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On the syllabus: GCSE PhysicsA-Level Physics
Your phone charger barely gets warm, but if you touched the element inside a toaster you'd be burned instantly. Both are resistors. Both are plugged into the same 230 V wall socket. So why does one almost melt and the other stay cool? The answer reveals why every electrical device on Earth is designed around exactly one equation.
Your phone charger barely gets warm, but if you touched the element inside a toaster you'd be burned instantly. Both are resistors. Both are plugged into the same 230 V wall socket. So why does one almost melt and the other stay cool? The answer reveals why every electrical device on Earth is designed around exactly one equation.
Both convert electrical energy to heat, but the toaster element has a far lower resistance and carries a much higher current. The power consumed — the rate of energy conversion — depends on both voltage and current. Understanding this relationship explains everything from why fuses blow to how electric heaters are rated.
Electric current (I) is the rate of charge flow. Ohm's Law states that voltage (V) equals current times resistance (I × R). Power is the rate of energy conversion — it equals voltage times current, or equivalently V²/R or I²R.
Georg Ohm discovered in 1827 that for many conductors, current is directly proportional to applied voltage. Resistors that follow this are 'ohmic'. The three forms of the power formula are equivalent via V=IR: P=VI=(IR)I=I²R, or P=VI=V(V/R)=V²/R. Power lines use high voltage (400,000 V) to transmit power efficiently: the same power at higher voltage means lower current, and since line losses scale as I²R, this can reduce losses by 99% compared to low-voltage transmission.