Energy and power in electric circuits

Heating up wires

As electrons flow through wires, they collide with the ions in the wire which causes the ions to vibrate more. This increased vibration of the ions increases the temperature of the wire. Energy has been transferred from the chemical energy store of the battery into the internal energy store of the wire.

The amount of energy transferred each second (power) can be calculated using the equation:

power = current × voltage

P = I \times V

This is when:

  • power (P) is measured in watts (W) and is energy transferred (J) / time (s)
  • current (I) is measured in amps (A)
  • voltage (V) is measured in volts (V)

One watt is equal to one joule per second (J/s).

Power can also be written as:

power = current2 × resistance

P= I^{2} \times R

This is when:

  • power (P) is measured in watts (W)
  • current (I) is measured in amps (A)
  • resistance (R) is measured in ohms ( \Omega)

Example

What is the power of a hair dryer if it takes a current of 2 amps (A) from the mains supply that has a potential difference of 230 volts (V)?

P= I \times V

P= 2 \times 230

P= 460\ W

Question

What power is dissipated by a current of 3 A through a 10 \Omega resistor?

P= I^{2} \times R

P = 3^{2 } \times 10

P = 9 \times 10

P = 90\ W

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