Unit 11: Electricity
Charge at rest and charge in motion: the force between two charges, and the current, voltage, and resistance that make a circuit actually work.
11 topics · 39 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Static Electricity and Charge
- How it worksHow does rubbing two materials together transfer electrons and leave one object positive and the other negative?
- Common mistakeWhy isn't a charged object always attracted to every other object it comes near?
- Going furtherWhat happens to a neutral piece of paper when a charged balloon is brought close but never touches it?
Your turn1 of 2
Rubbing a balloon vigorously against your hair and then holding it near a wall makes the balloon stick there on its own, clinging without any tape or glue involved. Why does rubbing a balloon on your hair make it stick to a wall all by itself?Coulomb's Law
- How it worksHow does Coulomb's law scale the electric force with both charges and shrink it with the square of the distance?
- Worked exampleFind the electric force between two 2 microcoulomb charges separated by 0.5 meters.
- Common mistakeWhy doesn't Coulomb's law only apply to charges that are touching each other?
- Going furtherWhat happens to the force between two charges when one of the charges is tripled?
Your turn1 of 2
Find the electric force between two charges of 13.96 and 13.01 microcoulombs separated by 4.66 meters. Use Coulomb's constant k = 8.99 times 109 N m2/C2.Electric Fields
- How it worksHow do electric field lines point away from positive charges and show the force a test charge would feel?
- Common mistakeWhy isn't an electric field something you could see sitting in empty space with your own eyes?
- Going furtherWhat happens to the electric field lines between two charges when one charge is flipped to the opposite sign?
Your turn1 of 2
A single isolated positive charge sitting alone in empty space creates a real, measurable electric field extending outward all around it, ready to exert a force on any second charge that might later enter that region. Why can a single charge change the space around it even before any other charge shows up?Electric Current
- How it worksHow does electric current measure the rate at which charge flows past a point in a wire?
- Worked exampleFind the current in a wire that carries 12 coulombs of charge past a point in 4 seconds.
- Common mistakeWhy doesn't current get used up as it travels around a circuit and back to the battery?
- Common mistakeWhy isn't the current measured right after the battery any bigger than the current measured just before it returns?
- Going furtherWhat happens to the current in a wire when the same amount of charge flows through it in half the time?
Your turn1 of 2
Find the current in a wire that carries 69.79 coulombs of charge past a point in 19.5 seconds.Voltage and Potential Difference
- How it worksHow does voltage measure the energy given to each unit of charge as it moves through a circuit?
- Common mistakeWhy isn't voltage something that flows through a wire the way current does?
- Going furtherWhat happens to the energy delivered to a charge when it moves through a higher voltage battery?
Your turn1 of 2
A 9 volt battery can push charge through a small motor with noticeably more force and effect than a single 1.5 volt battery could manage on that same motor, even though both are just ordinary batteries. Why can a 9 volt battery push charge with more force than a 1.5 volt battery?Resistance and Ohm's Law
- How it worksHow does Ohm's law tie voltage, current, and resistance together into one simple ratio?
- Worked exampleFind the current through a 6 ohm resistor connected to a 12 volt battery.
- Worked exampleFind the resistance of a device that draws 2 amps of current from a 120 volt outlet.
- Common mistakeWhy doesn't a higher resistance always mean less current no matter what the voltage is doing?
Your turn1 of 2
Find the current through a 349 ohm resistor connected to a 156 volt battery.Series Circuits
- How it worksHow does the same current flow through every component in a series circuit, one after another?
- Worked exampleFind the total resistance of three resistors of 2, 4, and 6 ohms connected in series.
- Common mistakeWhy doesn't current get weaker as it passes through each resistor in a series circuit?
- Going furtherWhat happens to the brightness of the remaining bulbs when one bulb is removed from a series circuit?
Your turn1 of 2
Find the total resistance of three resistors of 70, 65, and 93 ohms connected in series.Parallel Circuits
- How it worksHow does splitting current across parallel branches make the total resistance of the circuit drop?
- Worked exampleFind the total resistance of two 6 ohm resistors connected in parallel.
- Common mistakeWhy doesn't adding a parallel branch to a circuit increase its total resistance?
- Going furtherWhat happens to the total current drawn from the battery when a third branch is added in parallel?
Your turn1 of 2
Find the total resistance of two resistors of 140 and 130 ohms connected in parallel.Electric Power in Circuits
- How it worksHow does electric power combine voltage and current into the rate at which a device uses energy?
- Worked exampleFind the power used by a device that draws 5 amps from a 120 volt outlet.
- Common mistakeWhy doesn't a higher voltage rating by itself guarantee that a device uses more power?
Your turn1 of 2
Find the power used by a device that draws 20.92 amps from a 156 volt outlet.Conductors and Insulators
- How it worksHow do a conductor's loosely held electrons let charge move freely while an insulator's electrons stay locked in place?
- Common mistakeWhy isn't an insulator simply a material with zero electric charge inside it?
- Going furtherWhat happens to a charge placed on one end of a long conducting rod versus one end of a long insulating rod?
Your turn1 of 2
Walking across a carpeted room and then touching a metal doorknob can produce a small, sudden spark of static shock, an effect that doesn't happen the same way when touching a wooden door instead of the metal knob. Why does touching a metal doorknob after walking on carpet sometimes give you a spark of static shock?Electric Potential Energy
- How it worksHow does electric potential energy store the work done moving a charge against the electric force?
- Worked exampleFind the electric potential energy of two 3 microcoulomb charges separated by 0.2 meters.
- Common mistakeWhy isn't electric potential energy the same quantity as electric potential, or voltage?
Your turn1 of 2
Find the electric potential energy of two charges of 10.48 and 9.77 microcoulombs separated by 3.73 meters. Use Coulomb's constant k = 8.99 times 109 N m2/C2.