Series and parallel circuits — AQA GCSE Combined Science

This statement introduces the rules that apply when components are connected in series. In a series circuit there is a single loop, so the current is the same at every point: if 0.30 A leaves the cell, 0.30 A passes through each component. The total potential difference of the supply is shared between the components, so the p.d.s across the components add up to the supply p.d.; for a 6.0 V supply with two identical lamps, each lamp takes 3.0 V. The total resistance of the circuit is the sum of the individual resistances, so adding a component increases the total resistance and reduces the current. These rules let you calculate an unknown current, p.d. or resistance in a series circuit, and they explain why removing one series component breaks the circuit and stops the current everywhere.

In a series circuit, the current is the same at every point because there is only one path for charge to flow. Charge is conserved: the rate of flow of charge (current) cannot change unless charge accumulates, which it does not in a simple series loop. For example, if a cell, lamp and resistor are connected in series and an ammeter reads 0.30 A between the cell and lamp, then an ammeter placed between the lamp and resistor also reads 0.30 A, and between the resistor and cell it again reads 0.30 A. This holds regardless of component resistance or potential difference. The current is determined by the total resistance and the supply potential difference, but its value is identical through each component. This principle allows you to calculate current in any part of a series circuit once you know it at one point.

In a series circuit, the total potential difference (p.d.) of the power supply is divided among the components. The sum of the potential differences across each component equals the potential difference of the supply. This follows from conservation of energy: the energy transferred per unit charge by the source is shared as energy is transferred by each component. For example, if a 6.0 V battery is connected in series with two identical lamps, each lamp has a p.d. of 3.0 V. If the lamps have different resistances, the p.d.s differ but still sum to 6.0 V. The p.d. across a component is measured with a voltmeter connected in parallel. This rule allows you to calculate an unknown p.d. by subtracting known values from the supply p.d.

In a series circuit the current has only one path, so it passes through each component in turn. Each component resists that current, and the battery must drive the current through every resistance, so the total resistance is the sum of the individual resistances. For example, a 4 Ω resistor and a 6 Ω resistor in series give a total resistance of 4 Ω + 6 Ω = 10 Ω. Adding a component always increases total resistance and reduces the current for a fixed potential difference. This rule applies only to components connected in series, not in parallel. In parallel, the total resistance is less than the smallest individual resistance because there are additional paths for the current.

This equation gives the total resistance of two resistors connected in series. Rtotal is the total resistance in ohms (Ω), R₁ is the resistance of the first component and R₂ is the resistance of the second component. Because the same current passes through both components in turn, the potential differences add, and so the resistances add. For example, if R₁ = 5 Ω and R₂ = 7 Ω, then Rtotal = 5 Ω + 7 Ω = 12 Ω. The equation extends to more than two series components by adding each resistance in turn. It does not apply to parallel circuits, where the total resistance is less than the smallest individual resistance.

Resistance measures how strongly a component opposes the flow of electric charge. It is defined by R = V ÷ I, where V is the potential difference across the component in volts and I is the current through it in amperes. The unit of resistance is the ohm, symbol Ω; one ohm means one volt is needed to drive one ampere through the component. In a series circuit the same current passes through each component, so a larger resistance produces a larger share of the supply potential difference. For example, a resistor with 6 V across it and 2 A through it has R = 6 V ÷ 2 A = 3 Ω. Resistance arises because charge carriers collide with ions in the material, transferring energy and heating the component.

In a parallel circuit, components are connected across the same two points, so each branch has the full supply potential difference across it. The current from the supply splits between the branches: the current through each branch depends on that branch's resistance, and the total current is the sum of the branch currents. Because adding a branch provides another path for charge, the total resistance of the circuit is less than the resistance of any single branch. For example, with a 6 V supply and branches of 3 Ω and 6 Ω, the branch currents are 2 A and 1 A, giving a total current of 3 A and a total resistance of 6 V ÷ 3 A = 2 Ω. This is why domestic appliances are connected in parallel: each receives the full mains potential difference and can be switched independently.

This statement applies to components connected in parallel. In a parallel circuit, each branch is connected directly across the same two points of the supply, so every branch receives the full supply potential difference. If a 6 V battery feeds two lamps in parallel, the potential difference across each lamp is 6 V, even if the lamps have different resistances. This differs from a series circuit, where the supply potential difference is shared between components. The rule holds because all parallel branches share the same pair of connection nodes, so the electrical 'push' on each branch is identical. A voltmeter connected across any branch reads the same value as across the supply. This explains why household appliances, wired in parallel, each operate at the full mains potential difference and can be switched independently.

