Week 8

Electricity and Magnetism Chap 8,9


Sensors and Motors are often based on electronics and magnetism




E&M is a complex fundamental interaction. This interaction is responsible for most of the features we experience in our world. Basis for atoms and molecules. Determines material properties and interactions. Most phenomena (excluding gravity, falling and earth-sun-moon binding) can be seen as an E&M interaction.

2 charges +,-


Electron - , point particle

Proton + , made of quarks

No individual N or S poles found

Due to electric currents

drops with distance

act along line joining charges

like repel, unlike attract

r=distance between charges

= unit vector it shows direction only

= provides magnitude of force, sign

+ means repulsion, neg means attraction

Same characteristics as electric force only for N,S poles


While originally the interactions in the columns appeared similar they were considered to be distinct, unrelated interactions. However, the forces were linked when electric currents were found to have magnetic interactions (electromagnet).


Electric current

Measured in Amps A


Amount of charge that passes through an imaginary slice in a wire per second

1 A = 1 C/s

Analogs: water flows in rivers (flow amount passes by per second

Flow of cars on highway (flow # cars pass under bridge per second

Therefore flow depends on two quantities speed, amount

Few cars traveling fast = high flow

Many cars traveling slow high flow

Flow at 2:00 AM (90 MPH) i.e. the # under the bridge in a given time can be the same as the flow for a traffic jam (1 MPH).

Mississippi can have slow moving sections where the river is wide and faster sections where the river is narrow but the flow is the same.



Push that causes the flow

Analog: For rivers flow is due to gravity Flowing down hill

Analog: For skiers its due to gravity (vertical drop)

Analog: Gravity can be used to propel material along a path the amount of push can be related to the height h.

In trying to understand voltage we can consider things that flow down hill due to gravity. We can also identify the height that a skier descends or a river traverses as an important factor in determining flow. Indeed we can imagine that voltage can be directly linked to the height for a system that has material flowing due to gravity. This link allows us to develop some intuition about voltage and voltage distribution in a circuit.


Thursday -----------------------------


E&M film on Maxwells equations


Hand back the test.