Electricity

Explanation

• Circuit diagrams use standard symbols so that the components and their connections are unambiguous. Follow the conducting lines to decide which components are in the same loop and where branches begin.
• An ammeter measures the current through a component, so connect it in series with that component.
• A voltmeter measures the potential difference between two points, so connect it in parallel across the component.
• A working measuring circuit also needs a source of potential difference and a closed conducting path.
• The examiner expects recognisable standard symbols joined by lines, not pictures of apparatus.

Explanation

• Electric current is the rate of flow of electrical charge.
• Charge flow, current and time are linked by $Q=It$, where $Q$ is in coulombs, $I$ is in amperes and $t$ is in seconds.
• A source of potential difference and a closed circuit are required for charge to flow.
• At every point in a single closed loop, the current has the same value because charge cannot accumulate at one point.
• Current is not the amount of charge present; it tells you how many coulombs pass a point each second.

Explanation

• The current through a component depends on its resistance and the potential difference across it. Use $V=IR$, where potential difference is in volts, current in amperes and resistance in ohms.
• For a fixed potential difference, increasing resistance decreases current.
• Measure current with an ammeter in series and potential difference with a voltmeter in parallel; corresponding readings give $R=V/I$.
• Potential difference describes energy transferred per unit charge, while resistance describes how strongly the component opposes current.
• Use the values for the same component and the same operating condition.

Explanation

• At constant temperature, an ohmic conductor has current directly proportional to potential difference, so its resistance is constant and its current–potential difference graph is a straight line through the origin.
• A filament lamp heats as current increases, so its resistance rises and the graph becomes less steep.
• A diode conducts readily in one direction but has very high resistance in reverse.
• Thermistor resistance decreases as temperature increases, while LDR resistance decreases as light intensity increases.
• Investigate a component using an ammeter in series, a voltmeter in parallel and paired readings as the supply is varied.

Explanation

• In a series circuit, current is the same through every component, the supply potential difference is shared, and resistances add: $R_{\text{total}}=R_1+R_2+\ldots$.
• In a parallel circuit, the potential difference is the same across every branch and the total current equals the sum of the branch currents.
• Adding a parallel branch gives charge another path, so total resistance decreases and is less than the smallest branch resistance.
• Use junctions to decide whether components share one path or sit on separate branches; do not apply series rules just because symbols are drawn next to each other.

Explanation

• A direct potential difference has one polarity, so it drives current in one direction; cells and batteries provide dc.
• An alternating potential difference repeatedly reverses polarity, so the current also repeatedly reverses direction.
• The UK mains supply is approximately $230\,\text{V}$ ac at $50\,\text{Hz}$.
• A frequency of $50\,\text{Hz}$ means $50$ complete cycles each second, with a period of $1/50=0.020\,\text{s}$.
• On a potential difference–time graph, dc stays on one side of zero, whereas ac crosses zero and alternates between positive and negative values.

Explanation

• A three-core mains cable contains a brown live wire, blue neutral wire and green-and-yellow earth wire.
• The live wire carries the alternating potential difference from the supply; the neutral completes the circuit and is close to $0\,\text{V}$.
• The earth is a safety wire connected to a metal case and normally carries no current.
• If a fault makes the case live, a large current flows through the low-resistance earth path so the fuse melts or circuit breaker opens.
• The live wire is dangerous even when a switch is open, so switches and fuses must be placed in the live wire.

Explanation

• Power is the rate at which energy is transferred, so one watt is one joule per second.
• For an electrical component, use $P=VI$ when potential difference and current are known.
• Combining this with $V=IR$ gives $P=I^2R$, useful when current and resistance are known.
• A larger power rating means the appliance transfers more energy each second, not that it necessarily transfers more energy overall; total energy also depends on operating time.
• Use potential difference in volts, current in amperes and resistance in ohms to obtain power in watts.

Explanation

• Electrical appliances transfer energy from the mains or a battery to other energy stores. A motor increases a kinetic energy store, while a heater increases a thermal energy store.
• Calculate transferred energy using $E=Pt$, with power in watts and time in seconds for energy in joules.
• The alternative equation $E=QV$ links energy transferred to charge flow and potential difference.
• Appliance efficiency depends on how much of the input energy reaches the intended store; unwanted transfers usually heat the surroundings.
• A high-power appliance transfers energy quickly, but energy used also depends on its operating time.

Explanation

• The National Grid is the network of cables and transformers linking power stations to consumers. A step-up transformer increases potential difference before transmission.
• For a given power, $P=VI$ shows that a higher potential difference allows a smaller current.
• Cable heating depends on $P_{\text{loss}}=I^2R$, so reducing current greatly reduces energy wasted heating the cables.
• Near consumers, step-down transformers reduce the potential difference to safer, useful values.
• Transformers do not create energy: they change the potential difference and current so electrical energy can be transferred efficiently over long distances.

Explanation

• When certain insulating materials are rubbed together, electrons transfer from one material to the other. The material gaining electrons becomes negatively charged; the material losing electrons is left with an equal positive charge.
• Protons do not move between the materials.
• Like charges repel and unlike charges attract through non-contact electrostatic forces.
• Because charge cannot easily flow away from an insulator, it may build up until a spark transfers charge through the air.
• Static discharge can be useful, but it can also ignite flammable vapours or damage electronic components.

Explanation

• A charged object creates an electric field around itself. Another charged object placed in that field experiences a force; the interaction is stronger where the field is stronger.
• Field lines show the direction of force on a positive test charge.
• Around an isolated positive sphere they point radially outwards, while around a negative sphere they point radially inwards.
• Lines drawn closer together represent a stronger field, so the field is strongest near the charged object.
• Field lines never cross because the force on a positive test charge at one point has only one direction.