Circuits and Power Electricity
You must be able to: Explain the difference between series and parallel circuits Explain the effect of adding resistors in series and parallel. Explain what is meant by ‘power’ using examples Explain how the power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time.
• • • •
Series and Parallel Circuits • •
Electrical components can be connected in series or parallel. Some electrical circuits contain series and parallel parts. Series Circuits
Parallel Circuits V1
V1
3V
0.1A A 2
A1 0.1A A3
P
Q
0.3A A4
A1 0.3A Splits up
0.1A
V2
P
A2
0.1A
1.5V
3V
V2 1.5V
0.2A V3
Joins up 3V
Q
A3 3V
• • •
•
•
There is the same current through each component. The 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. This is because the current has to travel through each component in turn. Adding resistors in series increases the total resistance (R) in ohms (Ω): Rtotal = R1 + R2
• •
•
•
V3
The potential difference across each component is the same. The total current drawn from the power supply is the sum of the currents through the separate components. The total resistance of two resistors is less than the resistance of the smallest individual resistor. This is because, in parallel, there are more paths for the current to take – it can take one or the other, allowing it to flow more easily. Adding resistors in parallel reduces the total resistance.
You need to be able to calculate the currents, potential differences and resistances in d.c. parallel circuits. A circuit containing two resistors in series has a 12V supply. R1 is a 4Ω resistor and has a voltage of 8V across it. a) Work out the voltage across R2. Vtotal = V1 + V2 12 = 8 + V2 V2 = 12 – 8 = 4V
192
Remember, in a series circuit, the power supply voltage is shared. Substitute in the given values. Rearrange to find the voltage across the second resistor.
GCSE Combined Science Revision Guide
227841 AQA Higher Tier Combined Science All-in-One_P188_229.indd 192
6/29/16 12:07 PM