Series circuits

1. In a series circuit, the components are joined end to end so that they form a single loop with the power supply.
2. There is only one path around the circuit, so all of the charge that leaves the supply passes through every component in turn.
3. The current is the same at every point in a series circuit, because charge is not used up by the components and cannot leave the loop anywhere.
   1. An ammeter placed before the first lamp, between two lamps or after the last lamp gives the same reading.
4. The potential difference of the supply is shared between the components, so the potential differences across the components add up to the potential difference across the supply.
   1. For two components in series, this is written as $V_{supply}=V_1+V_2$.
   2. The component with the larger resistance takes the larger share of the potential difference.
5. In terms of energy, each coulomb of charge is given energy by the supply and then transfers part of that energy in each component it passes through.
   1. The energy transferred by each coulomb in all of the components adds up to the energy it was given by the supply.
6. If one component is removed, breaks or is switched off, the single loop is broken and every component stops working.
   1. Old-style strings of festive lights were wired in series, so one failed bulb made the whole string go dark.
7. One switch placed anywhere in a series circuit turns all of the components on or off together.

Parallel circuits

1. In a parallel circuit, each component is placed on its own branch, and every branch is connected between the same two points of the circuit.
2. The current from the supply splits at a junction, part of it flows along each branch, and the branch currents recombine at a second junction.
3. The total current from the supply is equal to the sum of the branch currents, written $I_{total}=I_1+I_2$ for two branches.
   1. A branch with a smaller resistance carries a larger current than a branch with a larger resistance.
4. Each branch is connected directly across the supply, so the potential difference across every branch is the same and equal to the supply potential difference.
5. In terms of energy, each coulomb of charge passes through only one branch, so it transfers all of the energy it was given by the supply in that branch.
6. Each branch can be switched on and off independently, and a break in one branch does not stop charge flowing through the others.
   1. The lights and sockets in a house are wired in parallel, so every appliance receives the full mains potential difference and switching off the kettle does not switch off the fridge.
7. Adding another identical lamp in series shares the supply potential difference between more lamps, so each lamp gets a smaller share and glows more dimly.
8. Adding another identical lamp in parallel gives it the full supply potential difference, so it glows as brightly as the lamps already there, while the total current from the supply increases.

Comparing the two circuits

1. Paths: a series circuit has one path for the charge, whereas a parallel circuit has a separate path through each branch.
2. Current: the current is the same everywhere in a series circuit, whereas in a parallel circuit the branch currents add up to the total current.
3. Potential difference: the supply potential difference is shared between series components, whereas each parallel branch has the full supply potential difference across it.
4. Energy: each coulomb transfers some of its energy in every series component, whereas it transfers all of its energy in the one parallel branch it passes through.
5. Faults and switching: one break stops a whole series circuit, whereas a break in one parallel branch leaves the other branches working.

Positioning measuring instruments

1. An ammeter measures the current through a component in amperes, $A$.
   1. To fit an ammeter, the circuit is broken at the point of interest and the ammeter is connected in series so that the same charge flows through it.
   2. An ammeter has a very low resistance, so it does not noticeably reduce the current it is measuring.
2. In a series circuit, an ammeter can go anywhere in the loop because the current is the same at every point.
3. In a parallel circuit, an ammeter in the main part of the circuit measures the total current, while an ammeter in a branch measures only the current in that branch.
4. A voltmeter measures the potential difference across a component in volts, $V$.
   1. A voltmeter is connected in parallel with the component, with one lead on each side of it, and the circuit does not need to be broken.
   2. A voltmeter has a very high resistance, so almost no current flows through it and it does not change the circuit it is measuring.
5. A voltmeter across the supply measures the supply potential difference, which equals the sum of the series voltmeter readings or the reading across any one parallel branch.
6. The positive terminal of each meter should be connected to the side of the circuit nearest the positive terminal of the supply, or a digital meter shows a negative reading and an analogue needle moves the wrong way.