Electrical Circuits Notes

Oxford AQA IGCSE Physics: Revision notes

Key facts

  • Current I=QtI = \dfrac{Q}{t}; pd V=EQV = \dfrac{E}{Q}.
  • Static electricity is a charge imbalance that is released by discharge.
  • V=I×RV = I \times R; a resistor at constant temperature obeys Ohm's law.
  • Thermistor resistance falls as temperature rises; LDR resistance falls as light rises; filament lamp resistance rises as it heats.
  • Series: same current, resistances add. Parallel: same pd across branches, lower combined resistance.

Charge, current and pd

Free electrons carry charge round a circuit: current is charge per second, and pd is energy per unit charge.

Metals conduct because of free electrons, which drift through the lattice when a potential difference is applied.

Current is the rate of flow of charge, in amps. Potential difference is the energy transferred per unit charge, in volts.

Here QQ is charge in C, tt time in s and EE energy in J.

246810125101520253035xy(10 s, 30 C)Q = It, I = 3 A
Charge against time for a steady current of 3 A: the gradient is the current
  • CurrentI=QtI = \dfrac{Q}{t}
  • Potential differenceV=EQV = \dfrac{E}{Q}

Worked example

A charge of 30 C flows past a point in 10 s. What is the current?

120 J of energy is transferred as 20 C of charge passes through a lamp. What is the pd across it?

Static electricity

Friction moves electrons between insulators, leaving opposite charges that stay put until they discharge.

Static electricity is an imbalance of charge on an object. Friction transfers electrons from one insulating surface to another. With no conducting route, the charge stays until it finds a path, then released in a discharge. Enough charge can jump a gap as a spark. Lightning is a large discharge between a cloud and the ground or between clouds.

  1. 1

    Friction

    Electrons move from one insulator to another

  2. 2

    Charge builds up

    The surfaces become oppositely charged

  3. 3

    Discharge

    Charge jumps across a gap as a spark

How a static discharge happens

Why does charge stay on an insulator instead of flowing away?

Circuit components

Circuits use standard symbols, and a thermistor or LDR lets a circuit respond to temperature or light.

Know the symbols for a switch, cell, battery, lamp, fuse, voltmeter, ammeter, diode, resistor, variable resistor, LDR, LED and thermistor.

A thermistor's resistance depends on temperature, so it is used in thermostats. An LDR's resistance depends on light intensity, so it switches lights on when it gets dark.

AL1V
A series circuit with an ammeter in series and a voltmeter across the lamp.

Which component switches a heater on when a room gets cold?

Resistance and I-V graphs

Resistance limits current for a given pd, and each component has its own I-V shape.

Resistance, in ohms, opposes current: for a given pd, greater resistance means smaller current. An I-V characteristic plots current against pd for a component. Charges colliding with atoms in a resistor cause heating.

An LED emits light only when current flows forwards, and uses much less current than other lighting.

−6−4−2246−3−2−1123pdcurrentresistor at constant temperaturefilament lamp
I-V characteristics (shapes only): resistor and filament lamp.
  • Potential differenceV=I×RV = I \times R

Resistor (constant temperature)

  • Straight line through origin: Ohm's law

Filament lamp

  • Curve flattens: resistance rises as it heats

Diode

  • Conducts forwards only

Thermistor and LDR

  • Resistance falls as temperature or light rises

Worked example

A 4 Ω resistor has a current of 3 A. What is the pd across it?

As a filament lamp warms up, what happens to its resistance?

Series and parallel

Series circuits share the pd and have one current; parallel branches each get the full pd.

In a series circuit components are in one loop; if one fails the whole circuit breaks. In a parallel circuit each branch works independently.

Series

Combined resistance:
Sum of the resistances
Current:
Same everywhere
Potential difference:
Shared between components

Parallel

Combined resistance:
Less than the smallest resistance
Current:
Splits between branches
Potential difference:
Same across each branch
L1L2
Parallel: each lamp is on its own branch, so each has the full battery pd.

Worked example

A 6 V battery is in series with a 2 Ω and a 4 Ω resistor. Find the current and the pd across the 4 Ω resistor.

Two identical lamps are in parallel. One lamp is removed. What happens to the other?

Try an exam question

A 12 V battery is connected in series with a 3 Ω resistor and a 5 Ω resistor. Calculate the current.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.