Understanding a series circuit

Understanding a series circuit

You can think of a circuit as a circular motorway …

… a bit like the M25

Along this circular road, cars (called ‘Q’ cars) slowly travel bumper-to-bumper all the way around …

… a bit like the M25

A break in the road anywhere in the loop …

… will mean all the cars stop moving.

If a bulb breaks or is taken out – it creates a break in the circuit. This will stop all the cars, like a switch.


Potential differenceand

Current

A petrol pump gives some energy to the cars, so they can drive round.

The cell in an electrical circuit acts a bit like an energy pump.

We can measure the petrol difference before and after they visit the pump …

… we call this the potential difference (measured in Volts).

It tells us how much energy is being transferred to the circuit.

P D

V6V

We can also measure how many ‘Q’ cars cross a particular point in the road every second …

The number of ‘Q’ cars each second tells us the flow of cars …

A

… this is the current (measured in Amps).It tells us how quickly electrical charge is flowing in the

circuit.

3A


Resistanceand

Lamps

We can measure how many ‘Q’ cars cross a particular point in the road every second …

If the cars meet a resistance in the road, the cars will slow down …

Because the cars are bumper to bumper …

… the resistance will slow down all the cars in the circuit.We measure resistance in Ohms.

3 Ohms

A 2A

A

2A

The bigger the resistance, the slower the cars will go …

The number of ‘Q’ cars per second will get smaller …The current (Amps) decreases.

6 Ohms

A 1A

An enterprising thief can take petrol from the cars as they slow down …

… leaving just enough for the cars to get back to the petrol pump.

The thief uses the stolen petrol to burn in a barrel …

… this gives out light and heat.

… by stealing petrol, the thief takes some energy from the cars as they pass – like a lamp in a circuit.

… we can measure how much energy is taken by measuring the petrol difference before and after the cars

pass the thief.

V

P D

6V

6V

If there are two thieves, they will have to share the petrol they steal from the cars as they pass …

… they have to leave enough petrol for the cars to get back to the pump.

… each thief can only take half as much petrol, so his fire burns less bright.

The energy transferred to each bulb will be halved.

V

P D

6V

3V

This means each thief can take more petrol from the cars again. So the bulbs will be brightly lit again.

6V 6V

6V 6V

… we could add another petrol pump, to give the cars more

energy.

We can measure the flow of cars with one bulb.

A 3A

… each extra bulb creates more resistance and will slow down the flow of cars. Reducing the current.

A 1.5 A

Understanding a parallel circuit

In a parallel circuit, the road splits creating two loops …

A break in a loop stops cars from moving in that loop …

… but they can use the other loop to travel back to the pump.

But, a break that prevents cars getting to the pump …

… will stop all of the cars in the circuit.


Current and

Resistance

All the cars leave the petrol pump …3 Ohms

3 Ohms

The traffic cop will see the full number of cars per second (e.g. current 2A)

Loop A

Loop B

A 2A

At the junction, the road splits – and half the cars go along each loop. 3 Ohms

3 Ohms

The traffic cop will see half the number of cars per second in loop A (e.g. current 1A)

Loop A

Loop B

A1A

The other half of the cars along the other loop …3 Ohms

3 Ohms

The traffic cop will see half the number of cars per second in loop B (e.g. current 1A)

Loop A

Loop B

A

1A

All the cars must come back to the petrol pump …3 Ohms

3 Ohms

The traffic cop will see the full number of cars per second again (e.g. current 2A)

Loop A

Loop B

A2A

Some loops may have more resistance than others …

Cars prefer to take roads with less resistance …

If one loop has more resistance than another …3 Ohms

6 Ohms

The cars will still divide up between the two loops, but more will travel along the loop with less resistance.

Loop A

Loop B

All the cars must travel via the pump …3 Ohms

6 Ohms

The traffic cop measures 3 ‘Q’ cars per second; a current of 3 Amps

Loop A

Loop B

A3A

If loop A has half the resistance of loop B …3 Ohms

6 Ohms

… then for every 2 cars per second that travel along loop A …

Loop A

Loop B

A2A

A3A

If loop A has half the resistance of loop B …4 Ohms

8 Ohms

… only 1 car per second will travel along loop B.

Loop A

Loop B

A1A

A3A

Whichever loop they took - all the cars must travel back to the pump …

4 Ohms

8 OhmsLoop A

Loop B

A1A

A3A

A2A


Voltageand

Lamps

An enterprising thief can take petrol from the cars as they slow down …

… leaving just enough for the cars to get back to the petrol pump.

With only one thief in each loop, each thief can take full amount of petrol from each car as they pass.

Loop A

Loop B 6V

6V

6V

If there are two thieves in the same loop, they will have to share the petrol they steal from the cars as they pass …

… they will only get half the petrol they would have on their own.

Thieves in the same loop have to share petrol. The thief in his own loop gets all the petrol to himself.

Loop A

Loop B 6V

3V

6V

3V

Try some practice questions

http://www.bbc.co.uk/bitesize/ks3/science/energy_electricity_forces/electric_current_voltage/quiz/q74171589/

http://www.twothirtyvolts.org.uk/education/revision-quiz/electric-circuits-11to14.html







Understanding circuits

Formula triangles

The energy transferred by each ‘Q’ car.

The number of ‘Q’ cars that pass a point each second.Roadworks slow the flow of cars.

The brightness of the bulb depends on how much energy is transferred each second… that is how much petrol can the thief steal each second

Power (Watts) = energy transferred each second (J/s) = current x voltage

Brightness of the bulb = The number of cars passing each second x the amount of petrol the thief can take from each car.

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Understanding a series circuit