Current, Potential Difference and Resistance

Below is the full set you need.

Drawing rules AQA marks reward:

1. Use straight lines, ideally drawn with a ruler.
2. Use 90° corners (not curves) where wires turn.
3. Don't draw arrowheads on wires.
4. Label values where given (e.g. '6 V cell', '10 Ω resistor').
5. Components should NOT touch each other — leave a gap of wire between them.

Meter placement (covered in detail in 4.2.1.3):

• Ammeter measures CURRENT — placed in series with the component you're measuring. Current must flow THROUGH the ammeter.
• Voltmeter measures POTENTIAL DIFFERENCE — placed in parallel with (i.e. across) the component. The voltmeter does NOT carry the main circuit current.

Mnemonic: Ammeter in series; Voltmeter across.

Diodes and LEDs. Both let current flow only in the direction the triangle points (anode → cathode). The LED also emits light. In a series circuit, putting an LED 'the wrong way' simply prevents current — no light, no damage at safe voltages.

Cells vs batteries. A single cell (e.g. AA = 1.5 V) is drawn as one short and one long line. A 'battery' is two or more cells joined in series — usually drawn as multiple pairs of lines, OR as one cell with broken lines.

Electric charge is a fundamental property of matter — measured in coulombs (C). Electrons carry a tiny negative charge; protons carry the equal positive charge.

Electric current is the rate of flow of charge through a circuit:

$Q = I \, t$

• $Q$ = charge (C)
• $I$ = current (A)
• $t$ = time (s)

So 1 amp = 1 coulomb per second. A 2 A current means 2 C of charge passes any point in the circuit every second.

Worked example. A current of 0.5 A flows for 4 minutes. How much charge has passed?

• $t = 4 \times 60 = 240\,\text{s}$
• $Q = I t = 0.5 \times 240 = 120\,\text{C}$

Worked example (rearranging). 60 C passes through a torch bulb in 30 s. What is the current?

$I = \frac{Q}{t} = \frac{60}{30} = 2.0\,\text{A}$

Conditions for current to flow:

1. A closed circuit (continuous loop of conductor — no break).
2. A source of potential difference (cell, battery, mains).

Without a closed circuit, charge has nowhere to go. Without a p.d., there's no 'push' to drive the charges.

In a series circuit (single loop, no branches), current is the same at every point — because charge has nowhere else to go.

If the ammeter at the start of the loop reads 0.5 A, the ammeter at any other point will also read 0.5 A.

This is a direct consequence of conservation of charge: charge doesn't pile up or disappear in a wire.

In a parallel circuit (covered in 4.2.2), the rule is different — current splits between branches.

Historically (before electrons were discovered), Benjamin Franklin guessed positive charges were the carriers. He defined current as flowing from + to − terminal. This conventional current convention is still used in all physics diagrams.

We now know that in metal wires, the actual mobile carriers are negatively charged electrons, which drift from − to +. So:

• Conventional current: + → − (outside the cell).
• Electron flow: − → + (outside the cell).

AQA exams use the conventional-current convention. When you draw an arrow on a circuit, it should point from the positive terminal, round the external circuit, to the negative terminal.

Potential difference, current and resistance are related by:

$V = I \, R$

• $V$ = potential difference (volts, V)
• $I$ = current (amperes, A)
• $R$ = resistance (ohms, Ω)

Rearranged: $I = V/R$ and $R = V/I$.

Worked example. A 12 V battery drives 0.3 A through a resistor. Find R.

$R = V/I = 12 / 0.3 = 40\,\text{Ω}$

Definitions:

• Potential difference (p.d., V) — the work done per unit charge by the cell driving the charge round the circuit. 1 V = 1 J/C.
• Resistance (R, Ω) — how strongly a component opposes current. Higher resistance → smaller current at the same p.d.

A component obeys Ohm's law if R is constant as V changes. Resistors and wires at constant temperature obey it. Bulbs and diodes do NOT (covered in 4.2.1.4).

Apparatus: length of resistance wire (e.g. constantan or nichrome) taped to a metre rule, two crocodile clips, low-voltage power supply (or cell), ammeter, voltmeter, switch.

Method:

1. Connect the wire in series with the cell, switch and ammeter; put the voltmeter across the wire.
2. Set the variable length, e.g. start at 100 cm.
3. Briefly close the switch, record V and I; open the switch immediately to avoid heating the wire.
4. Calculate R = V/I.
5. Repeat for shorter lengths (e.g. 90, 80, 70, …, 20 cm).
6. Plot R against L.

Result: R is directly proportional to L (a straight line through the origin).

Sources of error:

• Wire heating up changes R — keep switch closed only briefly.
• Length measurement: read at the inside edge of each crocodile clip.
• Junction contact resistance — keep clips tight.

Series-resistor part of RP3. Replace the wire with two resistors in series. Measure total resistance and confirm $R_{\text{total}} = R_1 + R_2$.

A fixed resistor kept at constant temperature is an ohmic conductor. Its I–V graph is a straight line through the origin:

$I = \frac{1}{R} \, V$

The gradient gives $1/R$ (so a steeper line means SMALLER resistance).

If you reverse the polarity (negative V), current also reverses — the line continues into the third quadrant.

This linear behaviour requires constant temperature. If the resistor heats up during measurement, R rises and the line curves.

A filament lamp (incandescent bulb) heats up as more current flows. A hotter metal filament has more vibrating ions colliding with electrons → higher resistance.

I–V graph shape: an S-curve through the origin. Steep near zero (low R when cold), flatter at high |V| (high R when hot).

The lamp is symmetric — reversed voltage produces a mirror-image curve.

This is why a torch bulb feels warm in your hand: a lot of electrical energy is dissipated as thermal store of the filament.

A diode allows current to flow in only one direction (forward bias). Below ~0.6 V (silicon) almost no current flows. Above 0.6 V, the current rises steeply.

Reverse the polarity: virtually zero current at any negative voltage (until extreme reverse breakdown — not assessed at GCSE).

Diodes are used in rectifiers (converting AC to DC) and many electronic devices.

Thermistor (NTC type) — resistance DECREASES as temperature rises.

• Used in: car engine temperature sensors, fire alarms, fridge thermostats.
• A high R at low temperature triggers a heater on; low R at high temperature switches it off.

Light-dependent resistor (LDR) — resistance DECREASES as light intensity rises.

• Used in: street lights (turn on at dusk), camera light meters, automatic car headlights.
• Dark → high R; bright → low R.

Both have characteristic curves: resistance falling smoothly as the sensed quantity (T or light) rises.

Method (for each component):

1. Set up a series circuit: cell, switch, variable resistor (rheostat), ammeter, and the component under test. Voltmeter goes across the component.
2. Adjust the variable resistor to set V at intervals (e.g. 0.5, 1.0, 1.5… up to ~6 V).
3. Record I at each V; reverse the cell polarity to repeat with negative V.
4. Plot I (y-axis) vs V (x-axis).

Expected results: straight line for resistor; S-curve for lamp; one-sided curve for diode.

Note: for the diode, include a protective series resistor to limit current at high V.