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Theme B — The Particulate Nature of Matter

B.5 Current and Circuits — IB Physics HL Revision Notes & Topical Questions

5 min read 13 parts Lesson 41 of 53 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Lesson summary

B.5 Current and Circuits — IB PHYSICS HL notes: Define current, potential difference, resistance, resistivity, power and e.m.f.

  • Energy source
  • Complete circuit
  • Components
  • Definition
  • Conventional current
  • Electron flow

Part 1 of 11Circuit diagrams and meters

Circuit diagrams and meters
Energy source
  • Energy source a cell, battery or power supply provides the potential difference
  • A cell converts chemical energy to electrical energy and a battery is several cells
Complete circuit
  • Complete circuit current flows only in a closed loop
  • A switch opens or closes the loop
Ammeter
  • Ammeter measures current and is connected in series
  • An ideal ammeter has zero resistance
Voltmeter
  • Voltmeter measures potential difference and is connected in parallel across the component
  • An ideal voltmeter has infinite resistance
Components
  • Components fixed resistor, variable resistor, LDR, thermistor, potentiometer, lamp, LED, heating element, motor and earth

Part 2 of 11Current and potential difference

1
Definition
  • Definition current is the rate of flow of charge, I = Δq/Δt
  • 1 A = 1 C s−1
2
Conventional current
  • Conventional current the flow of positive charge from the positive to the negative terminal
3
Electron flow
  • Electron flow electrons move the opposite way, from the negative to the positive terminal
4
Direct current
  • Direct current flows in one direction only and is produced by cells and batteries
5
Definition
  • Definition potential difference is the work done per unit charge moving a positive charge between two points, V = W/q
  • 1 V = 1 J C−1
6
Electronvolt
  • Electronvolt the energy needed to move an electron through a potential difference of 1 V
  • 1 eV = 1.6 × 10−19 J from W = eV
7
Cells
  • Cells charge gains electrical potential energy passing through the cell and transfers it to metal ions in the wire
8
Conductor
  • Conductor allows charge to flow easily, for example silver, copper, aluminium and steel
  • Metals have a lattice of positive ions in a sea of delocalised electrons
9
Insulator
  • Insulator has very few free charges, for example rubber, plastic, glass and wood
  • An insulator can still hold static charge on its surface

Part 3 of 11Conductors, resistance and Ohm's law

Conductors, resistance and Ohm's law
Cause of resistance
  • Cause of resistance electrons collide with lattice ions and transfer energy that heats the wire
Definition
  • R = V/I in ohms
  • 1 Ω = 1 kg m2 s−3 A−2
Ohm's law
  • Ohm's law for a component at constant temperature the current is proportional to the p.d.
  • The I–V graph is a straight line through the origin
Ohmic components
  • Ohmic components fixed resistors and metal wires at constant temperature
Non-ohmic components
  • Non-ohmic components filament lamps, diodes, LEDs, thermistors and LDRs
Gradient
  • Gradient with I on the y-axis, R = 1/gradient

Part 4 of 11Resistivity

Dependence
  • Dependence resistance is proportional to length L and inversely proportional to cross-sectional area A
Definition
  • ρ = RA/L in ohm-metres
  • It is a property of the material and depends on temperature
Wire area
  • A = πr2 so doubling the diameter quadruples the area and cuts the resistance to a quarter
Metals
  • Metals low resistivity so they make good wires
Insulators
  • Insulators very high resistivity so they safely cover wires

Part 5 of 11I–V characteristics

I–V characteristics
Fixed resistor
  • Fixed resistor a straight line through the origin
Diode and LED
  • Diode and LED forward-biased they show a sharp rise in current
  • Reverse-biased the current is zero
Filament lamp
  • Filament lamp nearly ohmic at low voltage
  • The filament heats up as V rises, its resistance increases and the graph curves with a decreasing gradient

Part 6 of 11Series and parallel circuits

FeatureSeriesParallel
CurrentSame everywhereSplits, I = I1 + I2 + …
Potential differenceSplits, V = V1 + V2 + …Same across each branch
Total resistanceR = R1 + R2 + … (larger than any one)1/R = 1/R1 + 1/R2 + … (smaller than any one)
One component openAll components stopOther branches continue
  • Equal resistors
    • two in series double the resistance and two in parallel halve it

