Power (OCR AS Physics): Revision Note

Two lamps are connected in series to a 150 V power supply.

Which statement most accurately describes what happens?

A. Both lamps light normally

B. The 15 V lamp blows

C. Only the 41 W lamp lights

D. Both lamps light at less than their normal brightness

Answer: A

Step 1: State the power equation

P = IV

Step 2: Rearrange for I

$I=\frac{\mathrm{P}}{\mathrm{V}}$

Step 3: Substitute in values to calculate

• For the 41W lamp:

 $I=\frac{41\mathrm{W}}{135\mathrm{V}}=0.3\mathrm{A}$

• For the 4.5W lamp:

 $I=\frac{4.5\mathrm{W}}{15\mathrm{V}}=0.3\mathrm{A}$

• For both to operate at their normal brightness, a current of 0.3 A is required

• Since the lamps are connected in series, the same current would flow through both

• Therefore, the lamps will light at their normal brightness

• This is option A

You can use the mnemonic “Twinkle Twinkle Little Star, Power equals I squared R” to remember whether to multiply or divide by resistance in the power equations. When a calculation links power and energy (E = Pt), always make sure the time is in seconds, because 1 W = 1 J s^-1