Electricity and Magnetism

In Grade 6 you said 'electricity flows'. Now you can describe it with three precise quantities.

Current is the flow of electric charge around the circuit. It is measured in amperes (A), often shortened to amps, using an ammeter. The bigger the current, the more charge flows each second.

Voltage is the electrical push that drives the current. It is supplied by the cell and measured in volts (V) using a voltmeter. A bigger voltage gives a bigger push.

Resistance is how much a component opposes the current. It is measured in ohms. A high-resistance component makes it hard for current to flow.

A water analogy helps. Picture water flowing round a pipe:

• the current is like the rate of water flowing,
• the voltage is like the pump that pushes the water,
• the resistance is like a narrow section that slows the water down.

These three are linked: voltage pushes, resistance holds back, and current is the result.

To study a circuit you need to measure it. Two meters do the job, and they are connected in different ways.

• An ammeter measures current. It is placed in series — right inside the loop — so the same current that flows through the components also flows through it.
• A voltmeter measures voltage. It is placed in parallel — connected across a component — to measure the push used by that component.

A simple way to remember it: the ammeter is part of the road the current travels on, while the voltmeter stands to one side, comparing two points.

Reading a circuit with these two meters lets you see exactly what current and voltage are doing.

There are two main ways to connect components, and they behave very differently.

In a series circuit, all the components sit in a single loop:

• the same current flows through every component,
• if one component breaks, the loop is broken and everything stops.

In a parallel circuit, components sit on separate branches:

• each branch is its own loop, so one bulb can be switched off or fail without affecting the others,
• the current splits between the branches and joins again.

This is why your home is wired in parallel. If one light fails, the rest stay on — something a single series loop could never do.

Resistance and current are closely linked. For a fixed voltage from the cell:

• more resistance in the circuit means a smaller current,
• less resistance means a larger current.

Think of the water analogy again. A wide pipe lets a lot of water through easily — low resistance, big flow. A narrow pipe holds the water back — high resistance, small flow. The pump (voltage) is the same; only the resistance has changed.

This idea explains a lot:

• a long, thin wire has more resistance than a short, thick one,
• adding a component such as a bulb adds resistance, so the current drops,
• a variable resistor lets you change the resistance on purpose — for example, a dimmer switch raises the resistance to lower the current and dim a light.

So if you want to control how much current flows, change the resistance.

Here is one of the most important discoveries in physics: an electric current produces a magnetic field. Whenever current flows through a wire, a magnetic field appears around that wire. This link between electricity and magnetism is called electromagnetism.

A single straight wire makes only a weak field. To make it useful, we build an electromagnet:

An electromagnet is a coil of wire, usually wound around an iron core. When current flows, it becomes a strong magnet. When the current is switched off, the magnetism disappears.

You can make an electromagnet stronger by:

• using more turns of wire in the coil,
• using a larger current,
• adding an iron core.

The big advantage is control: an electromagnet can be switched on and off. That is why giant electromagnets in scrapyards can pick up a car and then drop it just by switching the current.

Electromagnetism works the other way round too. If a current can make a magnetic field, can a magnet make a current? The answer is yes — and it is how almost all our electricity is made.

When a magnet moves near a coil of wire — or a coil moves near a magnet — a voltage is generated in the wire, and a current flows. This is called generating electricity. The key word is movement: the magnet and the coil must move relative to each other.

A generator uses this idea. Inside it, a coil is spun between magnets (or magnets are spun near a coil). The constant movement keeps generating a current. The faster the spinning, the bigger the current.

The clever part is what spins the generator. In a power station, an energy resource is used to turn it:

• burning coal or gas heats water into steam, and the steam spins the generator,
• moving water in a hydroelectric dam spins it,
• the wind spins the blades of a wind turbine, which spins the generator.

So a generator simply turns movement energy into electrical energy — the exact reverse of a motor, which turns electrical energy into movement.

Understanding circuits and electromagnetism now sets you up for a great deal of later science:

• Circuit calculations — current, voltage and resistance lead into Ohm's law and power.
• Mains electricity — parallel wiring and safe use of electricity build on these ideas.
• Motors and generators — these power transport, industry and the electricity grid.
• Energy and the environment — how electricity is generated links straight to energy resources.
• Electronics — components such as resistors and sensors all rely on these basics.

Whenever a topic about electricity feels tricky later, return to the three quantities: current is the flow, voltage is the push, resistance holds back. And remember the two-way link: current makes magnetism, and moving magnets make current. Take your time — these ideas power the modern world.