OCR A Level Physics

Revision Notes

A LevelPhysicsOCRRevision Notes6. Particles & Medical Physics6.4 Electric Potential & Energy6.4.4 Force-Distance Graph

6.4.4 Force-Distance Graph

Test Yourself

Force-Distance Graph for a Point Charge

  • The key features of the force-distance graph are:
    • The values for F are all positive
    • As r increases, F against r follows a 1/r2 relation (inverse square law)
    • The area under this graph is the work done ΔW
    • The graph has a steep decline as r increases

  • The area under the graph can be estimated by counting squares, if it is plotted on squared paper, or by splitting it into trapeziums and summing the area of each trapezium
    • The inverse square law relation means that if the distance r increases by a factor of 2, F would decrease by a factor of 4
  • This is a graphical representation of the equation:

F equals fraction numerator Q q over denominator 4 straight pi epsilon subscript italic 0 r to the power of italic 2 end fraction

  • Where Q, q and 4πε0 are constants

6-4-4-electric-force-distance-graph_ocr-al-physics

Exam Tip

Drawing, interpreting or calculations using graphs are common exam questions. The graph of F against r should start off steeper and decrease rapidly compared to that of V against r, to distinguish it as an inverse square law (1/r2) relation instead of just 1/r.

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