Calculations

• Power = energy transferred ÷ time
• Power = work done ÷ time
• Power = potential difference x current
• A TV runs on 3amps and 230V. Calculate the power of the TV

  Power = Potential difference x current

  P = 230 x 3

  = 690 Amps
• A hairdryer is rated 230V, 2kw. Calculate the current

  Power = Potential difference x current

  ( 2 x 1000 ) = 230 x I

  2000 = 230 x I

  I = 2000 / 230

  I = 8.696
• Power = current2 x resistance
• A hairdryer has 10ohm resistance and a current of 4amps. Calculate the power used.

  Power = current2 x resistance

  P = 4 squared x 20

  P = 16 x 10

  P = 160W
• A computer has a power rating of 198W and a resistance of 22. calculate the current.

  Power = current2 x resistance

  198 = I2 x 22

  I2 =198 / 22

  I2 = 9

  I = square root 9

  I = 3A
• Energy = power x time
• A kettle converts electrical energy into heat and has a power rating 250W, transforming 2500 joules of energy per second. If the kettle is on for 1 minute, what is the energy transformed?

  Energy = power x time

  E = 250 x ( 1 x 60 )

  E = 250 x 60

  150,000 joules

  ( remember you must convert minutes to seconds )
• Energy transferred = charge x voltage
• The motor in an electric toothbrush is attached to a 3V battery if a charge of 144c is passed, calculate the energy.

  Energy transferred = charge x voltage

  E = 144 x 3

  E = 432J
• Potential difference = current x resistance
• A diode has a current of 5A running through it, and a resistance of 5Ω. What is the potential difference across the diode?

  Potential difference = current x resistance

  V = 5 x 5

  V = 25V
• If you have a filament bulb and it has a current of 20A running through it, with the potential difference of 100V across it, What is the resistance of the bulb?

  Potential difference = current x resistance

  100 = 20 x R

  R = 100 / 20

  R = 5Ω
• Charge = current x time
• power = voltage x current
• A battery charger passes a current of 2.5A through a cell over 4 hours. Calculate how much charge is transferred to the cell.

  Charge = current x time

  Q = 2.5 x ( 4 x 60 x 60 )

  Q = 2.5 x 14400

  Q = 36,000 C