An electric current is a flow of charge round a complete circuit, and a current always produces a magnetic field. Those two ideas link the electrical half of this topic to the magnetic half.
Current is the flow of electric charge, measured in amperes (A) with an ammeter placed in series. A current needs a complete circuit. Opening a switch breaks the circuit, so the current stops throughout it.
In a series circuit there is one path, so the current is the same everywhere in it. In a parallel circuit there is more than one path, so the current splits between the branches and the branch currents add up to the total. Household wiring is parallel so each appliance works independently.
Potential difference, in volts, can be thought of as the electrical ‘push’ that drives the current. It is measured with a voltmeter connected in parallel across a component.
Resistance, in ohms, is how strongly a component opposes the current, and it is the ratio of potential difference to current: \(R = \frac{V}{I}\). For a fixed supply, more resistance means less current. In series the supply potential difference is shared between the components; in parallel each branch gets the full amount.
A conductor has low resistance and lets charge flow easily. Copper is a good conductor, so wires are made of it. An insulator has very high resistance and hardly lets charge through. Plastic, rubber and glass are insulators, so cables are sheathed in plastic.
Components differ too: a resistor opposes the current more than a plain wire, and a filament lamp's resistance rises as it heats up.
Rubbing two insulators together transfers electrons from one to the other. The one that gains electrons becomes negatively charged and the one that loses them is left positive. Only electrons move.
Like charges repel and unlike charges attract, and the force acts without contact. A charged object can also attract an uncharged one: bringing it near separates the charge in the neutral object, pulling the opposite charge closer. A charged balloon therefore sticks to a wall, and a charged rod picks up small pieces of paper.
A charged object is surrounded by an electric field, a region in which another charge feels a force. The field is strongest close to the object and weakens with distance.
The field is what lets the force act across a gap: nothing touches, but the second charge is still pushed or pulled.
Every magnet has a north and a south pole. Like poles repel and unlike poles attract, and again the force acts without contact.
Only a few materials are magnetic: iron, steel, nickel and cobalt. A magnet attracts these, but it does not attract aluminium or copper.
The region around a magnet where the force acts is its magnetic field. It is mapped with a plotting compass, whose needle lines up with the field at each point.
Field lines are drawn from north to south outside the magnet, and where they are closest together the field is strongest, which is at the poles.
The Earth behaves as though it contains a huge bar magnet, and the field is produced by movement in its liquid outer core.
That is what a compass responds to: the needle is a small magnet free to turn, and it lines up with the Earth's field so that it points north. Compasses have been used for navigation for centuries.
A current flowing in a wire produces a magnetic field around it. Winding the wire into a coil concentrates the field, and putting an iron core inside the coil makes it much stronger. This is an electromagnet.
An electromagnet can be switched off, and its strength changed by changing the current or the number of turns, which a permanent magnet cannot do. That is why cranes in scrapyards use them. A DC motor uses the same effect: a coil carrying a current in a magnetic field experiences a force that makes it turn.
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