Part 7 of 11Electrical power

Heating effect
  • Heating effect electrical energy is dissipated as thermal energy by current in a resistance
Equations
  • P = E/t = IV = I2R = V2/R
    What the symbols mean: P = power (W = J s−1) · W or E = work done / energy transferred (J) · t = time (s) · F = force (N) · v = velocity (m s−1)
  • E = VIt
    What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
Doubling
  • Doubling doubling the current or voltage through a fixed resistor gives four times the power

Part 8 of 11Sources of electrical energy

Chemical cells
  • Chemical cells single-use cells such as alkaline are non-rechargeable
  • Lithium-ion and lead-acid cells are rechargeable
Solar cells
  • Solar cells photons release electrons from the surface of photovoltaic cells
  • Output is variable, daytime only and needs large areas
Wind generators
  • Wind generators no fuel cost but the output is inconsistent and needs windy sites
Mains from fossil fuels
  • Mains from fossil fuels reliable with high energy density but unsustainable and polluting

Part 9 of 11E.m.f. and internal resistance

E.m.f.
  • E.m.f. the energy converted from chemical to electrical per coulomb passing through the supply
  • It equals the terminal p.d. when no current flows
Internal resistance r
  • Internal resistance r resistance inside the source that wastes energy and warms the cell
Equation
  • ε = I(R + r)
    What the symbols mean: E or ε = e.m.f. (V) · I = current (A) · R = external resistance (Ω) · r = internal resistance (Ω) · V = terminal p.d. (V)
  • Lost volts Vr = Ir and terminal p.d. V = ε − Ir
V–I graph
  • V–I graph the intercept is ε and the gradient is −r

Part 10 of 11Variable resistance and sensors

Thermistor
  • Thermistor an NTC thermistor has resistance that falls as temperature rises
  • It is used in ovens, fire alarms and digital thermometers
LDR
  • LDR resistance falls as light intensity rises
  • It is millions of ohms in the dark and tens of ohms in bright light and is used in automatic street lights
Potentiometer
  • Potentiometer a resistor with a sliding contact used as an adjustable potential divider
  • The output varies from zero to the supply p.d.
Potential divider
  • V1 = V × R1/(R1 + R2)

Part 11 of 11More from the specification

1
Circuit diagrams
  • Circuit diagrams standard symbols show cells, resistors, ammeters and voltmeters and how they are connected.
2
Current flowing
  • Current flowing current is the rate of charge flow, I = Q divided by t, the same at all points in series.
3
Charge passes
  • Charge passes the charge passing a point in a time equals current times time, measured in coulombs.
4
Combined resistance
  • Combined resistance in series it is the sum of resistances, in parallel the reciprocals add.
5
Kirchhoff's laws
  • Kirchhoff's laws currents at a junction sum to zero, and voltages around a loop sum to zero.
6
Combined resistance
  • Single resistance that could replace several resistors.
7
Circuit diagram
  • Drawing of a circuit using standard symbols.

Key terms

TermMeaning
Electric currentThe rate of flow of charge.
Potential differenceThe work done per unit charge in moving charge between two points.
Electromotive forceThe energy per unit charge transferred to electrical energy by a source.
ResistanceThe ratio of the potential difference across a component to the current in it.
ResistivityA material property equal to RA/L.
Internal resistanceThe resistance of the materials inside a source of e.m.f.
Ohmic conductorA conductor for which current is proportional to p.d. at constant temperature.
Potential dividerA series arrangement of resistors that provides a fraction of the supply p.d.

Avoid these mistakes

1
Connecting an ammeter in parallel or a voltmeter in series — the ammeter goes in series and the voltmeter in parallel.
2
Saying all conductors obey Ohm's law — only components with constant resistance at constant temperature do.
3
Treating resistivity as the same as resistance — resistivity is a material property while resistance depends on shape.
4
Using the load resistance alone in ε = I(R + r) — the internal resistance must be added.
5
Forgetting to convert mA and kV to A and V before finding resistance.

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