Chapter 19 - Electric Field

How are field lines drawn for + and - charges?

  • For + charges: lines pointing out

  • For - charges: lines pointing in

What is the equation for electric fields?

  • E=FeqE=\frac{Fe}{q}

  • Fe and E are in the same direction

What is electric potential energy?

  • PE=mghPE = mgh

  • KE=½mv2KE = ½ mv²

  • W=FdW = Fd =−ΔPE=ΔKE=-\Delta PE=\Delta KE

  • Electric field exists between two collections of charges → put a charge in the E → +q will go towards collection of - charges → as it moves, it loses PEe and gains KEe

    • Work is done because there is Fe and displacement

    • A + charge has high PEe when next to a collection of positive charges, low PEe when next to a collection of negative charges

    • A - charge will have low PEe when next to a collection of positive charges, high when next to negative charges

  • E=FeqE=\frac{Fe}{q}

  • W=Fe⋅dW=Fe\cdot d

  • W=−ΔPEW=-\Delta PE

  • We=Eq⋅dWe=Eq\cdot d

    • d is distance between plate a and plate b

  • ΔPE=PEb−PEa=−Eq⋅d\Delta PE=PEb-PEa=-Eq\cdot d

  • A charge in E will be accelerating because KEe is changing

What is electric potential?

  • What is a closed circuit?

    • Positive and negative of a battery connected through a bulb

    • Electrons move from - to + of the battery

      • Electron has - charge, therefore moves away from the - charges collected on the - side of the battery

      • As it does so, it does work, and PEe decreases

    • −ΔPEe=W-\Delta PEe=W → lights up a bulb attached to the circuit

  • What is electric potential? PEe lost per unit charge

    • Vba=ΔPEqVba=\frac{\Delta PE}{q} =Vb−Va=(PEb−PEa)q=Vb-Va=\frac{\left(PEb-PEa\right)}{q}

    • Va=PEaqVa=\frac{PEa}{q} at the a terminal

      • Same goes for b terminal

    • In volts: V=JCV=\frac{J}{C}

  • In a 12-volt battery, Vb−Va=12Vb-Va=12

    • Ground is always zero volts; high potential is 12 volts

  • W=−q(Vb−Va)=Eqd=FdW=-q\left(Vb-Va\right)=Eqd=Fd

  • ΔV=Ed\Delta V=Ed

  • E=ΔVdE=\frac{\Delta V}{d}

  • KEe=qVbaKEe=qVba

    • Electric field decreases as distance increases

What is a current?

  • Flow of charge

  • Depends upon a potential difference (charge will not move without this)

  • The larger the charge moving, the larger the PEe is

What are electron volts (eV)?

  • Energy gained by an electron when it moves to a potential difference of 1V

  • 1V=ΔPE1e−1eV=1.6⋅10−191V=\frac{\Delta PE}{1e^{-}}1eV=1.6\cdot10^{-19}

What is electric potential due to a point charge?

  • V=kQrV=\frac{kQ}{r}

  • Fe=kQqr2Fe=\frac{kQq}{r^2}

  • E=Feq=kQr2E=\frac{Fe}{q}=\frac{kQ}{r^2}

  • Q is a source charge (always positive)(ex: a Vandegraaff generator)

What is equipotential?