Current Electricity
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Electric Current and Drift
Part of: Current, Drift and Resistance
Current is charge passing a point per second. Inside a metal, free electrons collide constantly yet creep forward slowly when a field is applied; this average creep is the drift velocity, typically a fraction of a millimetre per second even for ampere-level currents.
Why it matters. Every circuit calculation starts from current, and the drift picture explains why wires of different thickness carry the same current with different electron speeds. Confusing drift speed with the near-light signal speed is the classic error.
Formulas
- Current: I = Q / t
- Drift link: I = n e A vd
- Mobility: mu = vd / E
Key words
- Electric current
- : Rate of charge flow past a point, measured in amperes.
- Drift velocity
- : Slow average velocity of electrons along a wire under an applied field.
- Mobility
- : Drift velocity gained per unit electric field, showing how freely charges move.
- Conventional current
- : The direction of positive charge flow, opposite to electron motion.
Try it in two minutes
- Divide charge by time to get the current in amperes.
- Note that electrons move randomly fast but drift slowly.
- Compare drift speeds in a thick and a thin wire carrying equal current.
Try at home. Estimate how long an electron takes to drift one metre at 10^-4 m/s.
Ohm's Law, Resistance and Resistivity
Part of: Current, Drift and Resistance
Ohm's law says current through many materials is proportional to voltage, with resistance as the constant. Resistance grows with length, shrinks with area, and scales with resistivity, a material property. In metals resistivity rises with temperature because hotter ions collide more with electrons.
Why it matters. Choosing wire thickness, heater materials and fuses all depend on these rules. Treating every device as ohmic, or mixing up resistance with resistivity, ruins predictions.
Formulas
- Ohm's law: V = I R
- Wire resistance: R = rho l / A
- With temperature: R = R0 (1 + alpha x delta T)
Key words
- Ohm's law
- : The rule that current through a conductor is proportional to the voltage across it at fixed temperature.
- Resistance
- : Opposition to current in a specific object, measured in ohms.
- Resistivity
- : Material property that sets resistance per unit length and area.
- Non-ohmic device
- : A device such as a diode whose current is not proportional to voltage.
Try it in two minutes
- Divide voltage by current to find resistance.
- Predict the effect of doubling length or halving area.
- Rank copper, nichrome and glass by resistivity.
Try at home. Compare why heater coils use nichrome while supply wires use copper.
Cells, EMF and Electric Power
Part of: Cells, EMF and Electric Power
A cell's emf is its open-circuit voltage; once current flows, part is lost across internal resistance, so the terminal voltage is lower while discharging and higher while charging. Cells in series add emf, while identical cells in parallel give the same voltage with longer life. Electric power is voltage times current.
Why it matters. Batteries sag under load, chargers must exceed emf, and bills are counted in kilowatt-hours. Confusing emf with terminal voltage misreads every loaded circuit.
Formulas
- Terminal voltage: V = E - I r (discharging)
- Charging: V = E + I r
- Power: P = V I = I^2 R = V^2 / R
Key words
- EMF
- : Work per unit charge done by a source, equal to its open-circuit voltage.
- Internal resistance
- : Resistance inside a cell that drops voltage whenever current flows.
- Terminal voltage
- : Actual voltage across a cell's terminals while it works in a circuit.
- Kilowatt-hour
- : Commercial unit of electrical energy equal to 3.6 million joules.
Try it in two minutes
- Subtract the drop across internal resistance from emf for a discharging cell.
- Add emfs of cells joined positive to negative.
- Multiply device voltage by its current for power.
Try at home. Note a torch dimming as cells age and link it to rising internal resistance.
Kirchhoff's Rules and Wheatstone Bridge
Part of: Kirchhoff's Rules and Wheatstone Bridge
Kirchhoff's junction rule says currents entering a point equal those leaving, expressing charge conservation. The loop rule says signed voltage changes around any closed loop sum to zero, expressing energy conservation. A Wheatstone bridge balances when opposite arm ratios match, leaving zero current in the middle branch.
Why it matters. Ohm's law alone cannot solve branched circuits; these two rules crack any network. Sign slips in loop equations and forgetting the zero-current condition are the standard pitfalls.
Formulas
- Junction: currents in = currents out
- Loop: sum of signed voltages = 0
- Balance: R1/R2 = R3/R4
Key words
- Junction rule
- : Currents entering a junction equal currents leaving it, conserving charge.
- Loop rule
- : Voltage changes around any closed loop add to zero, conserving energy.
- Wheatstone bridge
- : Four-arm network used to measure an unknown resistance at balance.
- Balance condition
- : R1/R2 = R3/R4, giving zero current through the middle branch.
Try it in two minutes
- At each junction, set incoming current equal to outgoing.
- Walk each loop, adding drops and subtracting rises to zero.
- Check bridge balance by comparing the two arm ratios.
Try at home. Solve a two-loop circuit on paper and verify currents at one junction.
What your child will practise
- Cells, EMF and Electric Power: Jungle Quick Fire (Quick Fire)
- Electric Current and Drift: Candy Quick Fire (Quick Fire)
- Electric Current and Drift: Space Puzzle (Clue Puzzle)
- Kirchhoff's Rules and Wheatstone Bridge: Desert Quick Fire (Quick Fire)
- Ohm's Law, Resistance and Resistivity: Jungle Puzzle (Clue Puzzle)
- Quiz: Cells, EMF and Electric Power (8 questions)
- Quiz: Electric Current and Drift (8 questions)
- Quiz: Kirchhoff's Rules and Wheatstone Bridge (8 questions)
- Quiz: Ohm's Law, Resistance and Resistivity (8 questions)