Series & Parallel Circuits

What 2 types of ways are there of joining electrical components?

• In series and in parallel

• some circuits include both series and parallel parts

What does a series circuit consist of?

A string of two or more components, connected end to end

What does a parallel circuit consist of?

2 or more components attached along separate branches of the circuit

Describe the current, total potential difference and total resistance in a series circuit

• the current is the same at all points i.e. through each component

• total potential difference of the power supply is shared between the components

• the total resistance of two components is the sum of the resistance of each component

Describe the total current, potential difference and total resistance of parallel circuits

• total current through the whole circuit is the sum of the currents through the separate components

• the potential difference across each component is the same

• the total resistance of two resistors is less than the resistance of the smallest individual resistor

Does the current always split equally in a parallel circuit?

When does it split equally?

• no, often more current in come branches than others

• Current in each branch will only be identical if the resistance of the components along each branch are identical

What is resistance measured in?

Ohms

What does increasing the number of resistors increase? why?

The overall resistance as the charge has now more resistors to pass through

When two or more resistors are connected in parallel, the combined resistance what?

why?

• decreases

• because each resistor creates an extra path along which the charge can flow

  • this allows more charge to flow overall

  • this leads to a smaller overall resistance

What are the advantages of parallel circuits?

• the components can be individually controlled, using their own switches

• if one component stops working the others will continue to function

What is current like in series circuits?

And in parallel circuits?

• current the same at all points

• current splits at junctions - some of it going one way and the rest going the other

What is the voltage like in series circuits?

And parallel circuits?

• the voltage of the power supply is shared between the components

• in a parallel circuit, the voltage across each component is the same

Why is the overall resistance larger than in series and smaller in parallel?

• in series circuits, the charge has to push through multiple components when flowing around the circuit, the more components the charge has to travel through, the higher the number of collisions that occur

• in parallel though, the charge has more than one pathway, with two or more components, to take, so only some charge will flow along each path, the more pathways there are, the smaller the amount of charge in each path

What is a direct current (d.c) defined as?

A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative

What direction(s) is the potential difference across a cell in a d.c. circuit?

In one direction only

A d.c. power supply has a … positive terminal and a … negative terminal

fixed, fixed

What produce direct current?

Electric cells, or batteries

Name 2 disadvantages of series circuits

• if one of the components breaks, all of the others will stop working

• the components cannot be controlled (switched on and off) separately

All the circuits you have studied so far are … circuits. Don't be put off by an exam question if you are asked to calculate the current, potential difference or resistance in a d.c. series circuits, you don't have to do anything different from what you have already learned!

d.c.

what is an alternating current (a.c.) defined as?

A current that continuously changes its direction, going back and forth around a circuit

An a.c. power supply has 2 … terminals

2

How does the potential difference between the terminals vary on a graph?

As a sine curve, or sinusoidally

What is the frequency of an alternating current?

the number of times the current changes direction back and forth each second

In the uk, what type of current is main electricity, with what frequency and what potential difference

• alternating current

• 50Hz

• around 230V

Describe the differences between direct and alternating current

• direct is continuous and in one direction while alternating current is constantly changing direction

• in current time graphs, direct current is just a horizontal line above the x axis while alternating current is a sine curve

• direct current is produced by cells and batteries while alternating current is produced by electrical generators i.e. mains electricity

• direct current has a positive and negative terminal while alternating current has 2 identical terminals

If you are asked to explain the difference between alternating and direct current, sketching and labelling the … shown above can earn you full marks.

graphs

What is mains electricity?

the electricity generated by power stations and transported around the country through the National Grid

What does everyone connect to when plugging in an appliance such as a phone charger or kettle

the mains

What type of power supply is mains electricity?

an alternating current supply

What does a frequency of 50Hz mean?

The direction of the current changes back and forth 50 times every second

Mains electricity, being an alternating current, does not have positive and negative sides to the power source

The equivalents to positive and negative are called what and what do they form?

• live and neutral

• form either end of the electrical current

What do the live and neutral wires do?

What does the earth wire do?

• deliver electricity to the device

• it is purely for safety

All electrical appliances are connected to the mains using at least what?

at least a live wire and a neutral wire

In the UK, most electrical appliances are connected to the mains using a three-core cable consisting of what?

• a live wire

• a neutral wire

• an earth wire

The insulation covering each wire is colour-coded for easy identification, what are these colours?

• live wire - brown

• neutral wire - blue

• earth wire - green and yellow stripes

Every country has a slightly different …, with many using two-pin plugs and plug sockets such as in the USA and mainland Europe

The earth wire is still present in the two-pin plugs, just more …

• configuration

• hidden

What is the purpose of the live wire?

How dangerous is it?

• Carries the alternating potential difference from the mains supply to a circuit

• it is the most dangerous of the three wires

• if it touches the appliance without the earth wire, it can cause electrocution

What is the purpose of the neutral wire?

How dangerous is it?

• Forms the opposite end of the circuit to the live wire to complete the circuit

• because of its lower voltage, it is much less dangerous than the live wire

What is the purpose of the earth wire?

What does this prevent?

• Acts as a safety wire to stop the appliance from becoming live

• this prevents electric shock from occurring if the appliance malfunctions or the live wire breaks off and touched the case of the plug

What is the potential difference of the different wires in mains electricity?

• live wire carries the most of the power to the circuit and as a result is the most dangerous wire - 230V

• neutral wire is much safer than the live wire although it can deliver a small shock- close to 0V

• earth wire only carries a current to the ground if there is a fault in the appliance causing a surge of current- 0V

Normally, the earth wire does not carry any electricity – it is there for … only

As a result, in most circumstances, its potential difference is 0 volts

safety

Many electrical appliances have metal cases that pose a potential safety hazard, why/how?

• if a live wire (inside the appliance) came into contact with the metal casing, the casing would briefly become electrified (live)

• if anyone touched it at this moment, they would get an electric shock

How can the earth wire reduce this risk if the casing becomes live?

• the earth wire completes the circuit, providing a low resistance path to the earth

• this causes a surge of current in the earth wire and also in the live wire

• this surge of current passes through the fuse in the circuit, causing it to melt and break

• this cuts off the supply of electricity to the appliance, making it safe to touch

Why, if the live wire and earth wire are connected together a very large current can be created?

Because of the large potential difference between the live (230 V) and the earth (0 V)

What happens if a person provides the connection between live and earth wire?

Then a large current can pass through them, providing a potentially lethal shock

Why will electricians always switch off the main electricity supply to the whole house, or section of a house when they are working with electrical appliances?

• This is because they will come into contact with live wires when they are working

• The potential difference of the live wire is 230 V and the potential of the electrician is 0 V

• Therefore, there is a large potential difference between the live wire and the electrician, so, a current would pass through the electrician's body to reach the earth

• Even if a device is switched off but the mains supply is on, the live wire can still cause an electric shock