Physics Lecture Notes: Magnetism and Electricity

Review flashcards covering simple phenomena of magnetism, electrical quantities, circuit rules, electrical safety, and electromagnetic effects based on lecture notes.

What are the four magnetic materials identified in the notes that are attracted to magnets?

Iron, steel, cobalt, and nickel.

What three methods can be used to magnetise a magnetic material by induced magnetism?

Stroking them with a magnet, hammering them in a magnetic field, or placing them in a coil with a direct current through it.

How do steel and soft iron behave differently when magnetised by induced magnetism?

Steel becomes permanently magnetised, whereas soft iron only forms a temporary (induced) magnet.

Why can temporary magnets be turned on and off while permanent magnets cannot?

Temporary magnets lose their magnetism when the magnetic field is removed, whereas permanent magnets produce their own magnetic field.

How is the direction of a magnetic field at a point defined and represented on field lines?

It is the direction of the force on the north pole of a magnet placed at that point, shown by arrows on the field lines pointing from the north pole to the south pole.

How is magnetic field strength represented in a magnetic field diagram?

By how close the field lines are to one another; closer lines indicate a stronger magnetic field.

What two methods are described to visualise and plot magnetic field lines?

Using a plotting compass and using iron filings.

What type of charge transfer is involved when an object is electrostatically charged?

The transfer of negative charge alone (electrons) by their addition or removal.

When an insulating rod is rubbed with a cloth, which object becomes positively charged and which becomes negatively charged?

The rod's end becomes positively charged and the cloth becomes negatively charged because electrons are transferred from the rod to the cloth.

How does a Gold-leaf electroscope (GLE) distinguish between a good conductor and a good insulator?

If the leaf falls upon touching the plate, the material is a good conductor; if the leaf remains in place, the object is a good insulator.

How are the electric field lines drawn for positive and negative point charges?

The field lines point radially outwards for a positive charge and radially inwards for a negative charge.

How is the electric field between two oppositely charged conducting plates drawn?

As straight lines directed from the positive plate to the negative plate that are equally spaced apart.

What is the definition and formula for electric current?

Electric current is the charge passing a point in the circuit per unit time, given by the formula $I = \frac{Q}{t}$, with units in amps ($A$).

How is conventional current defined?

The rate of flow of positive charge (flowing from a positive to a negative terminal).

How do analogue and digital ammeters compare in terms of their measurement range and precision?

Analogue ammeters measure currents over a larger range ($0.1\text{ to }5\text{ amps}$), whereas digital ammeters measure over a smaller range (milliamps or microamps) and are more precise.

What is the difference between direct current (d.c.) and alternating current (a.c.)?

In direct current (d.c.), electrons flow in one direction from the negative to the positive terminal; in alternating current (a.c.), electrons regularly change their direction of flow.

What is electromotive force (e.m.f.) and what is its equation?

The electrical work done by a source in moving a unit charge around a complete circuit, given by $E = \frac{W}{Q}$, measured in volts ($V$).

What is potential difference (p.d.) and what is its equation?

The work done by a unit charge passing through a component, given by $V = \frac{W}{Q}$, measured in volts ($V$).

How must a voltmeter be connected to measure potential difference across a component?

In parallel with the component (on a parallel branch connected on either side of it).

Why does the resistance of a filament lamp increase as current increases?

As current increases, temperature increases, causing electrons and ions to vibrate more and collide more, which increases resistance.

How do wire length and cross-sectional area affect the electrical resistance of a metal wire?

Resistance is directly proportional to the wire's length and inversely proportional to its cross-sectional area.

What equation is used to calculate the electrical power of a circuit component?

$$P = IV$$

What is a kilowatt-hour ($kWh$) and how is the cost of electricity calculated?

A kilowatt-hour is the electrical energy converted by a $1\,kW$ appliance used for $1\text{ hour}$; $\text{Cost} = \text{electrical energy used (kWh)} \times \text{cost per kWh}$.

How does resistance vary with temperature in an NTC thermistor and with light intensity in an LDR?

In an NTC thermistor, resistance decreases as temperature increases; in an LDR, resistance decreases as light intensity increases.

How do diodes and light-emitting diodes (LEDs) control current flow?

They allow current to flow in only one direction by maintaining a high resistance to current flow in the opposite direction.

What are the rules for current, potential difference, and combined resistance in a series circuit?

Current is the same at every point, the power supply's potential difference is shared across the components, and combined resistance is $R_T = R_1 + R_2$.

What are the rules for current, potential difference, and combined resistance in a parallel circuit?

Current is shared between branches (current entering a junction equals current leaving), potential difference is identical across every branch, and combined resistance is $\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2}$.

What formula describes the sharing of potential difference across resistors in a potential divider?

$$\frac{R_1}{R_2} = \frac{V_1}{V_2}$$

What are four common electrical hazards identified in electric circuits?

Damaged insulation, overheating of cables, damp conditions, and overloading plugs, extension leads, or single and multiple sockets.

What are the functions of the live, neutral, and earth wires in a mains circuit?

The live wire delivers current, the neutral wire carries current back to the power source, and the earth wire protects against sudden voltage fluctuations.

Why must a mains circuit switch be connected to the live wire rather than the neutral wire?

So that switching it off stops the flow of current into the circuit, making it safe; if placed on the neutral wire, current would still flow through the circuit.

What are the three most common fuse ratings in mains circuits?

$3\,A$, $5\,A$, and $13\,A$.

How does an earth wire work with a fuse to prevent electrocution in an appliance with a metal casing?

The earth wire provides a very low-resistance route for current in a short circuit, creating a current surge that melts the fuse and disconnects the appliance.

What factors increase the magnitude of an induced e.m.f. when moving a wire across a magnetic field?

Moving the wire more quickly, using a stronger magnetic field, or increasing the length of the wire.

At what coil positions in an a.c. generator is the induced e.m.f. at its maximum and at zero?

e.m.f. is at its maximum when horizontal ($90^\circ$ and $270^\circ$, cutting field lines fastest) and zero when vertical ($0^\circ$ and $180^\circ$, moving in line with the field).

How is the direction of the magnetic field around a straight current-carrying wire determined using the right hand grip rule?

The thumb points in the direction of the current, and the curled fingers show the direction of the concentric magnetic field lines.

What four factors increase the strength of the magnetic field inside a solenoid?

Increasing the number of turns on the coil, being closer to the wire, increasing the current, and adding a soft iron core.

What happens when a beam of charged particles enters an external magnetic field?

A force is exerted perpendicular to the velocity of the charge, altering its direction and resulting in circular motion (deflection).

What three modifications increase the turning effect on the coil of a d.c. motor?

Increasing the current, using a stronger magnetic field, and increasing the number of turns on the coil.

What is the role of a split ring commutator in a d.c. motor?

It reverses the current direction through the coil every half turn so that the coil continues to rotate in the same direction.

Why do transformers step up voltage in power lines during transmission?

Higher voltage reduces current, which minimises transmission power loss according to the formula $P = I^2 R$.

What equation relates the voltages and turns ratio across the primary and secondary coils of a transformer?

$$\frac{V_P}{V_S} = \frac{N_P}{N_S}$$