Electricity and Magnetism — Current Makes a Magnet

1.Electric charge and static electricity

All matter is made of atoms, and inside atoms there are protons with a (+) charge and electrons with a (−) charge. Normally the two charges are equal in amount, so matter is electrically neutral. Like charges repel each other, and unlike charges attract.

The crackling when you pull off a sweater in winter happens because friction moves electrons from one object to the other, leaving the two objects with opposite charges. Electricity that stays put in one place like this is called static electricity. It builds up more easily on dry days because there is little moisture in the air, so the charge doesn't leak away easily.

2.Current, voltage, and resistance

The movement of charge along a wire is electric current. Its unit is the A (ampere), and it is expressed as the amount of charge passing through a cross-section of the wire in 1 second. By convention, current flows from the (+) terminal to the (−) terminal, but the electrons that actually move in a metal wire travel in the opposite direction. That is because the direction was set before the electron was discovered.

Voltage is what drives current, and its unit is the V (volt). Resistance is how much the flow of current is opposed, and its unit is the Ω (ohm). For the same material, a longer or thinner wire has more resistance.

Electricity is often explained by comparison with flowing water. Voltage is like the difference in height of a water tank, current is like the amount of water flowing, and resistance is like the narrowness of the pipe. This is only an analogy to aid understanding; electricity doesn't actually flow like water.

3.Series and parallel connections

Connecting resistors one after another in a single line is called a series connection. The same current flows through every resistor, and the total resistance is the sum of the individual resistances. If the circuit breaks at any one point, no current flows anywhere in it.

Connecting resistors side by side in separate branches is called a parallel connection. Each branch has the same voltage across it, and the total current is the sum of the branch currents. The more branches you add, the smaller the total resistance actually becomes. The outlets and lights in a home are connected in parallel, so if one is turned off or breaks, the rest keep working, and all of them receive the same 220 V.

4.Energy use — electricity is billed in kWh

Power (W) is the rate at which electrical energy is used each second, and the amount of energy actually used is the energy use: power multiplied by the time used. Electricity bills count this energy in units of kWh (kilowatt-hours). 1 kWh is the amount used by running a 1,000 W device for 1 hour.

So even a high-power device uses little energy if it runs only briefly, and even a low-power device uses a lot if it stays on for a long time. A space heater with a lower power rating left on for several hours a day can use far more energy than an electric kettle (2,200 W) boiling water for 3 minutes. To estimate the cost, multiply the kWh you used by the price per kWh printed on your bill. The rate varies by region and by usage tier.

From an energy point of view, a light bulb converts electrical energy into light and heat. An LED bulb uses far less power than an incandescent bulb to produce the same brightness because the incandescent bulb gives off most of its energy as heat rather than light.

5.Magnetic fields made by current — electromagnets

When current flows through a wire, a magnetic field forms around it. You can see this by running current through a wire next to a compass and watching the needle move. Winding the wire into a coil with many turns and putting an iron core inside makes the magnetic field much stronger; this is an electromagnet.

An electromagnet becomes a magnet or stops being one just by switching the current on and off, and you can adjust the strength of its magnetic force with the strength of the current. Electromagnets are used in cranes that lift scrap metal, in speakers, and in doorbells. An electric motor is a device that runs current through a coil placed in a magnetic field so that the coil feels a force and spins; motors are found in fans, washing machines, and even the vibration in your phone.

6.Current made by a magnetic field — generators

Conversely, if you move a magnet near a coil or spin a coil in a magnetic field, the magnetic field passing through the coil changes and current flows. This is called electromagnetic induction. The faster you move the magnet and the more turns the coil has, the larger the current.

The generators in power plants work on this principle. Thermal and nuclear power plants spin turbines with steam, hydroelectric plants with falling water, and wind turbines with wind, and the generator connected to the turbine produces electricity. From an energy point of view, it is a device that converts kinetic energy into electrical energy. A motor goes from electricity → motion, and a generator from motion → electricity; only the direction is reversed.

Wireless phone charging and induction cooktops also use electromagnetic induction. They run rapidly changing current through a coil to create a changing magnetic field, and that field either produces current in a coil inside the phone or drives current through the bottom of a pot to produce heat.