4.3-Electric circuits

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Syllabus Objectives

4.3.1 Circuit diagrams and circuit components

CORE OBJECTIVES:

  • Draw and interpret circuit diagrams containing cells, batteries, power supplies, generators, potential dividers, switches, resistors (fixed and variable), heaters, thermistors (NTC only), light-dependent resistors (LDRs), lamps, motors, ammeters, voltmeters, magnetising coils, transformers, fuses and relays, and know how these components behave in the circuit

EXTENDED/SUPPLEMENT OBJECTIVES:

  • Draw and interpret circuit diagrams containing diodes and light-emitting diodes (LEDs), and know how these components behave in the circuit

4.3.2 Series and parallel circuits

CORE OBJECTIVES:

  • Know that the current at every point in a series circuit is the same
  • Know how to construct and use series and parallel circuits
  • Calculate the combined e.m.f. of several sources in series
  • Calculate the combined resistance of two or more resistors in series
  • State that, for a parallel circuit, the current from the source is larger than the current in each branch
  • State that the combined resistance of two resistors in parallel is less than that of either resistor by itself
  • State the advantages of connecting lamps in parallel in a lighting circuit

EXTENDED/SUPPLEMENT OBJECTIVES:

  • Recall and use in calculations, the fact that:
  • (a) the sum of the currents entering a junction in a parallel circuit is equal to the sum of the currents that leave the junction
  • (b) the total p.d. across the components in a series circuit is equal to the sum of the individual p.d.s across each component
  • (c) the p.d. across an arrangement of parallel resistances is the same as the p.d. across one branch in the arrangement of the parallel resistances
  •  Explain that the sum of the currents into a junction is the same as the sum of the currents out of the junction
  • Calculate the combined resistance of two resistors in parallel

4.3.3 Action and use of circuit components

CORE OBJECTIVES:

  • Know that the p.d. across an electrical conductor increases as its resistance increases for a constant current

Revision Notes

IGCSE Physics Notes /  Electric Circuits

 

 

Circuit Components, Series & Parallel Circuits, and Component Action

Click each blank to reveal the answer. Diagram-based items have been skipped, but every other important concept has been tested carefully for strong exam preparation.

14.3.1 Circuit Diagrams and Circuit Components

A single cell is a source of :
_______ electrical energy
.
Two or more cells connected together form a
_______battery
.
A switch is used to
_______open or close
a circuit.
A fixed resistor has a resistance that stays
_______constant
.
A variable resistor is used to change the
_______resistance
in a circuit.
A heater changes electrical energy into
_______thermal energy
.
An NTC thermistor has a resistance that
_______decreases
when temperature increases.
An LDR is a
_______light-dependent resistor
.
The resistance of an LDR
_______decreases
when light intensity increases.
A lamp changes electrical energy into
_______light and heat
.
A motor changes electrical energy into
_______kinetic energy
.
An ammeter measures
_______current
.
A voltmeter measures
_______potential difference
.
An ammeter is connected in
_______series
with the component.
A voltmeter is connected in
_______parallel
across the component.
A fuse protects a circuit by melting when the current is too
_______large
.
A relay is an electrically operated
_______switch
.
A transformer is used to change the size of an
_______alternating voltage
.
A magnetising coil produces a
_______magnetic field
when current flows through it.
A power supply provides electrical
_______energy
to a circuit.
A generator changes kinetic energy into
_______electrical energy
.
A potential divider is used to provide a chosen
_______potential difference
.

2Extended Components: Diodes and LEDs

A diode allows current to flow in only
_______one direction
.
LED stands for light-emitting
_______diode
.
An LED gives out
_______light
when current flows through it in the correct direction.
Like an ordinary diode, an LED works only when connected in the
_______forward direction
.

34.3.2 Series Circuits

Series-Circuits-IGCSE-Physics-

In a series circuit, the current is the
_______same
at every point.
Components in a series circuit are connected one after
_______another
.
The combined e.m.f. of cells in series is found by
_______adding
their e.m.f.s.
If two 1.5V cells are connected in series, the combined e.m.f. is
_______3.0V
.
The combined resistance of resistors in series is found by
_______adding
the individual resistances.
Two resistors of 2ohms and 3ohms in series have a total resistance of
_______5ohms
.
In a series circuit, the total p.d. across the supply is equal to the
_______sum
of the p.d.s across each component.
If one lamp in a simple series circuit breaks, the whole circuit usually stops
_______working
.

44.3.2 Parallel Circuits

igcse-physics-parallel-circuits

In a parallel circuit, there is more than one
_______branch
.
In a parallel circuit, the current from the source is
_______larger
than the current in each individual branch.
The current entering a junction is equal to the current
_______leaving
the junction.
The sum of currents into a junction is the
_______same
as the sum of currents out.
The p.d. across each branch in a parallel circuit is
_______the same
.
The combined resistance of two resistors in parallel is
_______less
than either resistor by itself.
Lamps in homes are connected in
_______parallel
.
One advantage of connecting lamps in parallel is that if one lamp fails, the others
_______stay on
.
Another advantage of connecting lamps in parallel is that each lamp gets the full supply
_______voltage
.
If currents of 2A and 3A leave a junction, the current entering the junction is
_______5A
.
If the p.d. across the supply is 6V, the p.d. across one branch of a parallel circuit is
_______6V
.

5Calculating Parallel Resistance

For two resistors in parallel, the combined resistance can be found using the reciprocal
_______formula
.
Two equal resistors of 4ohms in parallel have a combined resistance of
2ohms
.
Resistors in parallel make it easier for current to
_______flow
.

6Constructing and Using Series and Parallel Circuits

A series circuit has only
_______one
path for current.
A parallel circuit has
_______more than one
path for current.
Series circuits are useful when the same current is needed in every
_______component
.
Parallel circuits are useful when components need to work
_______independently
.

74.3.3 Action and Use of Circuit Components

For a constant current, the p.d. across a conductor increases as the
_______resistance
increases.
If resistance doubles while current stays constant, the potential difference also
_______doubles
.
This relationship follows the equation V =
_______IR
.
For a fixed current, a larger resistance needs a larger
_______potential difference
.
If current is constant, voltage is directly proportional to
_______resistance
.

Exam Summary

Cells and batteries supply electrical energy. Ammeters measure current in series, and voltmeters measure potential difference in parallel.

Thermistors and LDRs are variable-resistance components that respond to temperature and light respectively.

In series circuits, current is the same everywhere and resistances add directly.

In parallel circuits, current splits between branches, p.d. is the same across each branch, and total resistance is lower.

Diodes allow current in one direction only, and LEDs emit light when connected correctly.

For constant current, higher resistance means higher potential difference.

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