Unit 4: Electricity & Magnetism

Flashcards covering key concepts, definitions, and laws from the lecture notes on Electricity & Magnetism, including electrostatics, electrodynamics, circuits, components, electromagnetism, induction, and transformers.

How is an object defined as 'electrified'?

if it has a surplus or deficiency of electrons.

What are the three ways electrification can occur?

contact, friction, or induction.

What is the unit for electric charge, and approximately how many electron charges does it represent?

Coulomb, which represents approximately 6.25 x 10^18 electron charges.

According to the Laws of Electrostatics, what is the interaction between like charges and opposite charges?

Like charges repel one another; opposite charges attract one another.

What does Coulomb's Law state about the force between two charged objects?

The force is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

How are charges distributed on a conductor?

uniformly on the external surface of the object.

Where do charges concentrate on an object's surface?

the sharpest curvature

What is another term for the potential energy of electrical charges, and how is it most commonly known?

electromotive force (EMF), voltage.

What causes electric current, and what is it defined as?

Voltage (potential) causes charges to move, which is defined as electric current or charges in motion.

What is a 'conductor' in terms of electron movement?

a substance on which electrons easily move.

What is a 'semiconductor'?

a material whose conductivity depends on its conditions and is important to modern electronic circuits.

What is a 'superconductor'?

a material with no resistance to electron flow below a certain temperature, important to MRI main magnet systems.

What is an 'insulator'?

a substance that does not allow electrons to move easily.

What is the unit of measurement for electric potential?

volts (V)

What is the unit of measurement for electric current?

Amperes (A)

What is the unit of measurement for electrical resistance?

Ohms (Ω)

What factors does electrical resistance depend on?

material, length, diameter, and temperature of the conductor.

What is the relationship between power, current, and voltage in an electric circuit?

Power (P) is equal to current (I) multiplied by voltage (V), or P = IV.

What is 'Direct Current' (DC)?

flows in only one direction, from the positive pole/electrode to the negative pole/electrode.

What is 'Alternating Current' (AC)?

changes direction in a pattern of cycles, with a frequency of 60Hz in the USA.

What is the function of a battery in an electric circuit?

stores electrical charges that are released by chemical reaction to provide electric potential.

What is the function of a diode/solid-state rectifier?

allows electrons to flow in only one direction.

What is the purpose of circuit breakers and fuses?

protective devices that 'break' the circuit when a sudden high increase in current occurs.

What is the difference between a resistor and a rheostat?

Both inhibit electron flow to regulate current; resistors have a set value, while rheostats are variable.

What is the purpose of grounding?

connects charged object to the Earth, which acts as an infinite reservoir of electrons, to neutralize the object and prevent electrical shock.

What creates a magnetic field, according to electromagnetism?

Any charged particle in motion

In which direction do magnetic field lines run inside and outside a magnet?

Magnetic field lines run South to North inside the magnet and North to South outside the magnet.

What are the three laws of magnetism?

1. Every magnet has a north and south pole. 2. Like poles repel; opposite poles attract. 3. Magnetic force intensity is directly proportional to pole strength and flux line density, and inversely proportional to the square of the distance between poles.

What is the difference between paramagnetic and ferromagnetic materials?

Paramagnetic materials are weakly attracted to a magnet (e.g., platinum), while ferromagnetic materials are strongly attracted (e.g., iron, cobalt, nickel).

What is a 'solenoid'?

A solenoid is a coil of wire with a current in it, which creates a magnetic field around it.

What is the definition of electromagnetic induction?

An electric current is induced in a conductor in a changing or moving magnetic field.

What are the three ways to accomplish electromagnetic induction?

1. Moving a magnetic field near a conductor. 2. Moving a conductor through a magnetic field. 3. Changing the strength of a magnetic field near a conductor.

What factors influence the magnitude of the induced current in electromagnetic induction?

The magnitude is proportional to the strength of the magnetic field, the velocity/speed/frequency of the magnetic field movement, the angle of the conductor to the magnetic field, and the number of turns/loops in the conductor.

What is the primary function of a generator?

A generator changes kinetic/mechanical energy into electrical energy by moving a looped conductor in a magnetic field.

What is the primary function of a motor?

A motor changes electrical energy into kinetic/mechanical energy; an electric current supplied to a conductor in a magnetic field causes movement.

How does a transformer function based on electromagnetic induction?

Transformers use electromagnetic mutual induction: a primary solenoid supplied with AC induces an electric current in a nearby secondary solenoid.

What is a 'step-up transformer' and how does it affect voltage and current?

A step-up transformer has a turns ratio greater than 1, which increases voltage and decreases current (amperage).

What is a 'step-down transformer' and how does it affect voltage and current?

A step-down transformer has a turns ratio less than 1, which decreases voltage and increases current (amperage).

What are 'eddy current losses' in a transformer and how are they decreased?

Eddy current losses are currents induced in the ferromagnetic core, producing heat and losing energy; they are decreased by the material and volume of the core.

What are 'hysteresis losses' in a transformer and how are they decreased?

Hysteresis losses are caused by the constant reversal of magnetic fields in the core, producing heat; they are decreased by lamination of the core material.

Describe a 'shell-type transformer' and its advantage.

A shell-type transformer confines more of the magnetic field by having primary and secondary windings on top of one another, leading to greater efficiency.

What is an 'autotransformer'?

An autotransformer is a transformer with a single coil and core, but multiple points of connection to allow for varying the number of turns included in the circuit.