Physics notes · Chapter 9 of 11
Current Electricity
13 sections, 64 flashcards. Open a card to check your answer, or revise them in the app with spaced revision.
Electric Charge and Current
- Two kinds of charge, positive and negative: like charges repel, unlike charges attract.
- In a metal, charge is carried by free electrons (negative). One electron carries about 1.6 × 10⁻¹⁹ C; the unit of charge is the coulomb (C).
- Electric current is the flow of charge through a conductor: I = Q / t, in amperes (A): 1 A = 1 coulomb per second.
- Electrical Quantities and Units
| Quantity | Symbol | SI Unit |
|---|---|---|
| Charge | Q | coulomb (C) |
| Current | I | ampere (A) |
| Potential difference | V | volt (V) |
| Resistance | R | ohm (Ω) |
| Power | P | watt (W) |
Check yourself
Two kinds of electric charge
Positive (+) and negative (−); like charges repel, unlike charges attract.
What carries charge inside a metal?
Free electrons, which move freely and carry negative charge.
Define electric current
The flow of electric charge through a conductor.
Unit of charge; charge on one electron
Coulomb (C); one electron carries about 1.6 × 10⁻¹⁹ C.
Relate current, charge and time
I = Q / t.
SI unit of current
The ampere (A): 1 A = 1 coulomb per second. (Ampère)
Conventional Current vs Electron Flow
- Conventional current flows from the + terminal to the − terminal outside the cell. It is measured by an ammeter, always connected in series.
Current density (J = I/A) is a vector, though current itself is a scalar.
- Conventional Current vs Electron Flow
| Aspect | Conventional Current | Electron Flow |
|---|---|---|
| Direction | + to − (outside cell) | − to + (outside cell) |
| Carrier | Assumed positive charge | Actual electrons |
| Usage | Standard in circuits | Physical reality |
Check yourself
Direction of conventional current
From the + terminal to the − terminal outside the cell.
Actual direction of electron drift
The opposite way, from − to + — but the old convention is still used.
How is current measured?
By an ammeter, always connected in series.
Define current density
Current per unit cross-sectional area (J = I / A); it is a vector quantity.
History of Electromagnetism
- Volta (1800): the first battery (voltaic pile); the volt is named after him.
- Oersted (1820): a current-carrying wire deflects a compass needle, so a current produces a magnetic field.
- Ohm (1827): Ohm's law. Faraday (1831): electromagnetic induction, moving magnets creating electricity.
- Maxwell and Lorentz united electricity and magnetism into one theory, electromagnetism.
- History of Electromagnetism
| Year | Scientist | Contribution |
|---|---|---|
| 1800 | Alessandro Volta | First battery (voltaic pile); unit volt |
| 1820 | H. C. Oersted | Current produces magnetism |
| 1827 | Georg Simon Ohm | Ohm's law |
| 1831 | Michael Faraday | Electromagnetic induction |
Check yourself
Oersted's discovery (1820)
A current-carrying wire deflects a compass needle — proving an electric current produces a magnetic field.
Volta's contribution (1800)
The first battery (voltaic pile); the unit volt.
Ohm's contribution (1827)
Ohm's law.
Faraday's contribution (1831)
Electromagnetic induction — moving magnets create electricity.
Who united electricity and magnetism?
Maxwell and Lorentz, into one theory called electromagnetism.
The Electric Circuit
- An electric circuit is a complete closed loop for current; a bulb glows only when it is complete, and any gap stops the current.
- A cell provides the push; a battery is two or more cells joined for a bigger push. A switch or plug key opens or closes the circuit.
Check yourself
Define an electric circuit
A complete closed loop through which current can flow.
When does a bulb glow?
Only when the circuit is complete; any gap stops the current.
Cell vs battery
A cell provides the push; a battery is two or more cells joined for a bigger push.
What does a switch or plug key do?
Opens (breaks) or closes (completes) the circuit.
Potential Difference (Voltage)
- Potential difference (voltage): the electrical pressure that pushes current between two points; unit the volt (V): 1 V = 1 joule per coulomb.
