Physics: Circuits

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Atoms are made of neutrons (neutral), protons (positive) and electrons (negative). Neutrons and protons make up the nucleus of the atom, and electrons float around in shells on the outside. Electrons are the only ones that can move freely.

In order for the overall charge of an atom to be neutral, there must be an equal amount of protons and electrons.

If an atom has more protons than electrons (e.g. if it loses an electron), then it becomes positively charged. If an atom has more electrons than protons (e.g. if it gains an electron), then it becomes negatively charged.


Some materials can become charged very easily - these are referred to as conductors, and often include of metals (particularly transition metals, e.g. copper, iron). They are easily charged because they can lose electrons very easily - these are described as free electrons. 

Other materials don't charge very easily - these are referred to as insulators, and often include of non-metals. They do not have free electrons.

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Static Electricity

We can use friction to charge objects. For example, a polythene rod and a cloth:

  • The polythene rod becomes negatively charged because electrons are rubbed off the cloth and onto the polythene rod. The cloth has lost electrons and is left positively charged.

Using the principles of attraction (like charges repel, unlike charges attract), we can achieve a number of effects - such as charge separation.

  • By rubbing a balloon, you can charge it negatively.
  • Holding it near a stream of water will cause it to move - as like charges repel, the electrons in the water move as far as possible away from the negatively charged balloon, while the protons are attracted to it

This is called static electricity. 

Conductors cannot conduct static electricity because they gain and lose electrons too easily.

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Static Electricity: Uses and Hazards

Uses of static electricity

  • Inkjet printers:charging ink droplets in inkjet printers akkiws the droplets to be directed to particular places on the paper by deflecting them between charged plates.
  • Spray painters: Paint droplets are charged, and the object they are painted are connected to a supply of the opposite charge. This results in the droplets being attracted to the object.

Dangers of static electricity

  • Electric shock - cars become charged with static electricity and can shock people who touch them
  • Build up of static charge - when fuelling, planes and tankers can build up with static charge, which can result in a fire or explosion. Earthing is a way to prevent this - planes can be electrically earthed to the ground by a metal, which, as a conductor, can easily lead the unwanted charge away.
  • Damage to objects - workers handling electronic components must take care not to become charged by static as this can easily destroy expensive objects.
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A circuit must contain a battery. A battery is a group of cells, providing both a negative and positive terminal to charge the electrons. The batteries do not contain electrons which they send around the wires - they simply provide energy for the electrons, already in the wires, to travel around. Electrons travel from negative to positive, because they are repelled by negative charge and attracted to positive charge. However, conventional current flows from positive to negative, due to misconceptions in the past.

Series circuits

  • Adding a component will decrease the overall current of the circuit.
  • Voltage gets shared between components.
  • Opening a switch, or a component blowing, will break the circuit.
  • Resistance adds up.

Parallel circuits

  • With each new loop, the voltage sent to the component is equal to the supply voltage. Components all work as effectively as each other, just for not as long.
  • Current splits off at junctions. 
  • Components can be switched on and off separately, and one component blowing will have no effect on other components.
  • Resistance decreases.
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Voltage/Potential Difference

Voltage, or potential difference (PD), is the measure of energy difference between two certain points. It is measured in volts (V) - one volt is equal to one joule per coulomb. So, a 12V car battery supplies 12J of energy to each coulomb of charge it circulates.

Voltage can be measured using a voltmeter. To measure the voltage of a specific component, voltmeters must be placed in parallel with it. This is because the energy transferred between two certain points must be measured.

Equations for Voltage/PD:

  • Voltage = Current x Resistance (V=IR)
  • Voltage = Energy ÷ Charge (V=E/Q)

In a series circuit, the voltages of the components roughly add up to the supply voltage (not exact because electrons lose energy as they go through the circuit - in the form of heat energy).

In a parallel circuit, the voltage at each branch will be equal to the supply voltage. Each branch behaves as if it is connected independently to the cell (like multiple series circuits).

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Current is the rate of the flow of charge (negatively charged electrons). It is measured in amps (A) - one amp is equal to one coloumb per second. Current is measured using an ammeter.

Equations for current

  • Current = Voltage ÷ Resistance (I=V/R)
  • Current = Charge ÷ Time (I = Q/t)

So, if a battery delivers a current 1.2A to a circuit, after 2 minutes, the total charge that has moved around the circuit is 144 coloumbs.

In a series circuit, an ammeter will have the same reading no matter where you put it. This is because the position has no effect on the flow of electrons.

Adding components to a circuit will decrease the current because electrons' energy gets used up, resulting in a slower rate of flow of charge. 

A parallel circuit will increase the overall current of the circuit because essentially you are adding more circuits to the original one - you are providing more pathways for electrons to flow. In each branch of a parallel circuit, however, current will be halved (or more, depending on the amount of components).

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When current passes through a circuit, it doesn't flow freely. The electrons must flow past atoms. How hard it is for the electrons to pass the atoms is described as resistance. Resistance is measured in ohms.

The equation for resistance is: R = V ÷ I

Factors affecting resistance:

  • Wire length - in a longer wire, electrons must travel 
  • Wire thickness - a thicker wire allows more electrons to flow through, thus decreasing resistance.
  • Material - copper has a lower resistance than aluminium; gold has even lower resistance
  • Temperature - a higher temperature increases the movement of atoms, which increases the amount of collisions between them. So, increasing temperature increases resistance, because it is harder for electrons to pass through while colliding with other atoms.
  • Components - adding more components increases resistance because the electrons lose energy and end up moving slower, thus decreasing current.

The higher the resistance, the lower the current.

Resistance of a series circuit: R1 + R2 (etc.) = Rt

Resistance of a parallel circuit: 1/R1 + 1/R2 (etc.) = 1/Rt

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A resistor is a component that increases resistance in a circuit. There are multiple types of resistors:

  • Fixed resistor - these resistors have a fixed value
  • Variable resistor - these resistators have variable values
  • Light Dependent Resistor (LDR) - the brighter it is, the lower the resistance
  • Thermistor - the hotter it is, the lower the resistance (or vice versa)
  • Diode - these resistors resist the current in one direction

In a series circuit, adding resistors increases the overall resistance. The equation is R1 + R2 (etc.) = Rt

However, in a parallel circuit, adding resistors decreases the overall resistance. This is because parallel circuits are essentially multiple circuits, so the electrons have more pathways to flow. The equation is 1/R1 + 1/R2 (etc.) = 1/Rt

Some resistors are described as ohmic. This means that their resistance follows Ohm's law - which states that in a circuit, voltage and current are directly proportional. The voltage/current graph of an ohmic conductor would be a diagonal line through the origin.

Filament lightbulbs are not ohmic. The reason for this is that they heat up, which increases resistance, causing a curved line on a voltage/current graph.

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