User manual

12
Voltage, resistance and current
We already know that electrical voltage is measured in volts
(V). The battery has 9 V. Resistance is measured in ohms
(Ω) or kilo ohms (1 kΩ = 1,000 Ω). But there is another very
important measurement: electrical current is measured in am-
peres (A) or in milliamps (1 mA = 1/1000 A) in the case of low
currents. All of these names are originally the names of famous
researchers, who were researching electricity some 200 years
ago: Alessandro Volta, Georg Simon Ohm and André-Marie
Ampère.
You could use a meter to measure the current that is flowing
through the LED. However, if you know the actual battery
voltage and the voltage at the LED, you could calculate the
current flow yourself. If the battery is new, it has a voltage
of 9 V. The two LEDs together need about 4 V. We still have
5 V left for the resistor. Now you can calculate the greater
brightness:
Current = voltage / resistance
Current = 5 V / 4700 Ω
Current = 0.0011 A = 1.1 mA
That’s not much. The current flow is only 1.1 mA, while the
LED tolerates a current of 20 mA. The battery will last a long
time! The battery usually has a capacity of 500 milliamps
(500 mAh) and could deliver 500 mA for one hour or 1 mA for
500 hours. Or, the lamp can glow for about 450 hours at 1.1
mA; that is almost three weeks.