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Heating Effect of Current (DP IB Physics: SL)

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Katie M

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Heating Effect of Current

  • When electricity passes through a component, such as a resistor, some of the electrical energy is turned into heat, therefore, increasing its temperature
    • The heat that is produced will dissipate (spread out) into the environment via thermal conduction, convection and radiation

  • When electricity passes through a component, there is energy transferred to heat
  • This is due to collisions between:
    • Electrons flowing in the conductor, and
    • The lattice of atoms within the metal conductor

  • Electricity, in metals, is caused by a flow of electrons
    • This is called the current

  • Metals are made up of a lattice of ions
  • As the electrons pass through the metal lattice they collide with ions
    • The ions resist the flow of the electrons

Electrons and resistance, downloadable AS & A Level Physics revision notesAs electrons flow through the metal, they collide with ions, making them vibrate more

  • When the electrons collide, they lose some energy by giving it to the ions, which start to vibrate more
    • As a result of this, the metal heats up

  • This is used to an advantage to generate heat for appliances such as electric hobs

Heating, downloadable IGCSE & GCSE Physics revision notes

The heating effect of current can be used for many applications such as electric hobs

Power Dissipation

  • When an electrical current does work against electrical resistance:
    • Electrical energy is dissipated as thermal energy in the surroundings
    • The heat that is produced will dissipate via thermal conduction, convection and radiation

  • The amount of heat produced depends on two factors:
    • Current: The greater the current, the more heat that is produced
    • Resistance: The higher the resistance, the more heat that is produced (for a given current)

  • Note that reducing the resistance can cause the current to increase
    • This could actually increase the amount of heat produced

  • In mechanics, power P is defined as the rate of doing work
    • The potential difference is the work done per unit charge
    • Current is the rate of flow of charge

  • Therefore, the electrical power is defined as the rate of change of work done:

Power equation, downloadable AS & A Level Physics revision notes

  • The work done is the energy transferred so the power is the energy transferred per second in an electrical component

  • The power dissipated (produced) by an electrical device can also be written as

Power equation, downloadable AS & A Level Physics revision notes

  • Using Ohm's Law V = IR to rearrange for either V or I and substituting into the power equation, means power can be written in terms of resistance R

Power in terms of resistance, downloadable AS & A Level Physics revision notes

  • This means for a given resistor if the current or voltage doubles the power will be four times as great.
    • Which equation to use will depend on whether the value of current or voltage has been given in the question

  • Rearranging the energy and power equation, the energy can be written as:

E = VIt

  • Where:
    • E = energy transferred (J)
    • V = voltage (V)
    • I = current (A)
    • t = time (s)

Worked example

Two lamps are connected in series to a 150 V power supply.WE - power question image, downloadable AS & A Level Physics revision notesWhich 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

Worked example - power (2), downloadable AS & A Level Physics revision notes

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.