Physics 7C SS1, Lecture 8: Field Model Finish Electric Begin Magnetic

Potential Energy FieldForce Depend on source and test Depend on only source Vector Scalar PE=qV F=qE

Field Model: Electric A source charge q 1 creates an electric field E m1. Direction is toward negative charge OR away from positive charge. The net field E tot is the vector sum of all the source fields. A test charge q 2, placed in an electric field, experiences an electric  force Magnitude given by: q 2 E tot. Direction of force is: for +q 2 : same direction as E tot for -q 2 : opposite E tot. - +

Quiz 3

Electric Field and Potential: Constant Electric Field Slope of the potential constant as a function of distance. negative Electric field is constant as a function of distance positive

Electric Potential of a point charge: Positive and Negative Charge. Not all potentials are straight lines!

Equipotential Surfaces: Lines where V is the same. Equipotential surfaces for a point charge. Circles differ by 0.5 V. Distance between circles is NOT uniform! Circles get closer and closer toward center Potential grows like 1/r

Field Model: Magnetic A source creates a magnetic field B 1. Direction is. The net field B tot is the vector sum of all the source fields. A test, placed in a magnetic field, experiences an magnetic force Magnitude given by:. Direction of force is:.

A little background Compasses or bar magnets, if allowed, will always orient north-south Why?

A little background Compasses orient in the same direction as the magnetic field.

A little background Iron fillings also orient in the same direction as the magnetic field.

Magnetic Field from a wire If we allow iron fillings freedom to rotate, and put them near a current- carrying wire, this is how they align:

Magnetic Field from a wire If we place compasses around a long current-carrying wire, this is how they align (view is looking down wire)

Right Hand Rule # 1

Which direction is the magnetic field at point A? 1)Into the screen 2)Out of the screen 3)Towards the wire 4)Away from the wire 5)Points down 6)Points up 7)Another direction I  B  A  C

Which direction is the magnetic field at point B? 1)Into the screen 2)Out of the screen 3)Towards the wire 4)Away from the wire 5)Points down 6)Points up 7)Another direction I  B  A  C