# Magnetism and Electromagnetism

Computer Science I · Physics · https://tryals.app/learn/computer-science-i/magnetism-and-electromagnetism

## Electricity and Magnetism United

**Magnetism** and electricity, once thought separate, are two aspects of one **electromagnetic** force. This unification powers modern technology from hard drives to Wi-Fi.

| Phenomenon | Description | Application |
| --- | --- | --- |
| **Current to Field** | Moving charges create a magnetic field circling the wire. | Electromagnets |
| **Field to Force** | Magnetic fields exert a force on moving charges perpendicular to both. | Electric motors |
| **Induction** | Changing magnetic fields induce a voltage in a nearby loop. | Generators |

Maxwell's synthesis proved that oscillating electric and magnetic fields sustain each other and propagate as **electromagnetic waves**, which includes all light and radio signals.

## Faraday's Law and Induction

**Electromagnetic induction** relies entirely on change. A steady, motionless magnetic field induces **zero** voltage or current in a loop.

The induced voltage depends on the rate of change of magnetic flux through the circuit. **Faraday's law** dictates that motion or variation is strictly required.

| Field State | Effect on Circuit |
| --- | --- |
| **Steady / Motionless** | Zero current / no induction |
| **Changing / Moving** | Current induced |

> **Common pitfall:** Thinking a **steady** magnetic field induces a current. Only a **changing** field does. A magnet sitting motionless near a coil induces nothing; the current appears only while the field changes.

## Practice questions

6 of this lesson's 13 practice questions, with answers. The full set is in the app.

### 1. A hard drive stores bits as magnetic domains, and shrinking them indefinitely fails. What sets the floor?

A. Manufacturing precision, which improves each generation
B. The wavelength of light used to read the domains
C. The minimum size of a magnetic field that can be generated
D. Thermal stability, below which a domain flips on its own

**Answer:** D. Thermal stability, below which a domain flips on its own

**Why:** Below the superparamagnetic limit, kT at room temperature is comparable to the barrier holding the domain, so bits flip at random. Physics rather than engineering sets the density ceiling, which is why perpendicular recording and heat-assisted media were developed to raise the barrier instead.

Page: https://tryals.app/practice/computer-science-i/magnetism-and-electromagnetism/a-hard-drive-stores-bits-as-magnetic-domains-and-shrinking-them

### 2. A motor turns electrical energy into motion and a generator does the reverse, and the two are essentially the same device. Why does one mechanism run in both directions?

A. Because motors and generators contain different components arranged in opposite orders.
B. Because a generator is a motor running backwards in time.
C. Because the underlying relationship is symmetric in both directions.
D. Because energy conservation requires every device to be reversible.

**Answer:** C. Because the underlying relationship is symmetric in both directions.

**Why:** Two effects, two faces of one relationship: a current-carrying conductor in a field feels a force, and a conductor moved through a field has a voltage induced across it. Spin a small DC motor's shaft by hand with a voltmeter attached and it generates. Regenerative braking exploits the reversibility, which is why an electric car's range improves in hilly driving where a petrol car's does not.

Page: https://tryals.app/practice/computer-science-i/magnetism-and-electromagnetism/a-motor-turns-electrical-energy-into-motion-and-a-generator-does-the

### 3. A magnet is moved through a coil at four different speeds. Order the situations from the smallest induced voltage to the largest.

**Answer:**

1. The magnet is held completely still
2. The magnet is eased through very slowly
3. The magnet is moved through at a moderate speed
4. The magnet is thrust through very fast

**Why:** Faster motion means a faster change of flux and so a larger induced voltage, with the stationary case giving exactly zero. This is why a generator must be spun and why spinning it harder produces more voltage.

Page: https://tryals.app/practice/computer-science-i/magnetism-and-electromagnetism/a-magnet-is-moved-through-a-coil-at-four-different-speeds-order-the

### 4. Sort each phenomenon by which direction of the electromagnetic connection it demonstrates.

**Answer:**

- Electricity produces magnetism: A current-carrying wire deflects a nearby compass needle, An electromagnet lifts scrap metal when switched on
- Magnetism produces electricity: A generator turned by a turbine delivers current, A disk drive head reads data from a spinning magnetic platter, A moving magnet drives a current round a closed loop

**Why:** Currents create fields, and changing fields create currents, the two directions of one relationship. The disk read head is the payoff for computing: the platter's changing magnetisation induces the tiny voltages that carry back your stored bits.

Page: https://tryals.app/practice/computer-science-i/magnetism-and-electromagnetism/sort-each-phenomenon-by-which-direction-of-the-electromagnetic

### 5. Complete the condition for electromagnetic induction.

**Answer:** A voltage is induced in a loop when the magnetic flux through it **is changing**.

**Why:** Change is the requirement, and it is the only one. Magnitude and direction affect how much and which way, but a field that never varies induces nothing regardless of how strong it is or where it points.

Page: https://tryals.app/practice/computer-science-i/magnetism-and-electromagnetism/complete-the-condition-for-electromagnetic-induction

### 6. A strong magnet placed beside a coil and left stationary induces a steady current in the coil.

**Answer:** False

**Why:** False. Induced EMF is $-d\Phi/dt$, so a constant flux, however large, induces nothing. Current flows only while the magnet moves, the coil moves, or the field changes. This is why generators must rotate and why transformers work on alternating current and pass no direct current at all.

Page: https://tryals.app/practice/computer-science-i/magnetism-and-electromagnetism/a-strong-magnet-placed-beside-a-coil-and-left-stationary-induces-a