Sciences - Co-ordinated (Double) (0654) | Electricity and magnetism

P4 Electricity and Magnetism: Comprehensive IGCSE Study Notes

Welcome to the exciting world of electricity and magnetism! Don't worry if this topic feels complex—we're going to break it down into simple, manageable pieces. Everything from your phone charger to giant power stations relies on the principles you are about to learn. Let's get started!

---

P4.1 Simple Phenomena of Magnetism

Magnetism is all about forces and fields. It’s what makes fridge magnets stick and powers many of the motors we use every day.

1. Magnetic Poles and Forces (Core)

Every magnet has two ends called poles: a North pole (N pole) and a South pole (S pole).

• Attraction: Opposite poles attract (N attracts S).
• Repulsion: Like poles repel (N repels N, S repels S).

Think of magnetic poles like dating advice: Opposites attract, and likes repel!

2. Magnetic Materials (Core)

A material is magnetised if it acts as a magnet; it is unmagnetised otherwise.

• Magnetic materials: Materials that are attracted to a magnet (e.g., iron, steel, nickel, cobalt).
• Non-magnetic materials: Materials not attracted to a magnet (e.g., wood, plastic, copper).

3. Permanent vs. Temporary Magnets (Core)

We classify magnets based on how long they keep their magnetism:

Feature	Temporary Magnets (e.g., Soft Iron)	Permanent Magnets (e.g., Steel)
Magnetism	Easy to magnetise, but lose magnetism quickly.	Hard to magnetise, but retain magnetism for a long time.

4. Electromagnets (Core)

An electromagnet is a temporary magnet created by passing an electric current through a coil of wire wrapped around a soft iron core. It differs from a permanent magnet because its magnetism can be turned on and off by controlling the electric current.

5. Magnetic Fields (Supplement)

A magnetic field is defined as a region in which a magnetic pole experiences a force.

• The field direction at any point is the direction of the force experienced by a single N pole placed at that point.
• Field lines run from North to South (outside the magnet).

6. Induced Magnetism (Supplement)

Induced magnetism happens when a magnetic material (like an iron nail) becomes a temporary magnet when placed near a permanent magnet. The permanent magnet causes the magnetic domains inside the iron to align, making the iron object behave as a magnet itself.

---

P4.2 Electrical Quantities

To understand circuits, we need to define the fundamental quantities: charge, current, voltage, resistance, energy, and power.

1. Electrical Charge (P4.2.1)

• There are two types of charge: positive (+) and negative (-).
• Like charges repel (+ repels +, - repels -).
• Unlike charges attract (+ attracts -).
• Charge is measured in coulombs (C) (Supplement).

Charging by Friction (Core)

Electrostatic charging (by rubbing two solids together) involves the transfer of the negative charges (electrons) only. For example, rubbing a balloon on hair transfers electrons from your hair to the balloon, leaving your hair positive and the balloon negative.

Conductors and Insulators (Core)

• Electrical Conductors: Materials that allow charge (electrons) to flow easily (e.g., metals like copper, silver, and gold).
• Electrical Insulators: Materials that oppose the flow of charge (e.g., rubber, plastic, glass).

Electric Fields (Supplement)

An electric field is a region where an electric charge experiences a force. The direction of the electric field is defined as the direction of the force experienced by a positive charge at that point.

2. Electric Current (P4.2.2)

Electric current is the rate of flow of charge.

• In metals, current is the movement of delocalised (mobile) electrons (Supplement).
• Conventional Current: Flows from positive (+) terminal to negative (-) terminal.
• Electron Flow: Electrons actually flow from negative (-) to positive (+).

Don't get confused! In exams, unless specified, use conventional current (P4.2.2 Supp 7).

Calculation of Current (Supplement)

Current \(I\) is defined as the charge \(Q\) passing a point per unit time \(t\):

$$I = \frac{Q}{t}$$

The unit of current is the Ampere (A), where 1 A = 1 C/s.

Measuring Current (Core)

Current is measured using an ammeter, which must be connected in series with the component.

Types of Current (Core)

• Direct Current (d.c.): Current flows in one direction only (e.g., from a battery).
• Alternating Current (a.c.): Current continuously changes direction (e.g., mains electricity).

3. Voltage (e.m.f. and p.d.) (P4.2.3)

Voltage is the 'push' that drives the current.

• The voltage of the source is the cause of current in the circuit.
• Voltage is measured using a voltmeter, connected in parallel across the component.

