Galvanometer (Cambridge (CIE) AS Physics) : Revision Note

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The galvanometer

  • A galvanometer is a type of sensitive ammeter used to detect electric current

  • It is used in a potentiometer to measure e.m.f between two points in a circuit

  • The circuit symbol is recognised by an arrow in a circle:

 

Galvanometer circuit symbol

Galvanometer circuit symbol, downloadable AS & A Level Physics revision notes

The galvanometer circuit symbol is a circle with an arrow which deflects

  • A galvanometer is made from a coil of wire wrapped around an iron core that rotates inside a magnetic field:

 

A galvanometer

Galvanometer, downloadable AS & A Level Physics revision notes

A galvanometer contains a coil of wire wrapped around an iron core between magnets

  • The arrow represents a needle which deflects depending on the amount of current passing through

    • When the arrow is facing directly upward, there is no current

    • This is called null deflection

  • Ohm’s law tells us that the current through a conductor (wire) is directly proportional to the potential difference through it i.e. no p.d means no current flows through the galvanometer

  • A galvanometer has a p.d of zero when the potential on one side equals the potential on the other side

  • This is at the position at which it is connected on the wire (which varies with the sliding contact) gives a p.d equal to the EMF of the cell connected to the galvanometer

  • The cell should be connected such that its potential opposes the potential on the wire i.e. the positive terminal of the power supply faces the positive terminal of the cell:

A null galvanometer

Galvanometer circuit diagram, downloadable AS & A Level Physics revision notes

A circuit diagram showing a null galvanometer. The voltage V1V2

 

  • When the sliding contact moves along the potentiometer wire, you add or remove resistance from/to the external circuit. This changes the potential drop across X and Y

  • The location of the sliding point is adjusted until the galvanometer reads zero. This is until the potential difference equals E2

  • The direction of the two e.m.fs oppose each other and there is no current

Examiner Tips and Tricks

If you’re unsure as to whether the p.d will increase as the contact slider is moved along the wire, remember p.d is proportional to the length of the wire (from Ohm’s law and the resistivity equation). The longer the length of a wire, the higher the p.d.

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Ashika

Author: Ashika

Expertise: Physics Content Creator

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.