Circuit Currents Summary Guide
Cambridge IGCSE Sciences • Physics P5
Electricity & Magnetism
Coursebook Summary Sections P5.01 – P5.02
Revision guide covering electric charge, fields, circuits, Ohm's law, and component characteristics.
1
P5.01 Electric Charge & Electrostatics
Nature of Charge
- Charges: positive (+) & negative (−).
- Law: Like repel; opposites attract.
- Unit: Coulomb (C).
- Carried by electrons (\(-1.6 \times 10^{-19}\text{ C}\)).
Charging & Materials
Friction: Transfer of electrons only (+ loss, − gain).
- • Conductor: Free electrons flow freely (Cu, Al).
- • Insulator: Electrons tightly bound (polythene).
Electric Fields
Definition: Region where a charge experiences an electric force.
Direction: Arrow from positive (+) to negative (−) (force on a positive charge).
2
P5.02 Current, Voltage & Resistance
Current (\(I\))
Rate of flow of charge
\(I = \frac{Q}{t}\)
Unit: Amperes (A). Measured in series with an ammeter.
Voltage (\(V\))
Energy per unit charge
\(V = \frac{W}{Q}\)
Unit: Volts (V). Measured in parallel with a voltmeter.
Resistance (\(R\))
Ratio of p.d. to current
\(R = \frac{V}{I}\)
Unit: Ohms (\(\Omega\)). Opposition to electric charge flow.
Circuit Rules Comparison series vs parallel behaviour
| Feature | Series Circuit | Parallel Circuit |
|---|
| Current (\(I\)) | Same throughout: \(I_{\text{tot}} = I_1 = I_2\) | Splits across branches: \(I_{\text{tot}} = I_1 + I_2\) |
| Voltage (\(V\)) | Shared across loads: \(V_{\text{tot}} = V_1 + V_2\) | Same across branches: \(V_{\text{tot}} = V_1 = V_2\) |
| Total Resistance | \(R_{\text{tot}} = R_1 + R_2\) | \(\frac{1}{R_{\text{tot}}} = \frac{1}{R_1} + \frac{1}{R_2}\) (\(R_{\text{tot}} < R_{\text{smallest}}\)) |
| Break in Line | Opens entire circuit; all stop | Other branches keep working independently |
Wire Resistance Factors
- • Length (\(L\)): \(R \propto L\) (longer wire = greater \(R\)).
- • Area (\(A\)): \(R \propto \frac{1}{A}\) (thicker wire = lower \(R\)).
- • Material: Specific resistance / resistivity.
I-V Characteristics
- • Ohmic Resistor: Constant \(T\); straight line through origin.
- • Filament Lamp: As \(I\) rises, \(T\) increases; resistance rises.
- • Diode: Current flows in forward direction only.
Cambridge IGCSE Co-ordinated / Combined Sciences Coursebook (Physics) Page 1 of 2
Cambridge IGCSE Sciences • Physics P5
Electricity & Magnetism
Coursebook Summary Sections P5.03 – P5.04
Revision guide covering electrical power, energy, domestic safety, magnetic fields, and electromagnets.
3
P5.03 Electrical Energy & Electrical Power
Electrical Power (\(P\))
Rate of energy transfer (Watts, \(\text{W} = \text{J/s}\)).
\(P = I \times V\)
\(P = I^2 R \quad \text{or} \quad P = \frac{V^2}{R}\)
Electrical Energy (\(E\))
Total work done (Joules, \(\text{J}\) or \(\text{kWh}\)).
\(E = P \times t\)
\(E = I \times V \times t\)
Electrical Safety & Circuit Protection
Fuses & Circuit Breakers Placed on the live wire. Cuts off current when it exceeds safe rating to prevent overheating and fire.
Earth Wire & Earthing Connects metal casing safely to ground. Fault causes huge surge through earth, instantly melting the fuse.
Common Hazards Damaged cable insulation (electric shocks), damp conditions (water conducts), and overloaded extension sockets.
4
P5.04 Magnetism & Electromagnetism
Magnetic Fields
- Poles: North (N) & South (S).
- Law: Like poles repel; opposites attract.
- Field lines: Flow from N to S. Closer spacing = stronger field.
Magnetic Materials
Ferromagnetic: Iron, steel, nickel, cobalt.
- • Soft iron: Easy to magnetise/demagnetise (temporary magnets).
- • Hard steel: Retains magnetism (permanent magnets).
Electromagnets (Solenoids)
Coil with current creates a switchable magnetic field.
Increase strength by:
- Higher current (\(I\))
- More coil turns (\(N\))
- Adding soft iron core
Exam Formula Master Sheet & Tips Always state units!
Charge \(Q = I \times t\) \(C = A \cdot s\)
Resistance \(V = I \times R\) \(V = A \cdot \Omega\)
Power \(P = I \times V\) \(W = A \cdot V\)
Energy \(E = P \times t = IVt\) \(J = W \cdot s\)
Fuse Selection: Choose the standard rating just above normal operating current (\(I = \frac{P}{V}\)).
Parallel Rule: Total resistance in parallel is always strictly less than the smallest branch resistor.
Cambridge IGCSE Co-ordinated / Combined Sciences Coursebook (Physics) Page 2 of 2
Circuit Mastery Practice Worksheet
Cambridge IGCSE Sciences • Physics P5
Exam Practice Paper: Electricity & Magnetism
Duration: 45 Minutes Total: 30 Marks
Student Name:
Date:
Class / Group:
Score:
_____ / 30
Question 1: Electric Charge & Fields [7 Marks]
(a) A polythene rod is rubbed with a dry woolen cloth. The polythene rod becomes negatively charged.
(i) Explain, in terms of electron movement, how the rod acquires a negative charge. [2]
(ii) State the type of charge left on the cloth after rubbing. [1]
(b) An electric current of \(0.40\text{ A}\) flows through a resistor for \(2.5\text{ minutes}\). Calculate the total charge passing through the resistor. State the formula, show working, and state the unit. [3]
Working space:
Charge (\(Q\)) =
(c) Define the term electric field. [1]
Question 2: Circuits, Voltage & Resistance [8 Marks]
A \(12\text{ V}\) DC power supply is connected in series with two resistors, \(R_1 = 20\ \Omega\) and \(R_2 = 40\ \Omega\).
(a) Calculate total circuit resistance. [1]
\(R_{\text{total}} =\)
(b) Calculate the circuit current. [2]
\(I =\)
(c) Calculate the potential difference across the \(40\ \Omega\) resistor. [2]
\(V_2 =\)
(d) The two resistors are now reconnected in parallel across the \(12\text{ V}\) supply.
(i) Calculate effective parallel resistance: [2]
\(R_{\text{parallel}} =\)
(ii) Total current drawn compared to series: [1]
Cambridge IGCSE Physics Practice Worksheet • Chapter P5 Page 1 of 2
Cambridge IGCSE Sciences • Physics P5
Exam Practice Paper (Continued)
Coursebook Review Sections P5.03 & P5.04
Question 3: Electrical Energy, Power & Safety [8 Marks]
An electric kettle rated at \(1800\text{ W}\) is connected to a \(240\text{ V}\) mains AC electrical supply.
(a) Calculate the normal operating current of the kettle. Show your working. [2]
\(I =\)
(b) Available standard plug fuses are rated at \(3\text{ A}\), \(5\text{ A}\), and \(13\text{ A}\).
State the correct fuse rating: [1]
