Electricity (AQA GCSE Combined Science: Synergy: Physical Sciences): Flashcards

For an ohmic conductor at constant temperature, the current is directly proportional to the potential difference, so resistance remains constant.

False.

A greater resistance means a smaller current for a given potential difference.

V = 0.5 × 20 = 10 V

The unit of resistance (Ω), defined as one volt per ampere (V/A).

The resistance increases as the temperature of the metal conductor increases.

A component whose resistance is not constant — current and potential difference are not directly proportional. Examples include filament lamps, diodes, LDRs and thermistors.

Increasing current raises the temperature of the filament. Hotter atoms vibrate more, causing more collisions with free electrons and increasing resistance.

False.

A diode allows current to flow in one direction only (forward bias). In reverse bias, its resistance is very high and no current flows.

A component whose resistance decreases as temperature increases. Used in temperature sensors such as thermostats and fire alarms.

A component whose resistance decreases as light intensity increases. Used in light sensors such as automatic street lights.

For a filament lamp, the I–V graph is a curve (not a straight line), showing that resistance increases as current increases.

True.

Ohmic conductors obey Ohm's Law: current is directly proportional to potential difference, giving a straight-line I–V graph through the origin with constant resistance.

Linear (ohmic): fixed resistors, wires (at constant temperature)

Non-linear (non-ohmic): filament lamps, diodes

Linear components obey Ohm's Law; non-linear ones do not.

A graph of current (I) against potential difference (V) for a component, showing how the component behaves as voltage changes.

The variable resistor is used to change the potential difference across the component being tested, allowing a range of I–V readings to be taken.

True.

Reversing the power supply terminals reverses the direction of current and potential difference, allowing the I–V graph to be plotted for both positive and negative values.

The component is an ohmic conductor — it obeys Ohm's Law. Current is directly proportional to potential difference and resistance is constant.

In RP 15, the ammeter is connected in series to measure current, and the voltmeter is connected in parallel to measure potential difference.

The diode has a very low resistance and current flows through it. Current through a diode flows in one direction only.

True.

Heating changes resistance, which would make it a confounding variable. Switching off the circuit between readings keeps temperature controlled.

1. A fixed resistor (at constant temperature)

2. A filament lamp

3. A diode

Each produces a different shape of I–V graph.

The length of the resistance wire, measured in cm or m. The dependent variable is the resistance calculated from V/I.

A systematic error where a measuring instrument reads a non-zero value when it should read zero. In RP 16, this can occur if crocodile clips do not start at 0 on the ruler.

True.

The graph of resistance against length is a straight line through the origin, showing that resistance and length are directly proportional.

Resistance = potential difference ÷ current

R = V/I

where V is in volts (V) and I is in amperes (A), giving R in ohms (Ω).

In RP 16, the circuit must be switched off between readings to keep the temperature of the wire constant, preventing a change in resistance.

Resistors in series: combined resistance equals the sum of individual resistances — larger overall resistance.

Resistors in parallel: combined resistance is less than the smallest individual resistor.

True.

High current in a thin wire causes significant heating, which is a burn hazard. It also changes the wire's temperature and therefore its resistance, affecting results.

A variable other than the independent variable that affects the results. In RP 16, temperature is a confounding variable if the wire heats up during the experiment.

A circuit where components are connected in a single loop. The same current flows through every component; potential difference is shared between them.

A circuit where components are connected on separate branches. The potential difference across each branch is the same; current splits between branches.

True.

Adding branches creates extra paths for current, so more charge flows overall, meaning the overall resistance is lower than any single branch.

The potential difference (12 V) is shared between the two lamps. If the lamps are identical, each receives 6 V.

In a series circuit, the total resistance equals the sum of the individual resistances.

False.

In a series circuit, a break anywhere stops the current flowing in the whole circuit. In a parallel circuit, the other branches continue to operate.

The potential difference across each branch is equal to that of the supply. This is why parallel circuits are used in mains wiring.

A point in a parallel circuit where the current splits into separate branches. The total current entering a junction equals the total current leaving it.

The current splits at the junction. The total current entering equals the total current leaving:

I = I₁ + I₂

Electrons cannot be created or destroyed, so they divide between branches.

False.

Branch currents are only equal if the resistance in each branch is identical. If resistances differ, more current flows through the branch with lower resistance.

In a parallel circuit with two branches, if the total current is 6 A and branch 1 carries 2 A, then branch 2 carries 4 A.

Total current = I₁ + I₂, so I₂ = 6 − 2 = 4 A.

In a series circuit, current is the same at all points. All components share the same flow of electrons.

