Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

Theme B — The Particulate Nature of Matter

B.5.5 Electric Resistance — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 46 of 53 1 quiz

📚 IB Physics HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read
Lesson summary

B.5.5 Electric Resistance — IB PHYSICS SL notes: Define resistance, use R = V/I and describe why conductors resist current and what affects their resistance.Collisions — free electrons accelerated by the field collide with vibrating lattice ions and impuritiesEach collision trans…

  • Material differences
  • Every component
  • Unit conversion
  • Ohmic component
  • Definition versus law
  • Cross-sectional area

Section 1 of 10Why a conductor resists current

Why a conductor resists current
Collisions
  • Collisions free electrons accelerated by the field collide with vibrating lattice ions and impurities
  • Each collision transfers electrical potential energy to the positive ions.
  • This limits the drift speed of the electrons.
Heating
  • Heating the energy transferred raises the kinetic energy of the lattice atoms and so the internal energy of the metal
  • The macroscopic result is that the wire heats up, and this is the cause of resistance.
Material differences
  • Material differences some metals heat up more than others, and the greater the heating effect the higher the resistance
  • Copper has a low resistance, so it is an ideal material for wires.
Every component
  • Every component all components, including wires and batteries, have some resistance

Section 2 of 10Definition of resistance

Resistance
  • Resistance the ratio of the potential difference across a component to the current through it
  • R = V / I
  • R is in ohms (Ω), V in volts (V) and I in amperes (A).
Unit
  • Unit one ohm is one volt per ampere
  • 1 Ω = 1 V A−1
  • In SI base units, 1 Ω = 1 kg m2 s−3 A−2.
Size of R
  • Size of R the higher the resistance, the lower the current for a given p.d., and vice versa
Unit conversion
  • Unit conversion convert mA to A and kV to V before calculating R in ohms

Section 3 of 10Ohm's law and ohmic components

Ohm's law and ohmic components
Ohm's law
  • Ohm's law the current through a conductor is directly proportional to the p.d. across it, provided temperature and other physical conditions stay constant
Ohmic component
  • Ohmic component a component that obeys Ohm's law
  • Its V–I graph is a straight line through the origin.
  • Its resistance is constant.
Non-ohmic components
  • Non-ohmic components filament lamps and diodes do not obey Ohm's law
  • R = V/I still gives their resistance at any one operating point.
Definition versus law
  • R = V/I defines resistance, whereas Ohm's law states that R is constant for an ohmic conductor

Section 4 of 10Factors affecting resistance

Length
  • Length resistance is directly proportional to length L
  • Doubling the length doubles the number of collisions along the path.
Cross-sectional area
  • Cross-sectional area resistance is inversely proportional to area A
  • A wider wire gives more parallel paths for charge.
Material
  • Material each material has its own resistivity (B.5.6)
Temperature
  • Temperature the resistance of a metal increases as temperature rises
  • Greater ion vibrations cause more frequent collisions.
  • Together these give R = ρL/A.

Section 5 of 10Ideal meters

Ideal voltmeter
  • Ideal voltmeter it has infinite resistance, so no current passes through it
Ideal ammeter
  • Ideal ammeter it has zero resistance, so all the current passes through it
Convention
  • Convention unless stated otherwise, meters are treated as ideal
  • Their resistance is then ignored in the total circuit resistance.
  • If non-ideal meters are used, their resistance is constant.

Section 6 of 10Measuring resistance

1
Connect the component in series with a cell, ammeter and variable resistor (or use a potential divider).
2
Connect a voltmeter in parallel across the component.
3
Change the p.d. in steps and record V and I each time.
4
Keep the current small so that heating does not change R.
5
Plot V against I and draw a best-fit line.
Reading the graph
  • Reading the graph on a V–I graph the gradient of an ohmic resistor is R
  • On an I–V graph the gradient is 1/R, the reciprocal.
  • For a non-ohmic component R at a point is V/I at that point, not the gradient of the tangent.
Uncertainty
  • Uncertainty percentage uncertainties in V and I add to give the percentage uncertainty in R

Section 7 of 10Filament lamp

Rising resistance
  • Rising resistance the filament reaches about 2500 K, so ion vibrations are large and collisions are very frequent
  • The I–V graph of the lamp bends over as the current rises.
  • A cold filament has a much lower resistance than a hot one.
Switch-on
  • Switch-on the current is greatest when the cold filament is first switched on, which is when a bulb is most likely to fail

Section 8 of 10More from the specification

1Internal resistance of electrochemical cells
  • Internal resistance of electrochemical cells potential drop under load reveals internal electrical resistance within chemical battery cells.
2Electric Power
  • The rate at which electrical energy is transferred or dissipated in a circuit component, calculated using the power equation P = VI = I2 R = V2 / R and measured in watts.
3Negative Temperature Coefficient Thermistor
  • A semiconductor temperature sensor whose electrical resistance decreases as temperature increases due to the liberation of free charge carriers.
4Terminal Voltage and EMF
  • The potential difference measured across the terminals of a cell or DC source, given by V = epsilon - Ir,
  • which is lower than the electromotive force when current flows through internal resistance.
5Light-Dependent Resistor
  • A non-ohmic device whose resistance decreases markedly when the intensity of incident light increases, commonly integrated into automatic lighting sensor circuits.
6Thermistor
  • Resistor whose resistance falls as temperature rises.
7Terminal
  • Connection point of a cell, labelled + or −.
8Source
  • Reservoir at TH providing heat QH
9Combined resistance
  • Single resistance that could replace several resistors.
10Circuit diagram
  • Drawing of a circuit using standard symbols.
11Light intensity
  • How bright the light falling on a surface is.
12Fixed resistor
  • Resistor with a resistance that cannot change.

Section 9 of 10Key terms

TermMeaning
ResistanceR = V/I, the opposition of a component to current, in ohms.
OhmOne volt per ampere.
Ohm's lawFor a conductor at constant temperature, current is proportional to p.d.
Ohmic conductorA component for which V/I is constant.
Ideal voltmeterA voltmeter with infinite resistance.
Ideal ammeterAn ammeter with zero resistance.
LatticeThe regular array of positive ions in a solid.

Section 10 of 10Exam technique

1
Writing R = V/I as a proof of Ohm's law — it defines resistance, while Ohm's law says R is constant.
2
Using mA and kΩ without converting to base units — convert before substituting.
3
Believing resistance depends on the p.d. or current applied — for an ohmic conductor it does not.
4
Reading R as the gradient of an I–V graph — the gradient there is 1/R.
5
Forgetting that the resistance of a metal rises with temperature.

Last updated:

\n