This statement describes current in parallel circuits. Charge is conserved, so the current arriving at a junction equals the total current leaving it. In a parallel circuit, the current from the supply splits between the branches, and the total supply current equals the sum of the branch currents. For example, if branches carry 0.3 A and 0.5 A, the supply current is 0.8 A. This is the junction rule applied at each split and rejoin point. It contrasts with a series circuit, where current is the same at every point. Branch currents depend on branch resistance: a lower-resistance branch takes a larger share of the total current. This rule explains why adding parallel branches increases the total current drawn from a supply.

This statement requires a qualitative understanding of resistors in parallel. Students must explain why adding resistors in parallel decreases the total resistance, without using the reciprocal formula (which is not required for this specification). When a second resistor is added in parallel, it provides an additional pathway for the current to flow. Because the potential difference across each branch is the same as the power supply, the total current leaving the supply increases. According to the equation R = V / I, if the total current increases for a constant potential difference, the total resistance of the circuit must decrease. Therefore, the combined resistance is always less than the resistance of the smallest individual resistor.

Circuit diagrams are standardised drawings in which each component has its own symbol, so a diagram can be read and rebuilt by anyone. To construct a circuit, trace the diagram from the cell: connect the positive terminal to the first component, continue through each component in turn, and return to the negative terminal, keeping cells, lamps, switches, resistors, ammeters and voltmeters in their correct positions. Ammeters go in series with the component being measured; voltmeters go in parallel across it. To check a circuit, compare the built circuit with the diagram component by component, confirm each symbol matches the real component, and test that switches control the intended part. A lamp that stays lit when its switch is open shows a wiring error, such as a short circuit or a misplaced connection.

The difference lies in how components are connected and how current and potential difference behave. In a series circuit the components form one loop, so the same current passes through each component in turn; the cell potential difference is shared between them, and adding a component increases the total resistance, reducing the current. In a parallel circuit the components sit on separate branches connected across the same cell terminals; the current from the cell splits between the branches and recombines, while the potential difference across each branch equals the supply potential difference. Adding a branch therefore increases the total current drawn. A useful check is to remove one lamp: in series the whole circuit stops, whereas in parallel the other branches keep working.

In a series circuit, the same current passes through each component in turn, so each added resistor provides another obstacle to charge flow; the potential difference is shared and the total resistance is the sum of individual resistances, so adding resistors increases total resistance. In a parallel circuit, each added resistor provides an extra branch, giving charge additional paths between the same two points; the potential difference across each branch is the same, and the combined current is larger than through any single branch, so the total resistance decreases. For example, two identical resistors in parallel have half the resistance of one because the current doubles for the same potential difference. This qualitative explanation links circuit topology to current, potential difference and resistance without requiring the parallel resistance formula.

A dc series circuit has one continuous path, so the same current flows through every component and the supply potential difference is shared between them. This makes series circuits useful for measurement and testing: an ammeter is connected in series so that the current being measured passes through it, and a variable resistor in series can control that current. A test circuit might contain a cell, a switch, an ammeter, a component under test and a variable resistor. The switch makes and breaks the circuit safely, the ammeter monitors current, and the variable resistor allows the potential difference across the component to be varied so that current-potential difference characteristics can be investigated. A voltmeter is connected in parallel with the component whose potential difference is being measured, because it must compare the two points across that component.

In a dc series circuit there is a single loop, so the current is the same at every point: I₁ = I₂ = I₃. The supply potential difference is shared between components, so V_total = V₁ + V₂ + V₃, and the total resistance is the sum of the individual resistances, R_total = R₁ + R₂ + R₃. To calculate an unknown quantity, first identify what is known, then choose the relationship linking the known and unknown values. For example, a 6.0 V supply with a 2.0 Ω and a 4.0 Ω resistor in series gives R_total = 6.0 Ω, so I = V ÷ R = 6.0 V ÷ 6.0 Ω = 1.0 A; the potential differences are V = I × R = 2.0 V and 4.0 V, which sum to the supply value. Always include units and check that the shared potential differences add to the supply value.

Equivalent resistance is the single resistance that would replace a group of resistors without changing the current drawn from the supply. For resistors in series, the equivalent resistance is the sum of the individual resistances: R_eq = R₁ + R₂ + R₃. Once R_eq is known, the circuit behaves as one resistor, so I = V_supply ÷ R_eq gives the current through every component. For example, a 12 V supply with 3.0 Ω, 4.0 Ω and 5.0 Ω in series has R_eq = 12 Ω, so I = 12 V ÷ 12 Ω = 1.0 A. The equivalent resistance is always larger than the largest individual resistance in the series group, which is a useful check on the answer.