Electricity Part 3: Magnetic fields, Faraday’s Law, Electrical Generation

If a positive and a negative charge are sitting next to each other, which of the following is true? A.The charges will attract each other B.The charges will repel each other C.The charges will neither attract or repel D.None of the above

A material in which charges move freely is a A.Insulator B.Conductor C.Semiconductor D.Convector E.Radiator

A solar car runs on a 100 V battery pack. If the motor draws 10 A of current, how much power is the battery supplying? A.10 W B.100 W C.1000 W D W

A solar car runs on a 100 V battery pack. If the motor draws 10 A of current, what is the resistance of the motor? A.1  B.10  C.100  D.1000 

Magnetic Fields Similar to electric fields, but some differences. Instead of charges, poles called North and South  Similar to +/- charges  Different in that N/S poles always come as a pair.

Like poles repel and unlike poles attract each other. Similar to like charges repel, unlike charges attract Similar to like charges repel, unlike charges attract Magnetic field lines are always closed loops Notation: We usually use the symbol B to represent magnetic fields

Hans Oersted Until 1820, Electricity and Magnetism were considered to be separate phenomena. Oersted discovered that a current carrying wire (moving charges) deflected a compass needle, i.e. currents create magnetic fields

Magnetic Force on a Moving Charge If charge is not moving there is no force. If charge is moving, Force is  to both B and v. If v is || to B there is no force.

Magnitude of the force is F = qv  B v  is the part of the velocity  to the B field. F = qvBsin  F = qv × B Use Right Hand Rule to find the Direction of the force.

Magnetic Force A proton moves at 5.5 x 10 6 m/s at 60º to a magnetic field of 2.5 T. What are the size and direction of the force?

Force on proton moving at 5.5 x 10 6 m/s at 60º to 2.5 T B-field? A.1.7 x N B.8.8 x N C.8.8 x 10 9 N D.1.9 x N

B-field straight down, proton moving from left to right RHR gives direction of force as A.Out of page B.Into page C.To right D.Upward

Magnetic Force A proton moves at 5.5 x 10 6 m/s at 60º to a magnetic field of 2.5 T. What are the size and direction of the force? F = qvBsin  (1.602 x C)(1.0 x 10 5 m/s)(5.5 x T)(.866) F = 1.9 x N

Magnetic Force on Wires Since a current in a wire is moving charges, they also experience magnetic forces. If the wire is  B then the magnitude of the force is

Units of B

Application: Motors Current flow in each side of the wire loop produces a force in opposite directions Causes loop to rotate.

Motional Potential When a wire moves in a B- field the electrons in the wire are moving in the B- field. Results in a magnetic force on the charge of F = qvB In diagram, electrons will try to move down, this leaves + charges behind and creates an E-field along the wire.

Process continues until the electric force and the magnetic force balance each other. qE = qvB qE = qvB E = vB E = vB The voltage difference along the wire is V = Ed = vBd

Example: Tether Experiment

Estimation of tether voltage Orbital velocity of the space shuttle is 7600m/s (17,000 mph) B = 5  T d = 5000 m What is the voltage generated along the wire?

Voltage across tether A.4250 V B V C.1900 V D.425 V

Estimation of tether voltage Orbital velocity of the space shuttle is v = 7600m/s (17,000 mph) B = 5  T d = 5000 m Voltage = vBd = 1900 V It really works, but because B is so small, you either have to go really fast or have a really long wire.

Magnetic Flux Flux is a measure of how much magnetic field passes through a surface  = BA Actually only want the part of B that is perpendicular to the area.

More generally  = BAcos 

Faraday’s Law You can induce a voltage in a loop of wire by changing the magnetic flux through the loop.

Faraday’s Law You can induce a voltage in a loop of wire by changing the magnetic flux through the loop. Three way to change the flux 1.Change A (usually not practical.) 2.Change B (important for a lot of uses) 3.Change  (This is how we usually do it for power generation.)

Generators Basically a “backwards” motor. Instead of running current through the loop to get the shaft to rotate, rotate the shaft to get electrical current. Instead of running current through the loop to get the shaft to rotate, rotate the shaft to get electrical current. This is what is done in essentially all power plants. You run a heat engine/water wheel/wind mill to turn the shaft.