P3.2 - Simple cicuits

Circuit symbol: cell

Single cell — short line (negative) and long line (positive)

Circuit symbol: battery

Two or more cells in a row

Circuit symbol: switch (open vs closed)

Open switch: gap in the line; Closed switch: complete line

Circuit symbol: fixed resistor

Rectangle

Circuit symbol: variable resistor

Rectangle with an arrow through it diagonally

Circuit symbol: thermistor

Rectangle with a line bent at 45° through it

Circuit symbol: LDR

Circle with a resistor symbol inside and two arrows pointing INWARDS

Circuit symbol: LED

Diode triangle with two arrows pointing OUTWARDS (often in a circle)

Circuit symbol: diode

Triangle pointing to a vertical line (arrow shows direction of conventional current flow)

Circuit symbol: lamp

Circle with an X inside

Circuit symbol: ammeter

Circle with an A inside

Circuit symbol: voltmeter

Circle with a V inside

Circuit symbol: motor

Circle with an M inside

[DRAW: one reference image showing all key circuit symbols — cell, battery, switch, resistor, variable resistor, thermistor, LDR, LED, diode, lamp, ammeter, voltmeter, motor]

Function: cell/battery

Provides circuit with a source of potential difference (energy per unit charge); battery = two or more cells

Function: switch

Turns the circuit on (closed) or off (open)

Function: fixed resistor

Limits the flow of current; has a resistance that cannot change

Function: variable resistor

Resistor with a slider that can change its resistance; used in dimmer switches and volume controls

Function: thermistor

Resistance depends on temperature — as temperature increases, resistance decreases (and vice versa)

Function: LDR (light-dependent resistor)

Resistance depends on light intensity — as light intensity increases, resistance decreases (and vice versa)

Function: motor

Converts electrical energy into mechanical energy

Function: diode

Allows current to flow in one direction only; used to convert AC to DC

Function: LED (light-emitting diode)

Equivalent to a diode but emits light when current passes through; used in aviation lighting, TVs, road signs

Function: ammeter

Measures current in a circuit; connected in SERIES

Function: voltmeter

Measures potential difference across a component; connected in PARALLEL

Resistance of ammeter and voltmeter in exams

Taken as negligible

Key rules for drawing a circuit diagram

(1) Must have an energy source (cell, battery, or power supply).

(2) Must be a closed path/complete circuit — electrons need a complete loop for current to flow.

(3) Ammeter in series; voltmeter in parallel.

(4) Current flows from positive to negative terminal (conventional current)

What is potential difference?

The amount of energy transferred per unit of charge passing through the terminals; 1 V = 1 J/C

Units of potential difference

Volts (V); 1 V = 1 joule per coulomb (1 V = 1 J/C)

How is potential difference measured?

With a voltmeter connected in parallel across the component; described as measuring voltage "across" the component

Relationship between current and PD

Current and potential difference are directly proportional — more current means more PD (and more electrons moving around the circuit)

What is resistance?

The opposition to current; higher resistance = lower current

Good conductors vs insulators (resistance)

Good conductors: low resistance; Insulators: high resistance

Symbol and unit of resistance

Symbol: R; Unit: Ohms (Ω) — Greek capital letter Omega; 1 Ω = 1 volt per ampere (1 V/A)

How can resistance of a circuit be increased?

Adding resistors or variable resistors

Assumed resistance of wires and batteries in exam questions

Negligible; every component technically has resistance but wires and batteries are assumed to be zero

How does resistance affect current?

Greater resistance → lower current for a given PD; Lower resistance → greater current for a given PD

Equation linking V, I, R

V = I × R; where V = potential difference (V), I = current (A), R = resistance (Ω)

Worked example approach (V = IR)

Step 1: List known quantities; Step 2: Write equation; Step 3: Substitute values; Step 4: Calculate

Fixed vs variable resistors

Fixed: resistance stays constant once set; Variable: resistance can be changed by changing the length of wire used

How does wire length affect resistance?

Longer wire = more resistance; shorter wire = less resistance

Components whose resistance CHANGES with conditions

Lamps, diodes, LEDs, thermistors, LDRs — resistance changes with current, temperature, or light

Thermistor rule

Hotter → resistance decreases; Cooler → resistance increases

Uses of thermistors

Temperature sensors in ovens, fire alarms, digital thermometers

LDR rule

More light → resistance decreases; Less light → resistance increases

Typical LDR resistance values

Dark: resistance very large (millions of ohms); Bright light: resistance small (tens of ohms)

Uses of LDRs

Light sensors — automatic street lights, security lighting, garden lights

How to identify symbols with arrows

Arrows pointing TOWARDS = light-dependent (LDR); Arrows pointing AWAY = light-emitting (LED)

What is a linear component?