- It is measured by a voltmeter, always connected in parallel across the part.
- A charge moved through a p.d. V gains energy E = eV joules.
Check yourself
Define potential difference
The electrical pressure that pushes current between two points.
How is voltage measured?
By a voltmeter, always connected in parallel across the part.
Is electric potential a scalar or vector?
A scalar quantity.
Energy gained by a charge through p.d. V
E = eV joules.
SI unit of voltage
The volt (V): 1 V = 1 joule of energy per coulomb of charge.
Conductors, Insulators and Semiconductors
- Conductors have many free electrons: copper, silver, aluminium, the human body, the earth. Insulators have very few: glass, porcelain, plastic, rubber, dry wood, dry air.
- Semiconductors (silicon, germanium) conduct between the two, and their conduction can be controlled.
Distilled water is a bad conductor; salty or acidic water conducts.
- Conductors, Insulators, Semiconductors
| Type | Free electrons | Examples |
|---|---|---|
| Conductor | Many | Copper, silver, aluminium |
| Insulator | Very few | Glass, plastic, rubber |
| Semiconductor | Few, controllable | Silicon, germanium |
Check yourself
Why do conductors pass electricity easily?
They have many free electrons — e.g. copper, silver, aluminium, the human body, the earth.
Why do insulators resist current?
They have very few free electrons — glass, porcelain, plastic, rubber, dry wood, dry air.
Define semiconductors
Silicon, germanium — conduct between metals and insulators and can be controlled.
Effect of heating on metal vs semiconductor resistance
A metal's resistance increases; a semiconductor's resistance decreases.
Is distilled water a conductor?
No — pure water is a bad conductor; salty/acidic water conducts.
Resistance and Ohm's Law
- Resistance: a material's opposition to current; unit the ohm (Ω): 1 Ω lets 1 A flow when 1 V is applied.
- Ohm's law (1827): at constant temperature, current is directly proportional to voltage: V = I × R (so R = V/I, I = V/R).
- A rheostat is a variable resistor, used to change the current in a circuit.
Check yourself
Define resistance
The opposition a material offers to the flow of current.
State Ohm's law (1827)
At constant temperature, current is directly proportional to voltage.
Ohm's law in symbols
V = I × R, rearranged as R = V/I and I = V/R.
Define a rheostat
A variable resistor used to change the current in a circuit.
SI unit of resistance
The ohm (Ω): 1 Ω allows 1 A when 1 V is applied. (Ohm)
Factors Affecting Resistance
- Resistance increases with length, decreases with thickness (area), and depends on the material and its temperature: R = ρL / A.
- Resistivity (ρ) is a fixed property of the material, whatever the wire's shape.
- Factors Affecting Resistance
| Factor | Effect on Resistance |
|---|---|
| Longer length | More resistance |
| Greater area (thicker) | Less resistance |
| Higher temperature (metal) | More resistance |
| Material | Depends on resistivity |
Check yourself
Effect of length and thickness
Resistance increases with length and decreases with thickness (area).
Other factors affecting resistance
The material of the wire and its temperature.
Combine them in one formula
R = ρL / A, where ρ is resistivity.
Define resistivity (ρ)
A fixed property of the material, independent of wire shape.
Resistors in Series and Parallel
- Series: resistors end to end, the same current through each; R = R₁ + R₂ + R₃, larger than the biggest.
- Parallel: both ends joined, the same voltage across each; 1/R = 1/R₁ + 1/R₂ + 1/R₃, smaller than the smallest. Adding resistors in parallel increases the total current.
- Series vs Parallel
| Property | Series | Parallel |
|---|---|---|
| Current | Same in all | Splits among branches |
| Voltage | Splits across each | Same across each |
| Total R | R₁+R₂+… (larger) | 1/R = 1/R₁+1/R₂ (smaller) |
Check yourself
Series combination: current and formula
Resistors joined end-to-end; the same current flows through each. R = R₁ + R₂ + R₃, always larger than the biggest one.
Parallel combination: voltage and formula
Both ends joined; voltage is the same across each. 1/R = 1/R₁ + 1/R₂ + 1/R₃, always smaller than the smallest one.
