Not every circuit contains both parallel and series sections, but the ones in your exam probably will. Start with short sections that are clearly just series or parallel and merge these resistors together. Then, when the diagram is simplified, you will be able to combine the new resistors.
Key Concepts
Combining resistors in parallel is slightly more complicated than in series:
- Resistors in parallel have the same potential difference across them. This is a result of Kirchhoff's second law, as the potential difference in any complete loop of the circuit must equal the EMF.
- However, current splits at a parallel junction in inverse ratio to the resistance.
- The resistance of a parallel combination is less than the individual resistors.
The equation is \({1\over R_T}={1\over R_1}+{1\over R_2}+...\)
NB: Think the algebra looks simple? Think again. You will need to combine the total right hand side before you flip.
Use quizzes to practise application of theory.
START QUIZ!
The diagram shows 4 bulbs a battery and 2 switches
Which bulb doesn't light when switch 1 is closed?
A is short circuited no current will pass through it.
The diagram shows 4 bulbs a battery and 2 switches
Which bulb is brightest when switch 1 is closed?
Twice as much current passes through B than D and C
The diagram shows 4 bulbs a battery and 2 switches
If switch 1 is closed then switch 2 opened what will happen to the lights?
A is no longer short circuited so the resistance of the circuit increases all 3 bulbs get dimmer.
Type the total resistance in Ohms (no need to type the units) before the combinations move out of view.
How much of Parallel resistors have you understood?
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