E.m.f. and P.d. (Supplement)

Both are measured in volts (V).

• Electromotive Force (e.m.f.): The electrical work done by a source in moving a unit charge around a complete circuit. (This is the total voltage supplied by the battery/cell).
• Potential Difference (p.d.): The work done by a unit charge passing between two points in a circuit. (This is the voltage dropped across a component).

4. Resistance (P4.2.4)

Resistance (R) is the opposition to the flow of current. It causes electrical energy to be converted into heat/light energy.

Ohm's Law:

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

Where V is voltage (V), I is current (A), and R is resistance (Ohm, \(\Omega\)).

Resistance Factors (Supplement)

For a metallic conductor, resistance depends on:

1. Length (L): Resistance is directly proportional to length. (Longer wire = more resistance).
2. Cross-sectional Area (A): Resistance is inversely proportional to area. (Thicker wire = less resistance).

Investigating Resistance (Core)

To find the resistance of a component, you measure the current \(I\) through it using an ammeter (series) and the voltage \(V\) across it using a voltmeter (parallel), then calculate \(R = V/I\).

Did you know? A resistor that has a constant resistance, meaning it obeys Ohm's law, has a V-I graph that is a straight line passing through the origin (Supplement).

5. Electrical Energy and Power (P4.2.5)

Electric circuits transfer energy from the source (like a battery) to the components and then into the surroundings (usually as heat or light).

1. Electrical Power (P): The rate at which energy is transferred.

$$P = IV$$

Where P is power (W, Watts), I is current (A), and V is voltage (V).

2. Electrical Energy (E): Power multiplied by time.

$$E = IVt$$

Where E is energy (J, Joules), and t is time (s, seconds).

The Kilowatt-hour (kWh) (Core)

The standard unit for energy used by electricity companies is the kilowatt-hour (kWh).
1 kWh is the energy transferred by a 1 kW appliance running for 1 hour. This is used to calculate your electricity bill.

Calculation Tip: Cost = kWh used \(\times\) cost per kWh.

---

P4.3 Electrical Circuits

Circuits allow electrical current to flow safely. We use standard symbols to draw diagrams.

1. Circuit Components (P4.3.1)

It is vital to know the symbols for components:

• Source: Cell, Battery (two or more cells), Power supply, Generator (Supplement).
• Components: Fixed resistor, Variable resistor, Lamp/Heater, Motor.
• Safety: Fuse.
• Measurement: Ammeter (series), Voltmeter (parallel).
• Diodes: Light-Emitting Diode (LED) (Supplement) - allows current in one direction only, emitting light.

2. Series Circuits (P4.3.2)

Components are connected one after another, forming a single loop.

1. Current (I): The current is the same at every point in the circuit.
2. Voltage (V): The total voltage from the source is shared between the components (Supplement: \(V_{total} = V_1 + V_2 + \dots\)).
3. Resistance (R): The total resistance is the sum of individual resistances.

   $$R_{Total} = R_1 + R_2 + \dots$$

Common Mistake: If one bulb blows in a series circuit, the entire circuit breaks because there is only one path for the current.

3. Parallel Circuits (P4.3.2)

Components are connected across the same two points, forming separate branches.

1. Voltage (V): The p.d. across each branch is the same as the source voltage (Supplement).
2. Current (I): The total current from the source splits between the branches. The sum of currents entering a junction equals the sum of currents leaving (Supplement: \(I_{source} = I_{branch 1} + I_{branch 2} + \dots\)). Note that the current from the source is larger than the current in each branch (Core).
3. Resistance (R): The combined resistance is less than the resistance of any single branch. Adding more resistors in parallel actually decreases the total resistance, making the total current larger.

   Calculation (Supplement): For two resistors in parallel:

   $$\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2}$$

Advantages of Parallel Circuits (Core)

Lamps are usually connected in parallel because:

1. If one lamp blows, the others stay lit.
2. All lamps receive the full supply voltage, so they shine brightly.

---

P4.4 Electrical Safety (Core Only)

Electricity is powerful and must be handled carefully. Safety devices and procedures protect us from the heating effect of current (which causes fires) and electric shock.

1. Electrical Hazards

• Damaged Insulation: Exposed wires can cause electric shock or short circuits.
• Overheating Cables: Too much current (often due to worn cables or excessive use) causes heat, leading to fire.
• Damp Conditions: Water conducts electricity, increasing the risk of shock.
• Excess Current/Overloading: Connecting too many high-power appliances to a single socket or extension lead draws excessive current, leading to overheating.