In a parallel circuit, the total current equals the sum of the currents in each branch:

I = I₁ + I₂

Current splits at junctions.

False.

Branch currents are only equal if the resistance in each branch is identical. If resistances differ, more current flows through the branch with lower resistance.

In series, all electrons travel the same single path, so current is identical everywhere.

In parallel, electrons split at junctions between branches, so current divides (but is conserved: total in = total out).

Total current = 1 + 3 + 2 = 6 A

Using I = I₁ + I₂ + I₃ — the total current equals the sum of all branch currents.

The combined resistance of resistors in series equals the sum of their individual resistances:

R = R₁ + R₂ + R₃ ...

where R is in ohms (Ω).

Total resistance = 10 + 20 + 30 = 60 Ω

Using R = R₁ + R₂ + R₃ for resistors in series.

True.

Adding a parallel branch creates an extra path for current. More charge can flow, so the overall resistance decreases below the value of any individual resistor.

Series: all charge passes through every resistor — more components to push through means more opposition to current.

Parallel: extra branches give charge more paths to take, so more total current flows and overall resistance is lower.

Two resistors in parallel have resistances of 6 Ω and 12 Ω. The total resistance is less than 6 Ω.

The total resistance in parallel is always less than the smallest individual resistor.

False.

At GCSE you only need to explain qualitatively that parallel resistors reduce total resistance. You are not required to use the parallel resistance formula.

R₃ = 60 − 30 − 10 = 20 Ω

Using R_total = R₁ + R₂ + R₃, rearranged to find the unknown resistor.

The single resistance value that would have the same effect on a circuit as a combination of resistors. For series: R_total = R₁ + R₂; for parallel: total is less than the smallest.

A safety device in a circuit. If current becomes too large, the fuse wire melts, breaking the circuit and cutting off electricity to the appliance.

A resistor with adjustable resistance, used in dimmer switches and volume controls. Its circuit symbol shows a resistor with an arrow through it.

Ammeter: connected in series with the component being measured.

Voltmeter: connected in parallel across the component being measured.

False.

A diode allows current to flow in one direction only (forward bias). In reverse bias the resistance is very high and no current flows.

A thermistor's resistance decreases as temperature increases. An LDR's resistance decreases as light intensity increases.

The earth wire is a safety wire. If the appliance casing becomes live, the earth wire provides a low-resistance path to ground, causing a surge of current that blows the fuse and breaks the circuit.

True.

By definition, conventional current flows from positive to negative. Note: electrons actually flow in the opposite direction (negative to positive).

A type of diode that emits light when current passes through it in forward bias. Used in displays, road signs and aviation lighting.

A current that flows in one direction only at a steady rate. Produced by cells and batteries. Potential difference is constant in one direction.

A current that continuously reverses direction, going back and forth in a circuit. Mains electricity in the UK is a.c. at 50 Hz and about 230 V.

False.

Batteries and cells produce direct current (d.c.), which flows in one direction only. Alternating current (a.c.) is produced by generators such as those in power stations.

Frequency: 50 Hz (direction changes 50 times per second)

Potential difference: approximately 230 V

UK mains is an alternating current (a.c.) supply.

On an oscilloscope, d.c. appears as a flat horizontal line, while a.c. appears as a sinusoidal (wave-shaped) curve.

True.

In any series circuit (including d.c.), the current is the same at every point. All charge flows through one path with no junctions to split it.

Frequency is the number of times per second the current changes direction back and forth. It is measured in hertz (Hz). UK mains has a frequency of 50 Hz.

1. If one component breaks, all others stop working.

2. Components cannot be individually switched on or off.

Parallel circuits avoid both of these problems.

The brown wire in a mains cable. It carries the alternating potential difference (~230 V) from the supply and is the most dangerous wire.

False.

A live wire is dangerous even when the switch is open. The wire itself remains at ~230 V; touching it can still cause a lethal electric shock.

The blue wire in a mains cable. It completes the circuit with the live wire and is at a voltage close to 0 V, making it much less dangerous.

The three wires in a mains cable are: live (brown), neutral (blue), and earth (green and yellow).

The green and yellow striped safety wire in a mains cable. It is at 0 V and only carries current if there is a fault, preventing the appliance casing from becoming live.

The earth wire provides a low-resistance path to ground. A large current surges through the earth and live wires, blowing the fuse and breaking the circuit, making the appliance safe to touch.

False.

The earth wire carries current only if there is a fault. Normally it carries no current and is at 0 V. It is a safety device, not part of the working circuit.

The live wire is at approximately 230 V while the neutral wire is close to 0 V. If the live wire touches a person (at 0 V), there is a large potential difference and a dangerous current flows through them to earth.

