Jugami › Physical Science › Electricity and Magnetism
Moving charges light our homes, and electricity and magnetism are two sides of one force.
Why does a balloon stick to the wall after you rub it on your hair? What makes a light bulb glow, a doorbell ring, or a speaker play music? On this page you'll learn about electric charge, current, voltage, and resistance, how circuits work, how to stay safe around electricity, and how magnets and electricity work together in motors and generators.
All matter is made of atoms. Atoms contain protons, which have a positive charge, and electrons, which have a negative charge. Most objects have equal numbers of each, so they are neutral.
Like charges repel (push apart) and opposite charges attract (pull together). Two negatively charged balloons push away from each other, but a negatively charged balloon pulls on your positively charged hair.
Static electricity is charge that builds up on an object. Rubbing a balloon on your hair moves electrons from your hair to the balloon. When you shuffle across a carpet and touch a doorknob, the extra charge jumps across as a tiny spark: that's the zap you feel.
When charges flow, that's electric current. In wires, it's electrons moving. Think of a circuit like water in a pipe:
The three are linked by Ohm's law:
V = I × R
You can rearrange it to find current: I = V ÷ R. More voltage means more current; more resistance means less.
Worked examples (made-up numbers)
A flashlight: a 3 V battery pack runs a bulb with a resistance of 6 Ω. I = V ÷ R = 3 V ÷ 6 Ω = 0.5 A.
A toy motor: 2 A of current flows through a motor with a resistance of 4.5 Ω. V = I × R = 2 A × 4.5 Ω = 9 V.
Finding resistance: a 12 V circuit carries 3 A. R = V ÷ I = 12 V ÷ 3 A = 4 Ω.
A circuit is a complete loop that current can flow around: a power source (like a battery), wires, and something that uses the energy (like a bulb).
Pick a battery voltage and a bulb resistance, then wire two identical bulbs in series or in parallel. This is a simplified model: real bulbs change resistance as they heat up.
Series: total current = 6.0 V ÷ 20 Ω = 0.30 A.
The switch opens or closes the circuit. Bulb brightness here is based on the power each bulb uses (P = I × V).
Your body can conduct electricity, and electric current through your body can cause burns, stop your heart, or kill. Household outlets have far more voltage than a battery: in the United States, about 120 volts.
Every magnet has two poles: a north pole and a south pole. Like poles repel and opposite poles attract, just like electric charges. If you cut a magnet in half, you get two smaller magnets, each with its own north and south pole.
A magnet is surrounded by a magnetic field, the region where its force acts. You can see the field's shape by sprinkling iron filings around a magnet: they line up in curved lines from pole to pole. Magnets attract iron, nickel, cobalt, and steel, but not aluminum, copper, paper, or plastic.
Earth is a giant magnet. Its magnetic field comes from moving molten iron deep in the outer core. A compass is a tiny magnet that's free to spin, so it lines up with Earth's field and points roughly north. Earth's magnetic poles slowly wander and aren't exactly at the geographic North and South Poles.
In 1820, the Danish scientist Hans Christian Oersted noticed that a wire carrying current made a nearby compass needle move. That showed electric current makes a magnetic field.
An electromagnet is a coil of wire, often wrapped around an iron core, that becomes a magnet when current flows and stops being one when you switch it off. More turns of wire or more current make it stronger. Electromagnets ring doorbells, move the cone in speakers to make sound, lift scrap metal in junkyards, and make the strong magnetic fields in hospital MRI scanners. Maglev ("magnetic levitation") trains use powerful magnets to float above the track and move with very little friction.
An electric motor uses electricity and magnets to make motion: the push between an electromagnet and other magnets makes a shaft spin. Fans, blenders, and electric cars all use motors.
In 1831, the English scientist Michael Faraday discovered the reverse: moving a magnet near a coil of wire makes current flow. That's how a generator works, turning motion into electricity. Most power plants spin huge generators using steam, falling water, or wind.
Ask an adult before you start. Use only small batteries (AA) or fruit. Never use wall outlets or car batteries.
Keep magnets and balloon pieces away from little kids. They are a choking hazard, and swallowed magnets are dangerous.
You need: a balloon, your hair or a wool sweater, small bits of tissue paper.
Rub the balloon on your dry hair for about 10 seconds. Hold it near the tissue bits, then press it against a wall and let go.
Why: Rubbing moves electrons onto the balloon, giving it a negative charge that attracts the paper and the wall.
You need: 3–4 lemons, the same number of galvanized (zinc-coated) nails and copper coins or strips, alligator-clip wires, a small red LED, a dark room.
Push one nail and one copper piece into each lemon, not touching. Connect the copper of one lemon to the nail of the next. Then connect the free copper end to the LED's long (positive) leg and the free nail to its short leg. Turn off the lights: the glow is faint, so look in a dark room.
Why: Chemical reactions between the two metals and the lemon juice push electrons through the wire. One lemon gives under 1 volt, so you need several in series to light the LED. If it doesn't glow, flip the LED around. Throw the lemons away afterward; don't eat them.
You need: a sewing needle, a magnet, a small piece of cork or a leaf, a bowl of water. An adult should handle the needle.
Stroke the needle along the magnet about 30 times, always in the same direction. Lay it on the cork and float it in the water.
Why: Stroking turns the needle into a weak magnet. Floating lets it turn freely, so it lines up with Earth's magnetic field and points north–south. Check it against a phone compass.
You need: a large iron nail, about 1 meter of thin insulated copper wire, one fresh ordinary alkaline AA battery (not lithium or rechargeable), tape, paper clips.
Wrap the wire tightly around the nail 20–30 times, leaving the ends free. Have an adult strip the ends. Hold the wire only by its plastic coating, and hold or tape the bare ends to the battery's two terminals and touch the nail to the paper clips.
Why: Current in the coil makes a magnetic field, and the iron nail makes it stronger. Safety: the wire and battery get warm fast. Connect for only a few seconds at a time, then disconnect. Stop if anything gets hot.
Credits: All illustrations on this page are original, hand-coded SVG by Galileo. The share image was drawn in code.
