Study Guide

Electromagnetism - Common Mistakes

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Common Mistakes

Electric Fields

For multiple point charges, we have to be very careful to add up their contributing electric fields as vectors, or there’s no hope of arriving at a sensible (and correct) answer.

Continuous charge distributions have their own lurking traps: their shapes. Check and double check that the equation used is the equation which best represents the problem.

Electric Potential and Beyond

Gauss’ Law has spared many the student the headache of deriving the electric field of some continuous charge distribution by calculus. It’s there to help.

Watch out for negative signs in electric potential and work calculations. There be monsters.

Lastly, the angle involved in the electric dipole is defined with respect to the electric field lines, and not equipotential lines.

Magnetic Fields

Again, just as for electric fields, make sure the situation described matches the equation chosen. So many parallels. And then again, so many perpendiculars. Remember the right hand rule works only with the right hand.

Remember that a moving point charge does NOT produce a constant magnetic field. We need to consider current distributions in order to apply the laws of magnetostatics.

Magnetic Field Relatives

Double check the constants and their meanings in an equation: many symbols have multiple meanings in electricity and magnetism, which makes it harder to keep track yet keeps us on our toes.

Also, don’t confuse the vector potential A with the scalar potential V. The scalar potential is electric potential, which is the work needed to move a positive electric charge q from infinity to a point in an electric field E.

Electricity and Magnetism Together at Last

We didn’t do much math in this section, so it’s the concepts we need to double check.


The most likely problems are with the math: dividing or taking square roots of numbers in scientific notation has ever been a challenge for the humble physics student. Run through the numbers twice. Always. Even outside of circuits.

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