Draw and interpret circuit diagrams.
BEEZYREVISOR
GCSE Physics Revision
Learn it. Recall it. Revise it.
GCSE Physics revision
Standard circuit diagram symbols
Current, potential difference and resistance
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Interpret results or representations related to standard circuit diagram symbols.
Analyse an unfamiliar problem involving standard circuit diagram symbols and justify the method or conclusion.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
What is a Series Circuit?
- A series circuit has only a single loop, with all components connected one after the other in sequence.
- If any one component is disconnected or broken, the entire circuit stops working, which is a major disadvantage of series circuits.
- In practice, very few real-world devices are wired in series because of this reliability issue.
Voltage in a Series Circuit
- The total potential difference of the cell or battery is shared across all components in the circuit.
- This can be written as Vtotal = V1 + V2 + ... where each term represents the voltage across an individual component.
- For example, if a battery supplies 12 V and one lamp has 8 V across it, the other lamp must have 4 V, since 8 + 4 = 12.
- Identical components with the same resistance share the voltage equally, so two identical lamps on a 12 V supply would each receive 6 V.
Current in a Series Circuit
- Current is the same at every point in a series circuit, regardless of where it is measured.
- Current is measured using an ammeter, which is placed in series (within the main loop) and can be positioned anywhere in the loop.
- Ammeters have negligibly small resistance, so they do not affect circuit calculations and are ignored when working out total resistance.
Resistance in a Series Circuit
- The total resistance in a series circuit is the sum of the individual resistances of each component: Rtotal = R1 + R2 + ...
- The symbol Σ (sigma) means 'sum of' and is sometimes used in physics and maths to represent this total.
- For example, a lamp with 4 Ω and a lamp with 2 Ω give a total resistance of 4 + 2 = 6 Ω.
Using Ohm's Law to Find Current
- V
- Ohm's law states V = IR, which can be rearranged to I = R to calculate current.
- To find the current in a series circuit, divide the total potential difference by the total
- Vtotal resistance: I = R .
- total
- Using the example of a 12 V battery and a total resistance of 6 Ω gives a current of I = 12
- 6 = 2 A.
Calculating Voltage Across a Single Component
- Because current is the same throughout a series circuit, you can use V = IR with the current and a component's individual resistance to find its share of the voltage.
- For example, a 2 Ω lamp with a current of 2 A has a potential difference of V = 2 × 2 = 4 V across it.
- Once the voltage across one component is known, the voltage across the remaining components can be found by subtracting from the total: V2 = Vtotal − V1 .
Measuring Voltage with a Voltmeter
- A voltmeter is connected in parallel across the component whose voltage you wish to measure.
- Even though the voltmeter is connected in parallel, the overall circuit is still treated as a series circuit for calculation purposes.
- A voltmeter has a very high resistance so it draws negligible current and does not significantly affect the circuit.
Resistance and Voltage Share
- Components with greater resistance receive a larger share of the total voltage, as described by Ohm's law.
- This can be understood as more force (potential difference) being needed to push charge through regions of higher resistance.
- In the example, the 4 Ω resistor received 8 V whilst the 2 Ω resistor only received 4 V, reflecting their ratio of resistances.
Why We Use Circuit Diagram Symbols
- Circuit diagram symbols are standardised representations used to draw electrical circuits clearly and simply, replacing complex drawings of real components.
- There are 15 key circuit symbols that GCSE Physics students need to know and recognise.
Switches, Cells, and Batteries
- An open switch breaks the circuit so no current can flow, whilst a closed switch completes the circuit allowing current to flow.
- A single unit that provides electrical energy is called a cell, whereas a battery refers to two or more cells joined together.
- The longer line in the cell symbol represents the positive terminal, indicated by a plus sign.
Measuring Components: Ammeters and Voltmeters
- An ammeter is represented by a circle containing the letter 'A' and is used to measure current flowing through a circuit.
- A voltmeter is represented by a circle containing the letter 'V' and is used to measure potential difference (voltage) across a component.
Resistors and Their Variations
- A fixed resistor reduces the current in a circuit and has a constant, unchanging resistance value.
- A variable resistor can change its resistance value and is commonly used in dimmer switches to control the brightness of a lamp by varying the current.
- A thermistor is a temperature-dependent resistor whose resistance decreases as temperature increases, allowing it to act as a temperature sensor in a circuit.
- A light-dependent resistor (LDR) changes its resistance depending on light levels, making it useful as a light sensor to switch circuits on or off.
Lamps, LEDs, and Diodes
- A lamp (often a filament lamp) is a component that converts electrical energy into light energy.
- An LED (light-emitting diode) emits light and is shown with an arrow pointing outwards away from the symbol, indicating light is being given out.
- A diode allows current to flow in one direction only within a circuit, and unlike an LED, it does not emit light.
Protective Components: Fuses
- A fuse acts as a deliberate weak point in a circuit, designed to break (melt) if the current exceeds a safe level, protecting the rest of the circuit.
- Fuses are commonly found in three-pin plugs used in everyday household appliances.