Where is each used?
Series where one switch must control all; parallel is used in home wiring.
Adding more resistors in parallel does what?
Increases total current (lowers resistance).
Electric Power and Energy
- Electric power: the rate at which electrical energy is used, in watts: P = V × I = I²R = V²/R.
- Energy = power × time; the commercial unit is the kilowatt-hour (kWh), 1 “unit”: the energy a 1 kW device uses in 1 hour.
- Example: 5 bulbs of 100 W for 20 hours = 5 × 100 × 20 = 10,000 Wh = 10 kWh (10 units).
Check yourself
Define electric power and its unit
The rate at which electrical energy is used; SI unit the watt (W).
Three power formulas
P = V × I = I²R = V²/R.
Which forms do NOT give power?
V × Q or I × t — those denote other quantities.
Electrical energy and its commercial unit
Energy = Power × Time; commercial unit is the kilowatt-hour (kWh).
Define 1 unit of electricity
1 kWh = energy used by a 1 kW device in 1 hour.
Example: 5 bulbs of 100 W for 20 hours
5 × 100 × 20 = 10,000 Wh = 10 kWh (10 units).
Heating Effect and Safety Devices
- Heating effect: a current through a resistor produces heat, H = I²Rt, used in heaters, irons, geysers and bulb filaments.
- A fuse protects equipment by melting and breaking the circuit when the current is too high.
- In a short circuit the resistance falls and the current becomes dangerously large. Earthing gives leaked current a safe path to the ground, protecting users from shock.
Check yourself
Define the heating effect of current
Current passing through a resistor produces heat — H = I²Rt.
Where is the heating effect used?
Electric heaters, irons, geysers and bulb filaments.
What does a fuse do?
Protects equipment by melting and breaking the circuit when current is too high.
What is fuse wire made of?
A low-melting alloy of lead and tin with high resistance.
What happens in a short circuit?
Resistance falls and the current becomes very large (dangerously high).
What does earthing do?
Protects users from electric shock by giving leaked current a safe path to the ground.
AC Concepts: Reactance
- Capacitive reactance, the opposition a capacitor offers to alternating current: Xc = 1 / (2πfC), in ohms.
- Inductive reactance: Xl = 2πfL, which rises with frequency.
- Reactance and Frequency
| Element | Reactance | Effect of frequency |
|---|---|---|
| Capacitor | Xc = 1/(2πfC) | Decreases as f rises |
| Inductor | Xl = 2πfL | Increases as f rises |
Check yourself
Define capacitive reactance
The opposition a capacitor offers to current in AC circuits.
How does reactance relate to frequency?
Inversely proportional — higher frequency gives lower reactance.
Does a capacitor pass AC or DC?
It blocks DC (zero frequency → infinite reactance) but passes AC.
Define inductive reactance
Xl = 2πfL, which rises with frequency.
Capacitive reactance formula
Xc = 1 / (2πfC), measured in ohms.
Thermoelectric and Photoelectric Effects
- Thermoelectric (Seebeck) effect: temperature differences along a conductor create a potential difference; a thermocouple works on it.
- Photoelectric effect: electrons are emitted when light falls on a surface; solar cells use it to turn light into electricity.
Check yourself
Define the thermoelectric (Seebeck) effect
Temperature differences along a conductor cause potential variation.
What device works on the Seebeck effect?
A thermocouple.
Define the photoelectric effect
The emission of electrons when light falls on a surface.
Best photoelectric device for digital applications
A photodiode, due to its fast response.
How do solar cells work?
They use the photoelectric effect to convert light into electricity.
All 11 chapters of Physics notes
- Units, Measurement & Physical Quantities11 sections
- Motion, Laws of Motion & Forces13 sections
- Gravitation11 sections
- Work, Power, Energy & Conservation13 sections
- Properties of Matter & Fluids12 sections
- Heat & Thermodynamics12 sections
- Waves & Sound11 sections
- Light & Optics15 sections
- Current Electricity13 sections
- Magnetism & Electromagnetism15 sections
- Modern & Nuclear Physics16 sections