through it?
Consider the diagram below – is the resistance the same in both cases?
Example 4
Does the resistance of an object depend on the path current takes through it?
Yes, resistance depends on path taken, resistance is not the same in both cases.
Consider the diagram below – is the resistance the same in both cases?
No, Dimensions of conductor change, so length has changed. This will change the resistance.
Ohmic vs non-ohmic
Ohmic resistors - resistance is constant for wide range of voltages and currents (e.g. wire, resistor)
Obeys Ohm’s Law
• Non-ohmic resistors – resistance is not constant, varies depending on current and voltage (e.g. diodes)
• Does not obey Ohm’s Law
Side note - superconductors A superconductor is a material which,
when cooled below a critical temperature exhibits zero electrical resistance
This means a current flowing through a loop of superconducting wire can persist indefinitely with no power source
Promising future applications include electric power transmission, transformers, magnetic levitation devices and superconducting magnetic refrigeration.
ResourcesAV
Royal Institution - Levitating Superconductor on a Möbius strip (7:30)
Further Reading
Physics Classroom - Resistance
BBC Bitesize Science - Resistance