成人快手

Key points

  • Models can be used to help us understand electrical circuits.

  • Each model has its advantages and limitations.

Back to top

Video - Introduction to circuits

Can you answer these questions based on the video?

  1. In an electrical circuit, what are bulbs, switches and motors collectively known as?

  2. Why does it help to think of an electrical circuit a bit like the central heating system in a house?

Back to top

The water model

An example of the water model of electricity is a central heating system:

  • the pump acts like the cell or battery

  • the pipes are like the wires

  • the radiator is like a component, for example a lamp, transferring energy to the room

  • the water flow is like the electrical current

In many ways, electrical current behaves like water flowing through a closed ring of piping. The movement of the water through the pipe is like the movement of electrons through a circuit.

The diagram shows a central heating system.

A model of a water system within a house showing the pump, pressure gauge, valve, a radiator and flow meter.
Figure caption,
Water flowing through a closed ring of piping behaves like electrical current in a circuit

This circuit diagram represents the water model.

A circuit diagram of a voltmeter, switch, lamp, ammeter and battery.

The pump acts like a cell or battery. When the pump is switched on, it produces a pressure difference across the pump. This pressure difference causes water to flow around the loop 鈥 just like the potential difference produced by a cell or battery causes electrical current to flow in a circuit.

The amount of water flowing past any point in the loop each second is the same. There are no leaks in the pipe.

The amount of current flowing in a series circuit (a circuit with only one loop) is also the same in all parts of the circuit. Find out more about series circuits.

The radiator creates resistance to the flow of water, like a component (eg a lamp).

The flow meter measuring the rate of flow of water is like an ammeter which measures the rate of flow of electrical charge.

The valve acts like a switch.

Limitations of the water model

  • If a pipe breaks then water leaks out. If a wire breaks the electrical current does not leak out, instead it stops flowing altogether.

  • Wires always contain electrons, even if they are not connected into a circuit, but a water pipe could be empty.

Back to top

The rope model

The rope model is a useful tool to help us understand and in a circuit. Find out more about resistance in circuits.

Imagine a group of students and their teacher are lightly holding a loop of rope. If the teacher starts to move the rope around at a steady rate, the rope everywhere in the loop moves at this same rate. If the students are only holding the rope lightly then it will move easily 鈥 just like current flows easily through a wire.

If student A grips more tightly then it will be more difficult for the rope to move. The student is causing resistance, just like a (eg a resistor) causes resistance in an electrical circuit. The student鈥檚 hands will become warm as energy is transferred to them, just like a resistor becomes warm when current flows through it.

A teacher and three students standing a circle, holding a rope between them which forms a ring.
Figure caption,
The teacher represents the cell and each of the students represent the resistors.

The circuit diagram represents the rope model. The cell represents the teacher. The three resistors, labelled A, B and C, represent the students.

A circuit diagram with a cell and three resistors.
Figure caption,
A circuit diagram representing the rope model

Limitation of the rope model

  • The rope cannot easily be used to model circuits.

Find more about parallel circuits.

A sign which says 'remember'

The rope does not represent the wires. It represents the electrons in the wires.

Back to top

The distribution model

The distribution model is also known as the donation model or the delivery van model.

In this model:

  • the vans represent the charges () moving through the circuit

  • the bread represents energy which is transferred to the (eg a lamp or motor)

  • the bakery supplies the bread, just like the battery/cell supplies the energy for the charges to move

  • the bread is delivered to the shop by vans 鈥 just like energy is transferred to a component (eg a lamp or motor) by electrons

A circular diagram of the distribution model showing the bakery, vans delivering bread and empty vans returning to the bakery.
Figure caption,
The distribution model illustrates how energy is transferred to a component

The circuit diagram illustrates the distribution model. The battery represents the bakery. The lamp represents the shop.

A circuit diagram of a lamp and a battery.
Figure caption,
The battery represents the bakery and the lamp represents the shop

Limitations of the distribution model

  • Energy is not a physical substance but bread is.

  • The vans come from the bakery, but electrons are always in the wires. Electrons don鈥檛 come from the cell.

Back to top

Motorway model for parallel circuits

Imagine travelling along a motorway. If the motorway has more lanes then it is easier for the traffic to flow. Each lane of the motorway provides a parallel path for the traffic.

In the motorway model:

  • the lanes represent separate resistors in parallel in a circuit

  • the cars represent free electrons moving around a circuit

  • the traffic flow represents the current of an electrical circuit

  • adding lanes to a motorway reduces the resistance to traffic flow, just as adding resistors in parallel reduces electrical resistance.

Learn more about parallel circuits.

A three-lane motorway represents three resistors in parallel.

Cars travelling on a three-lane motorway.
Image caption,
A three-lane motorway represents three resistors in parallel
A circuit diagram of three resistors in parallel.
Figure caption,
Three resistors in parallel is represented by a three-lane motorway

A four-lane motorway represents four resistors in parallel.

A photo of a motorway with four lanes
Image caption,
A four-lane motorway represents four resistors in parallel
A circuit diagram of four resistors in parallel.
Figure caption,
Four resistors in parallel is represented by a four-lane motorway

Limitations of the motorway model

  • The model only represents part of a circuit. It doesn鈥檛 include a model of the battery.

  • Cars join and leave the motorway and the number of cars in any section varies. Electrons are already in the wire and components don鈥檛 join or leave the circuit.

Back to top

Test your knowledge

Quiz

Back to top

Play the Atomic Labs game! game

Try out practical experiments in this KS3 science game.

Play the Atomic Labs game!
Back to top

More on Electricity

Find out more by working through a topic