2. Fuses and Trip Switches

These devices break the circuit if the current becomes dangerously high.

• Fuse: A thin piece of wire that melts if the current exceeds its specified rating. Once blown, it must be replaced.
• Trip Switch (Circuit Breaker): An electromagnetic switch that flips open when the current exceeds a limit. It can be reset and reused.

When choosing a fuse, select the rating slightly higher than the normal operating current of the appliance (e.g., for a 10 A current, use a 13 A fuse).

3. Earthing and Insulation

• Double Insulation: Appliances that have plastic casings (non-conducting) and no exposed metal parts are double-insulated and do not require an earth wire.
• Earthing: A protective measure where the appliance's metal casing is connected to the ground via the earth wire (green/yellow). If a live wire touches the metal casing, a huge current flows directly to the earth, blowing the fuse immediately and protecting the user from shock.

---

P4.5 Electromagnetic Effects

This section explores how electricity creates magnetic fields and how magnetic fields create electricity—the core of modern technology.

1. Magnetic Effect of Current (P4.5.3)

When a current flows through a wire, it produces a magnetic field around it.

• Straight Wire: Concentric circles of field lines are formed around the wire.
• Solenoid (Coil): The field produced looks similar to that of a bar magnet (a strong, uniform field inside, looping fields outside).

The magnetic field can be affected by:

1. Magnitude: Increasing the current increases the strength of the magnetic field.
2. Direction: Reversing the current reverses the direction of the magnetic field.

2. Force on a Current-Carrying Conductor (Motor Principle) (P4.5.4 & P4.5.5)

When a current-carrying wire is placed in a magnetic field, it experiences a force (the motor effect).

The direction of the force is perpendicular to both the current and the magnetic field. The force is reversed by reversing either the current or the direction of the field.

The D.C. Motor (P4.5.5 Supplement)

A simple D.C. motor works by using the turning effect (force) on a current-carrying coil placed between two strong magnets.

• Turning Effect Factors: The turning effect (torque) increases if you increase the number of turns on the coil, the current, or the strength of the magnetic field.
• Operation: The split-ring commutator and brushes ensure the current direction in the rotating coil is reversed every half-turn. This reversal keeps the turning force in the same direction, allowing the coil to rotate continuously.

3. Electromagnetic Induction (P4.5.1 & P4.5.2)

This is the reverse of the motor effect: using magnetism to generate electricity.

An e.m.f. is induced across a conductor if the conductor cuts across magnetic field lines or if the magnetic field linking with the conductor changes (P4.5.1 Supp 1).

Factors Affecting Induced E.m.f. (P4.5.1 Supplement)

The magnitude (size) of the induced e.m.f. increases if:

• The conductor moves faster.
• The magnetic field is stronger.
• The coil has more turns.

The A.C. Generator (P4.5.2 Supplement)

The A.C. generator (dynamo) uses electromagnetic induction to produce alternating current.

• A coil rotates within a magnetic field.
• It uses slip rings and brushes. The slip rings maintain electrical contact while allowing the coil to rotate fully, leading to an alternating e.m.f.
• The graph of e.m.f. against time is a wave shape (sinusoidal), showing the voltage alternates between positive and negative values.

4. The Transformer (P4.5.6 Supplement)

A transformer is used to change (step-up or step-down) the voltage of an A.C. supply.

Construction and Operation

• It consists of a primary coil (input voltage \(V_p\), turns \(N_p\)) and a secondary coil (output voltage \(V_s\), turns \(N_s\)) wrapped around a soft-iron core.
• Step-up Transformer: Increases voltage (\(V_s > V_p\)). Requires \(N_s > N_p\).
• Step-down Transformer: Decreases voltage (\(V_s < V_p\)). Requires \(N_s < N_p\).

Transformer Equations (Supplement)

The ratio of voltages equals the ratio of turns:

$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$

Assuming 100% efficiency (meaning no power loss: \(P_p = P_s\)), the relationship between current and voltage is:

$$I_p V_p = I_s V_s$$

High-Voltage Transmission (Supplement)

Transformers are essential for efficient power transmission. Power loss (P) in cables is given by:

$$P_{loss} = I^2 R$$

If power (P = IV) is transmitted at a very high voltage (V), the current (I) must be very low. Because power loss depends on \(I^2\), reducing the current significantly reduces the energy loss during transmission across long distances.

Step-up transformers increase the voltage before transmission; step-down transformers decrease it back to a safe level for household use.