A component whose I-V graph is a STRAIGHT LINE — obeys Ohm's Law, has constant resistance

What is a non-linear component?

A component whose I-V graph is NOT a straight line — does not obey Ohm's Law, resistance changes

Examples of linear components

Fixed resistors, wires, heating elements

Examples of non-linear components

Filament lamps, diodes, LEDs, LDRs, thermistors

I-V graph: fixed resistor

Straight line through the origin (directly proportional); gradient = 1/R; constant resistance

[DRAW: I-V graph showing straight line through origin for a fixed resistor]

Why is a fixed resistor's I-V graph linear?

Resistance is constant — current is directly proportional to potential difference

I-V graph: filament lamp

S-shaped curve through origin; gradient decreases as |V| increases (resistance increases)

[DRAW: I-V graph for filament lamp showing S-shaped curve]

Why is a filament lamp's I-V graph non-linear?

As current increases, temperature of filament increases → atoms vibrate more → harder for free electrons to pass through → resistance increases → current increases at a slower rate

Filament lamp: reversing PD

Makes no difference to the shape of the curve — graph is symmetrical about origin

I-V graph: diode

Zero current for reverse bias; sharp increase in current for forward bias (above a threshold PD)

[DRAW: I-V graph for semiconductor diode showing flat line for negative V and sharp curve up for positive V]

What is forward bias?

When current flows in the direction of the diode's arrow symbol — diode allows current; shown by sharp increase in current on right side of graph

What is reverse bias?

When the diode is switched around — very high resistance, no current flows; shown by zero current on left side of graph

Interpreting I-V graph gradient

Straight line, steep gradient = low resistance (high current for given V); shallow gradient = high resistance

[PAG] Aim of I-V characteristics experiment

Use circuit diagrams to construct appropriate circuits to investigate the I-V characteristics of a variety of circuit elements (fixed resistor at constant temperature, filament lamp, diode)

[PAG] Independent variable

Potential difference, V

[PAG] Dependent variable

Current, I

[PAG] Control variables

Potential difference of the power supply; use of the same equipment (wires, diodes)

[PAG] Equipment list

Ammeter, voltmeter, variable resistor, fixed resistor (100-500 Ω), filament lamp, diode, voltage supply, connecting wires

[PAG] Resolution of measuring equipment

Variable resistor: 0.005 Ω; Voltmeter: 0.1 V; Ammeter: 0.01 A

[PAG] Circuit setup

Fixed resistor (or lamp/diode) in series with ammeter, variable resistor, and voltage supply; voltmeter in parallel across the component being tested

[DRAW: circuit diagram with voltage supply, ammeter in series, variable resistor in series, fixed resistor with voltmeter in parallel]

[PAG] Method (step-by-step)

(1) Set up circuit with fixed resistor.

(2) Vary voltage/PD across component using variable resistor (8-10 readings).

(3) Record current value 3 times and calculate average.

(4) Increase voltage in steps of 0.5 V, repeat.

(5) Switch off circuit between readings to prevent heating.

(6) Reverse terminals to get negative readings.

(7) Repeat for filament lamp and diode.

[PAG] Why switch off between readings?

To prevent heating of the component, which would change its resistance

[PAG] Why take 3 readings and average?

To reduce random error and improve accuracy

[PAG] Why reverse the terminals?

To take negative voltage readings and plot the full I-V graph on both sides of the origin

[PAG] Analysis of results

Plot graph of average current against PD;

Straight line through origin = ohmic conductor (obeys Ohm's Law, V=IR);

Curve = non-ohmic conductor

[PAG] Expected I-V shape: fixed resistor

Straight line through origin (ohmic)

[PAG] Expected I-V shape: filament lamp

S-shaped curve (non-ohmic)

[PAG] Expected I-V shape: diode

Flat (zero current) in reverse bias, sharp curve up in forward bias

[PAG] Systematic errors

Voltmeter and ammeter should start from zero to avoid zero error

[PAG] Random errors

Voltmeter and ammeter have some resistance, so readings may be slightly inaccurate;

temperature of equipment could affect resistance (must be controlled);

take multiple readings to reduce random error and uncertainty

[PAG] Safety considerations

(1) High current in thin wire makes wire very hot — don't touch when switched on.

(2) Switch off power supply if burning smell detected.

(3) No liquids near equipment.

(4) Components get hot — careful handling, especially filament lamp.

(5) Disconnect power supply between readings to avoid overheating.