Electricity & Magnetism
Electricity and magnetism are two aspects of the same fundamental force — electromagnetism — one of the four fundamental forces of nature.
Electric Charge & Current
Atoms contain positive protons and negative electrons. Like charges repel; opposite charges attract. Electric current (I) = flow of charge, measured in amperes (A).
Voltage & Resistance
Voltage (V) = electric potential difference, measured in volts (V). It's the "push" driving current.
Resistance (R) = opposition to current flow, measured in ohms (Ω).
Ohm's Law
V = IR (Voltage = Current × Resistance)
A circuit has 12V battery and 4Ω resistance.
I = V/R = 12/4 = 3 A
Series vs Parallel Circuits
Series: components in one loop. If one fails, all fail. Resistance adds: Rₜ = R₁ + R₂
Parallel: components in separate branches. One can fail without affecting others. Used in home wiring.
Magnetism
Moving charges create magnetic fields. Permanent magnets have north and south poles — opposites attract. Earth has a magnetic field that protects us from solar radiation. Electromagnets use coiled wire with current.
Working With Ohm's Law
Ohm's law rearranges three ways, and problems will hand you any two of the three quantities.
V = I × R— find voltageI = V / R— find currentR = V / I— find resistance
A 60 Ω bulb draws 2 A. What voltage is it running on?
V = I × R = 2 × 60 = 120 V
Adding electrical power
Power is P = V × I, measured in watts. That same 120 V bulb drawing 2 A uses 120 × 2 = 240 W.
This is why appliance labels list watts — it tells you the rate at which energy is being used, and it is what your electricity bill actually charges for.
Comparing Series and Parallel
| Series | Parallel | |
|---|---|---|
| Path for current | One only | Several branches |
| Total resistance | Adds up | Less than the smallest |
| Current | Same everywhere | Splits between branches |
| Voltage | Divides across parts | Same across each branch |
| If one part fails | Everything stops | Others keep working |
Houses are wired in parallel for exactly that last reason: switching off one lamp must not darken the whole building, and every socket needs the full supply voltage.
How Electricity and Magnetism Connect
They are not two topics that happen to share a chapter. Each one creates the other, and that relationship powers almost all modern technology.
Current makes magnetism
Any wire carrying current is surrounded by a magnetic field. Coil the wire and the field concentrates; add an iron core and you have an electromagnet strong enough to lift a car — and one you can switch off.
Magnetism makes current
Move a magnet through a coil of wire and current flows in that coil. This is electromagnetic induction, and it is how essentially all electricity is generated: something spins a coil inside a magnetic field, whether the spinning comes from steam, falling water, or wind.
Static Versus Current Electricity
- Static: charge builds up and stays put, then discharges suddenly — a shock from a doorknob, or lightning.
- Current: charge flows continuously through a conductor, which is what circuits use.
Common Mistakes
- Saying current "gets used up" in a circuit. Current is the same all the way round a series loop; it is energy that gets transferred.
- Confusing voltage with current. Voltage is the push; current is the flow that results.
- Assuming parallel resistors give more total resistance. Extra paths make it easier for current to flow, so total resistance drops.
- Mixing up magnetic poles with electric charges. They behave similarly but are different phenomena.
Summary
- Current is charge in motion, voltage is the push behind it, resistance opposes it.
V = IRlinks all three;P = VIgives the power used.- Series circuits share one path; parallel circuits give current a choice.
- Moving charge creates a magnetic field, and a moving magnet creates current.
- Induction is the basis of every generator and transformer.
FAQ
Why do birds sit on power lines without being electrocuted? Current flows from high to low potential. Birds aren't connected to ground, so no current flows through them.
What is the difference between AC and DC? Direct current flows one way, as from a battery. Alternating current reverses direction many times a second and is what mains sockets supply, because it is easier to transmit over long distances.
Why are some materials conductors and others insulators? Conductors such as copper have loosely held electrons free to move. In insulators such as rubber, electrons are bound tightly in place.
What does a fuse do? It is a deliberately weak link. If current climbs too high, the fuse wire melts and breaks the circuit before the wiring overheats.
Can you have a magnet with only one pole? No. Cut a magnet in half and each piece immediately has its own north and south pole.
Quick Quiz
Test what you just learned. Choose the best answer for each question.