PHY 1020 – Chapter 6: Electricity and Magnetism

These flashcards cover major concepts from PHY 1020 Chapter 6, including fundamental forces, electric charge and current, AC/DC power, safety devices, power transmission, lightning, magnetism, transformers, motors, generators, superconductivity, and magnetic levitation.

What are the four fundamental forces of nature?

Strong, Electromagnetic, Weak, and Gravitational forces.

Which particles mediate the electromagnetic force?

Photons.

What is the range and relative strength of the strong nuclear force?

Range ≈ 10⁻¹⁵ m; it is the strongest of the four forces (relative strength ≈ 25 compared with electromagnetism = 1 at 3×10⁻¹⁷ m).

What conserved quantity comes in positive and negative forms and is analogous to mass?

Electric charge.

State the law of charge conservation.

The total electric charge in an isolated system remains constant.

What is the fundamental unit of electric charge?

The elementary charge e = 1.6 × 10⁻¹⁹ C.

Define electric current (I).

The rate of flow of charge, measured in coulombs per second (amperes).

What is electric potential (voltage)?

Electric potential energy per unit charge; 1 volt = 1 joule per coulomb.

Give the formula that relates voltage, current, and resistance.

V = I R (Ohm’s law).

Why do static‐electric shocks usually feel harmless despite high voltages?

Because the current is extremely small (~1 mA), so the power delivered is tiny.

Household electrical power limit for a 110 V, 100 A service is ___.

P = V I = (110 V)(100 A) = 11 kW.

Distinguish between alternating current (AC) and direct current (DC).

AC reverses direction periodically; DC flows in one constant direction.

Who championed DC power distribution in the ‘Current War’?

Thomas Edison.

Who promoted AC transmission over long distances?

Nikola Tesla.

Why is AC preferred for long-distance power transmission?

Transformers can easily step AC voltages up or down, enabling high-voltage, low-current transmission with lower power losses.

Explain the purpose of fuses or circuit breakers.

They interrupt the circuit when current exceeds safe levels, preventing overheating and fire.

In Europe the mains voltage is 220 V. How does this affect current and danger compared with 110 V systems?

Higher voltage allows lower current for the same power, reducing I²R losses, but the higher voltage is more hazardous to humans.

Write the power loss formula for transmission lines.

P_loss = I² R.

Why do power companies favor very high transmission voltages?

High voltage allows smaller current for the same power, greatly reducing I²R losses.

Calculate the power lost when 300 A flows through a 2 Ω line.

P_loss = (300 A)² × 2 Ω = 180 kW.

What makes lightning so destructive?

It combines extremely high voltage (~10 MV) with enormous current (~100 kA), giving power ≈ 10¹² W.

What fundamental cause produces magnetism at the atomic scale?

Moving charges and the intrinsic spin of electrons.

State the rule for interaction of magnetic poles.

Like poles repel; unlike poles attract.

What is a ferromagnet?

A material whose atomic magnetic moments align spontaneously, creating a permanent magnet.

Define Curie temperature.

The temperature above which a ferromagnetic material loses its permanent magnetism.

Have isolated magnetic monopoles been observed?

No; magnets always appear as dipoles in nature so far.

Why are ‘rare-earth’ magnets so strong?

Rare-earth elements have many unpaired outer-shell electrons, producing large atomic magnetic moments that align strongly.

How is a magnetic field generated by an electric current?

A current-carrying conductor produces concentric magnetic field lines around it (right-hand rule).

What shields Earth from harmful charged solar particles?

Earth’s planetary magnetic field (magnetosphere).

What device uses two coils to step voltages up or down?

A transformer.

If Np/Ns = 1000 in a transformer and Vp = 110 kV, what is Vs?

Vs = Vp ÷ 1000 = 110 V (step-down).

Describe briefly how a DC electric motor works.

Current through a coil in a magnetic field experiences a torque; a commutator reverses current each half-turn, making continuous rotation.

What is the difference between a dynamo and an alternator?

Both convert mechanical work to electricity; a dynamo outputs DC, an alternator outputs AC.

How is binary information stored in magnetic media?

Each tiny region’s magnetic dipole orientation (up/down) encodes 0 or 1.

What are eddy currents?

Circulating currents induced in conductors by changing magnetic fields or moving magnets.

Define superconductivity.

A state in which a material’s electrical resistance drops to zero below a critical temperature (T_c).

Why can superconductors enable magnetic levitation?

They expel magnetic fields (Meissner effect) and can carry large persistent currents, generating strong opposing magnetic fields for levitation.

Give one application of superconducting magnets mentioned.

Maglev trains that float above tracks with minimal friction.

What is the approximate date when high-temperature cuprate superconductors were discovered?

Mid-1980s (1986).

How do fuses and circuit breakers differ?

A fuse melts open once; a circuit breaker can be reset after tripping.

Why are refrigerators considered paramagnets?

Their atoms align only in the presence of an external magnetic field (e.g., from a fridge magnet).

What is the purpose of brushes in a traditional DC motor?

They conduct current to the rotating commutator.

Explain why magnets always have both a north and a south pole when broken.

Breaking splits aligned dipoles; each fragment still has complete north–south pairs, never an isolated monopole.

Which constant (ε₀) appears in Coulomb’s law and what is its value?

Permittivity of free space, ε₀ ≈ 8.854 × 10⁻¹² F/m.

What is the basic SI unit of power and how is it defined electrically?

Watt (W); P = V I.