04.08 Circuits and Current exam review

Module 04 โ€” Circuits and Current ยท quiz worth 70 pts ยท 2026-09-03

The big idea The 70-point exam covers electrical charge and fields, current and magnetism, Ohm's law, capacitors, resistance, circuit components and schematics, and series versus parallel circuits.

Formula map

Coulomb force
Fe=kq1q2d2F_e=\frac{kq_1q_2}{d^2}

k = 8.99 ร— 10โน Nยทmยฒ/Cยฒ; charges in coulombs, distance in meters, force in newtons.

Electric field
E=FqE=\frac{F}{q}

E in N/C. Rearrange to F = Eq when force is unknown.

Ohm's law
V=IRV=IR

Voltage in volts, current in amperes, resistance in ohms.

Power
P=IVP=IV

Power in watts.

Equation method

1. Name the unknown

Write the requested quantity and unit before choosing an equation.

2. Convert units

Convert milliamperes to amperes and centimeters to meters before substitution.

3. Rearrange symbolically

Solve the equation for the unknown first. Then substitute values with units.

4. Check direction and size

A negative charge feels force opposite the electric-field direction. At fixed voltage, higher resistance should produce lower current.

Worked examples

Flashlight bulb from 04.02

  • Resistance. R=5.0โ€‰ฮฉR=5.0\,\Omega
  • Current. I=300โ€‰mA=0.300โ€‰AI=300\,\text{mA}=0.300\,\text{A}

Find the battery voltage.

  1. Use Ohm's law.
    V=IRV=IR
  2. Substitute the current in amperes.
    V=(0.300โ€‰A)(5.0โ€‰ฮฉ)V=(0.300\,\text{A})(5.0\,\Omega)

V = 1.5 V.

Electric field from 04.01

  • Force. F=3.0ร—10โˆ’9โ€‰NF=3.0\times10^{-9}\,\text{N}
  • Charge. q=1.25ร—10โˆ’19โ€‰Cq=1.25\times10^{-19}\,\text{C}

Find the electric-field magnitude.

  1. Use force per charge.
    E=FqE=\frac{F}{q}
  2. Substitute and divide.
    E=3.0ร—10โˆ’91.25ร—10โˆ’19E=\frac{3.0\times10^{-9}}{1.25\times10^{-19}}

E = 2.4 ร— 10ยนโฐ N/C.

Charge, current, and magnetism

Like charges repel and opposite charges attract. Coulomb force grows with the product of the charges and falls with the square of distance. A closed conducting path allows charge to move. Conventional current points from positive to negative, opposite electron flow. A current-carrying wire creates a magnetic field. With the right-hand rule, the thumb follows conventional current and the curled fingers show the magnetic-field direction. Reversing battery polarity reverses current and compass deflection.

Capacitors, resistance, and components

A capacitor has two conducting plates separated by an insulating dielectric and stores charge. Capacitance depends on plate area, plate separation, and the material between the plates; pressing a keyboard key moves its capacitor plates closer and increases capacitance. Capacitance is measured in farads. A resistor impedes current and is measured in ohms. A battery supplies electrical energy, a switch opens or closes the path, a bulb converts electrical energy to light, a motor converts it to mechanical energy, an ammeter measures current, and a voltmeter measures voltage. Schematics use standard symbols instead of pictures of these components.

Series and parallel

Series means one path and higher total resistance as resistors are added. The current is the same through the single path, and one break stops the whole circuit. Parallel means components connect across the same two points, creating multiple paths. Adding parallel branches lowers overall resistance and raises total current at fixed voltage. A complete branch can continue working when another branch opens. Complex circuits can combine both arrangements.

Mixed exam practice

Work every calculation on paper with units. Use the explanations only after committing to an answer.

Final checklist

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