7.3 Electricity and Magnetism
Key Takeaways
- Ohm's law, V = IR, links voltage, current, and resistance; holding voltage constant, current is inversely proportional to resistance.
- In a series circuit the current is identical everywhere and resistances add, so one break opens the whole loop; in a parallel circuit the voltage across each branch is the same and the branches operate independently.
- Charge is conserved in every electrical process: rubbing transfers electrons from one object to another rather than creating charge, and a bulb transforms energy without consuming current.
- An electric current always produces a magnetic field, and a changing magnetic field always induces a voltage — the paired effects behind motors and generators respectively.
- Household wiring is parallel so that each appliance receives full line voltage and operates independently, with fuses or breakers opening the circuit when current exceeds a safe value.
Quick Answer: Electricity and magnetism are two faces of one electromagnetic interaction. Charge that is stationary produces static effects; charge in motion is a current governed by Ohm's law (V = IR) and by the series-versus-parallel rules; a current always creates a magnetic field, and a changing magnetic field always induces a voltage. Those paired effects are what motors, generators, speakers, and transformers exploit.
Work, power, kinetic energy, and potential energy are developed in "Work, Power, and Mechanical Energy."
Heat, temperature, specific heat, and the three modes of heat transfer are developed in "Thermal Energy: Heat, Temperature, and Heat Transfer."
Electricity: Charge, Current, Voltage, Resistance
Electric charge is measured in coulombs (C); the charge on one electron is −1.6 × 10⁻¹⁹ C. Current (I) is the flow of charge per unit time, measured in amperes (A) = C/s. Voltage (V), or potential difference, is the energy per unit charge, in volts (V) = J/C — it is the "push" that drives current. Resistance (R) opposes current flow, in ohms (Ω).
Ohm's law: V = I × R. A typical exam-style calculation: a 6 V battery drives current through a 3 Ω resistor — I = V/R = 6/3 = 2 A. If the resistance doubles to 6 Ω, the current falls to 1 A.
Series and Parallel Circuits
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Current | Same everywhere (I_total = I₁ = I₂) | Splits across branches; I_total = I₁ + I₂ + … |
| Voltage | Splits across resistors; V_total = V₁ + V₂ + … | Same across each branch (V_total = V₁ = V₂) |
| Resistance | R_total = R₁ + R₂ + … (adds) | 1/R_total = 1/R₁ + 1/R₂ + … (total is less than smallest) |
| A bulb burns out | Whole circuit opens, all bulbs go dark | Other branches keep working |
| Household wiring | Not used for wall outlets | Used so each appliance works independently |
Texas homes are wired in parallel so that turning off one lamp does not kill power to every other outlet on the same circuit. Series circuits are useful for switches and fuses that must break the whole loop.
Magnetism and Electromagnetism
A magnet has two poles (north and south); like poles repel and unlike poles attract, and magnetic field lines run from N to S outside the magnet. Electromagnetism is the interaction of electric current and magnetic fields: a current-carrying wire creates a circular magnetic field (right-hand rule), and a wire carrying current in an external magnetic field feels a force — the basis of a motor.
| Device | What it does | Core principle |
|---|---|---|
| Electromagnet | A coil (solenoid) wrapped around an iron core that becomes magnetic only when current flows | Current → magnetic field; iron core amplifies it |
| Electric motor | Converts electrical energy to rotational mechanical energy | Current-carrying coil in a magnetic field experiences a torque |
| Generator | Converts mechanical energy to electrical energy (the inverse of a motor) | Moving a coil through a field induces a voltage (electromagnetic induction) |
| Transformer | Steps AC voltage up or down | Changing current in the primary coil induces a voltage in the secondary coil; turns ratio sets the voltage |
| Audio speaker | Converts electrical signal to sound | Current in a coil attached to a cone interacts with a permanent magnet, pushing the cone to make sound waves |
| Nerve impulse | Electrical signal along a neuron | Movement of Na⁺ and K⁺ ions changes the membrane potential; an action potential is an electrical wave, not a current in a wire |
| Lightning | A massive static discharge | Charge separation in a thundercloud builds a huge voltage until air ionizes and current flows to ground |
These applications connect the abstract electricity/magnetism ideas to Texas 4-8 contexts: students can build a simple electromagnet from a nail, a D-cell, and insulated wire, and observe that more coil turns or more current produce a stronger field.
Static Electricity and Charge Transfer
Before current electricity, students meet static electricity: an imbalance of charge on an object's surface. Rubbing a balloon on hair transfers electrons from the hair to the balloon, leaving the balloon negative and the hair positive. Three principles govern what follows:
- Charge is conserved. Electrons move; they are never created or destroyed. The balloon gains exactly what the hair loses.
- Like charges repel, unlike charges attract, and a charged object attracts a neutral one by induction, pushing like charges to the far side of the neutral object.
- Grounding neutralizes an object by providing a path for excess charge to Earth, which is why lightning rods and antistatic wrist straps work.
Lightning is static discharge at enormous scale: charge separation within a thundercloud builds a potential difference of hundreds of millions of volts until the air ionizes and a current flows.
Conductors, Insulators, and Electrical Safety
Conductors — copper, aluminum, salt water, the human body — contain charges free to move. Insulators — rubber, glass, dry wood, plastic — hold their electrons tightly. This distinction is the basis of every safety rule in a 4-8 electricity unit: use insulated wire, never operate electrical equipment with wet hands, and use only low-voltage battery sources rather than wall current for student circuits. A fuse or circuit breaker protects a circuit by opening it when current exceeds a safe value, and a short circuit — a low-resistance path that bypasses the load — produces exactly that dangerous surge.
Earth's Magnetic Field and Electromagnetic Induction
Earth behaves as though a giant bar magnet sits near its center, generated by convection of molten iron in the outer core. A compass needle aligns with this field, which is why the needle's north-seeking pole points toward Earth's magnetic north pole — a location that is geographically near, but not identical to, the geographic North Pole, and that migrates measurably from year to year.
Electromagnetic induction is the reverse of the motor effect: moving a magnet through a coil, or moving a coil through a magnetic field, induces a voltage in the coil. Faraday's discovery underlies every generator, and therefore nearly all electricity in the Texas grid. A coal, gas, nuclear, or wind plant differs only in what turns the turbine; the turbine spins a coil in a magnetic field, and induction does the rest. That single idea connects this section to the energy-transformation material in Section 8.2.
A student rubs a balloon on their hair, and the balloon then attracts small paper scraps that carry no net charge. What explains the attraction?
A 9 V battery drives current through a 3 Ω resistor. What current flows, and what is the current if the resistance is tripled to 9 Ω?
In a string of old holiday lights wired in series, one bulb burns out and the entire string goes dark. In a modern string wired in parallel, one bulb burns out but the others stay lit. Why?