Electricity and MagnetismElectric FieldElectric FieldElectric FieldThe Electric FieldElectric Field DemoDoes Dipole feel a net Force?Superposition PrincipleExample: Superposition principle for 3 chargesExample: Superposition principle for continous charge distributionExample: Superposition principle for continous charge distributionMore on Fields and Field LinesMore on Fields and Field LinesFeb 15 2002Electricity and Magnetism• Electric field continuedFeb 15 2002Electric Field• New concept – Electric Field E• Charge Q gives rise to a Vector Field•E is defined by strength and direction of force on small test charge qFeb 15 2002Electric Field• For a single charge• Visualize using Field Lines– Cartoon!– Strength -> Density of Lines– Direction -> Direction of Lines • away from positive charges+QFeb 15 2002Electric Field• Field can be used to accelerate charged particlesF = Q E-> Particle AcceleratorsFeb 15 2002The Electric Field• Electric Field also exists is test charge q is not present•We can say:The charge Q gives rise to a property of space itself – the Electric Field-> In-Class Demo...Feb 15 2002Electric Field Demo•Use a Van-der-Graaf Generator• Much more powerful than rubbing glass rods• Not really dangerous (I’ve been told) – but potentially painful• Creates large electric fields• Really big ones were used in Particle Accelerators (still in use in some labs)Feb 15 2002Feb 15 2002Torque τ = p x Ep = Q l DipolemomentFeb 15 2002Does Dipole feel a net Force?NoYesFeb 15 2002Superposition Principle • Field of many charges is Vector Sum of individual fieldsFeb 15 2002Example: Superposition principle for 3 chargesxy+L-L+++0Q1Q2Q3x0E1E2E3r1r2r3)Feb 15 2002Example: Superposition principle for continouscharge distributionxy+L-L0x0Total Charge Q = 2 λ LdEdQ = λ dyrFeb 15 2002Example: Superposition principle for continouscharge distributionxy+L-L0x0Total Charge Q = 2 λ LdEdQ = λ dyrFeb 15 2002More on Fields and Field Lines• What’s wrong with this picture?• Magnitude and direction of field have to be unique at each point!• Field lines can’t cross!EFeb 15 2002More on Fields and Field Lines• Very close to surface of charged object• Field lines perpendicular to surface (if we go close enough)! • Symmetry left and right (like an infinite plane)++ +++++
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