The circuit
On the board: a 9 V battery, a switch, a resistor and a capacitor in one loop. A lamp beside the capacitor glows brighter as the capacitor's voltage rises. The switch is open, so nothing flows yet.
Resistors
A resistor makes it harder for current to flow. Its resistance is measured in ohms (Ω). For the same voltage, more ohms means less current. Ours is 1,000 ohms, written 1 kΩ (k means kilo, a thousand).
Coloured bands print the value. Brown = 1 and black = 0 give 10. The third band, red, means multiply by 100: 10 × 100 = 1,000 Ω. The gold fourth band is the tolerance: within 5%, so 950 to 1,050 Ω.
Capacitors
Close the switch and current flows from the battery through the resistor into the capacitor. The capacitor starts empty, so at first the whole 9 V sits across the resistor: 9 V ÷ 1 kΩ = 9 mA (milliamps).
Inside, a capacitor is two metal plates with a thin insulator between them. Charge cannot cross the gap, so it piles up: positive on one plate, an equal negative charge on the other. The more it holds, the higher its voltage.
Capacitance C says how much charge Q a capacitor stores per volt: Q = C × V. It is measured in farads (F). One farad is huge, so parts use microfarads (µF, millionths) and nanofarads (nF, billionths). Ours is 1000 µF.
The RC curve
The time constant τ (tau) is R × C: 1,000 Ω × 0.001 F = 1 second. After one τ the capacitor has reached about 63% of the battery voltage: 5.7 V of 9 V. The lamp glows brighter as it climbs.
As the capacitor fills, its own voltage pushes back against the battery. Less voltage is left across the resistor, so less current flows and filling slows. That is why the curve bends over. After 3 τ it is at 95%.
After 5 τ, 5 seconds here, the capacitor is above 99%: 8.94 V. The current has almost stopped. A bigger resistor or a bigger capacitor makes τ longer, so the same curve takes more time.
Uses
Capacitors smooth the ripple out of power supplies, set the timing of blinkers and timers through R × C, and sit right next to chips as decoupling capacitors: a tiny local reserve for sudden bursts of current.
In detail
;; step 1/10 · circuit Loop: battery + -> switch -> resistor -> capacitor -> battery - Battery: 9 V Resistor: R = 1 kΩ (1,000 ohms) Capacitor: C = 1000 µF Lamp: wired across the capacitor; it only shows the voltage (treated as an ideal meter that draws no current).
The board is a teaching model: the 9 V battery is ideal (no internal resistance) and the lamp is treated as an ideal meter that draws no current. 1 kΩ and 1000 µF are picked so that τ = R × C = 1 s. The colour code is standardised in IEC 60062: two digit bands, a multiplier band and a tolerance band (gold ±5 %, silver ±10 %, brown ±1 %). The tolerance band usually sits a little apart from the others: start reading from the other end. Charging through a resistor: V(t) = V_bat (1 − e^(−t/RC)) and I(t) = (V_bat / R) e^(−t/RC) (OpenStax University Physics Volume 2, section 10.5). After 1 τ the voltage is 63.2 % of the way there, after 5 τ 99.3 %.