The rate of energy transfer, or energy transferred per unit time. Measured in watts (W).

P = E/t

The unit of power. One watt equals one joule per second:

1 W = 1 J/s

Power = energy transferred ÷ time

P = E/t

where P is in watts (W), E in joules (J) and t in seconds (s).

True.

Power = E/t. If the same energy is transferred in a shorter time, the denominator decreases, so power is greater.

An iron with a power rating of 1000 W is used for 3 minutes (180 s). The energy transferred is 180 000 J.

E = P × t = 1000 × 180 = 180 000 J.

Power = E/t = 40/5 = 8 W

Using P = E/t, with E in joules and t in seconds.

True.

The watt is defined as one joule per second. If time is given in minutes or hours, it must be converted to seconds before substituting into P = E/t.

The power at which an appliance is designed to operate, shown on a label. A higher power rating means more energy is transferred per second.

The rate of energy transfer in an electrical circuit, equal to potential difference multiplied by current:

P = VI

Measured in watts (W).

P = I²R

where P is power in watts (W), I is current in amperes (A), and R is resistance in ohms (Ω).

True.

Using P = VI: V = P/I = 48/4 = 12 V.

Power, potential difference and current are linked by P = VI.

Power can be calculated as P = V × I, or as P = I² × R, depending on which quantities are known.

P = I²R = 6² × 20 = 36 × 20 = 720 W

Using the equation P = I²R for power and resistance.

True.

Power is the rate of energy transfer: 1 W = 1 J/s. This means a 1 W device transfers 1 joule of energy every second.

An equation for electrical power in terms of current and resistance. Derived by combining P = VI with V = IR.

P in watts, I in amperes, R in ohms.

Power = V × I = 230 × 10 = 2300 W

Using P = VI. This is equivalent to 2.3 kW.

A label value showing how much energy an appliance transfers per second. A higher power rating means faster energy transfer.

The 2 kW iron takes half the time of the 1 kW iron, because it transfers energy twice as fast.

True.

The electric motor in a washing machine transfers energy electrically to the kinetic energy store of the drum, rotating it to wash clothes.

Everyday appliances transfer energy from the mains to either the kinetic energy store of an electric motor, or the thermal energy store of a heating device.

A device that transfers energy electrically to a kinetic energy store. Found in appliances such as vacuum cleaners, washing machines and refrigerators.

False.

Energy transferred depends on both the power rating and the time the appliance is used.

1. Kettle — transfers energy electrically to the thermal energy store of the water

2. Toaster — transfers energy electrically to the thermal energy store of the heating element

Other examples: iron, electric oven, radiator.

1. E = P × t (energy = power × time)

2. E = Q × V (energy = charge × potential difference)

where E is in joules, t in seconds, Q in coulombs, V in volts.

The equation linking energy transferred, charge and potential difference:

E = Q × V

where E is in joules (J), Q in coulombs (C), and V in volts (V).

True.

E = P × t = 2000 × (3 × 60) = 2000 × 180 = 360 000 J.

Remember to convert minutes to seconds before calculating.

A current of 0.7 A flows through a potential difference of 4 V for 60 s. The energy transferred = 0.7 × 4 × 60 = 168 J.

Using E = I × V × t.

E = Q × V = 30 × 12 = 360 J

Using E = QV, where Q is charge in coulombs and V is potential difference in volts.

True.

Both equations calculate the same quantity — energy transferred. They are equivalent because P = IV and Q = It, so both reduce to the same underlying relationship.

The power rating of an appliance (in watts) gives the energy transferred per second. Higher power rating = more energy used in the same time.

Because power is measured in watts = joules per second. Using minutes or hours would give an incorrect answer. Always convert: 1 min = 60 s, 1 hour = 3600 s.

The National Grid is a system of cables and transformers linking power stations to consumers across the UK, used to transfer electrical power.

A step-up transformer increases potential difference and decreases current. Used in the National Grid to reduce energy losses during transmission.

High potential difference means low current (for the same power, using P = IV). Low current reduces heating in the cables, so less energy is wasted.

False.

A step-down transformer decreases the potential difference (and increases the current) to a safe level suitable for domestic use in homes and offices.

In the National Grid, a step-up transformer increases voltage and decreases current before transmission.

The cables have resistance. When current flows through a resistance, energy is dissipated as heat (thermal energy in the surroundings). The higher the current, the greater the heating effect and wasted energy.

True.

From P = IV: for the same power, increasing V means I must decrease. Lower current means less heating in cables and less energy wasted.

A step-down transformer decreases potential difference and increases current. Used at the end of National Grid transmission to make electricity safe for domestic use.