Electric and magnetic forces

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Apuntes de la lección

Electric Charge and Electric Force

Electric charge is a physical property of particles (protons positive, electrons negative) that causes attraction or repulsion.
Opposite charges attract (positive and negative), like charges repel (positive-positive or negative-negative).
Electric force is the attraction or repulsion between charged objects; it acts without contact.
Strength of electric force depends on the amount of charge and the distance between objects: more charge increases force, greater distance decreases force.
Static electricity is a buildup of charge on objects, often from friction transferring electrons (e.g., rubbing a balloon on hair).
Objects become negatively charged when they gain electrons, positively charged when they lose electrons; protons do not move.

Testing charge with perspex rods and watch glasses

Magnetic Force and Magnetic Fields

Magnetic force is a non-contact force exerted by magnets on certain materials (like iron, nickel, cobalt) and other magnets.
All magnets have two poles: north and south; opposite poles attract, like poles repel.
A magnetic field is the region around a magnet where magnetic force acts; it is invisible but can be visualized with iron filings.
Field lines show the direction and strength of the magnetic field; they go from north to south pole, and closer lines indicate stronger field.
Magnetic field is strongest near the poles and weakens with distance from the magnet.
Magnetic forces can act through materials like paper or wood, but some materials (e.g., thick iron) can block or weaken them.

Magnetic field lines between two bar magnets with opposite poles facing each other, showing attraction.

Electromagnets

An electromagnet is made by wrapping a wire coil around an iron core and connecting it to a battery; current creates a magnetic field.
Electromagnets have two advantages: they can be very strong and can be turned on/off by controlling the current.
The strength of an electromagnet depends on the number of coils, the current (batteries), the core material, and distance to objects.
Data shows: more coils (30 vs 25) pick up more paper clips; more batteries (2 vs 1) increase strength; thicker iron nail strengthens the magnet.
If the circuit is incomplete (only one wire connected) or the coil is too small (3 turns), the electromagnet does not work.
Electromagnets are used in electric motors, generators, doorbells, and lifting heavy scrap metal.

A copper coil wrapped around an iron nail, connected to a battery

Factors Affecting Electric and Magnetic Force Strength

Distance affects both electric and magnetic forces: the closer objects are, the stronger the force; farther apart, weaker.
Amount of charge affects electric force: more charge (more electrons/protons) increases force.
Number of coils in an electromagnet: increasing turns of wire increases magnetic strength.
Current (batteries) in an electromagnet: higher current increases magnetic strength.
Core material: iron core increases strength compared to no core or non-magnetic core.
Thickness of iron core: thicker nail provides more material for magnetic alignment, increasing strength.

Applications and Phenomena

Lightning is a large electrostatic discharge between charged clouds or between clouds and the ground.
Earth's magnetic field acts like a giant bar magnet, allowing compasses to work; it also causes auroras (Northern/Southern Lights).
Static electricity causes hair to stand up on a plastic slide because electrons transfer, creating opposite charges that attract.
Electromagnets are used in maglev trains for magnetic levitation, and in electric generators to produce current.
Magnetic materials include iron, nickel, cobalt, and steel (an alloy of iron); other metals like copper and aluminium are not magnetic.

Lightning: a huge electrostatic discharge

Experimental Evidence from Electromagnet Data

Connecting both wire ends to the battery is essential; one end only gives 0 paper clips.
Increasing coils from 25 to 30 raised pickup from 3 to 5 paper clips; decreasing to 3 coils gave 0.
Distance matters: at 20 cm away, 0 clips; at 2 cm, 3 clips.
Using 2 batteries instead of 1 increased pickup from 3 to 4 clips.
A thicker iron nail picked up 4 clips vs 2 for a thinner nail.
Plastic-coated paper clips were not picked up (0), while metallic ones were (3), showing magnetic force affects certain metals.

Testing an electromagnet

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Preguntas de práctica

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1.Which of the following is the correct definition of electric charge?
Easy
- AA physical property that causes objects to attract or repel each other without touching
- BA force that only acts when objects are in contact
- CA property that only exists in metals
- DA measure of the amount of matter in an object
2.Which of the following is the correct definition of a magnetic field?
Easy
- AThe region around a magnet where magnetic forces act
- BThe region around a charged object where electric forces act
- CThe force between two magnets
- DThe area where iron filings are placed
3.Which of the following are magnetic materials? (Select all that apply)
Easy
- AIron
- BNickel
- CCobalt
- DCopper
- EAluminium
4.Like poles of magnets attract each other.
Easy
True or false?
5.An object with more electrons than protons is negatively charged.
Easy
True or false?
6.What is the charge of a proton?
Easy
- APositive
- BNegative
- CNeutral
- DIt depends on the atom
7.The figure shows four pairs of charged particles, each pair a set distance apart. Which pair has the greatest attractive electric force between them?
Medium
- APair 1 (+1 and +1, 5 mm apart)
- BPair 2 (+1 and +1, 10 mm apart)
- CPair 3 (+1 and −1, 5 mm apart)
- DPair 4 (+1 and −1, 10 mm apart)
8.Which of the following is NOT a field force?
Easy
- AFrictional force
- BGravitational force
- CElectrostatic force
- DMagnetic force

Más preguntas

9.The strength of an electromagnet can be increased by increasing the number of coils of wire.
Easy
True or false?
10.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Electromagnet 2 cm from the paper clips | 3 | | Electromagnet 20 cm from the paper clips | 0 | In the electromagnet experiment, what happens to the number of paper clips picked up when the distance between the electromagnet and the paper clips is increased from 2 cm to 20 cm?
Easy
- AIt decreases from 3 to 0
- BIt increases from 3 to 5
- CIt stays the same
- DIt becomes negative
11.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Both wire ends connected to the battery | 3 | | Coil turns increased to 30 | 5 | | 2 batteries connected | 4 | | Thicker iron nail | 4 | In the electromagnet experiment, which change resulted in the greatest increase in the number of paper clips picked up?
Easy
- AIncreasing coil amount to 30
- BUsing a thicker iron nail
- CUsing 2 batteries
- DMoving the electromagnet closer
12.An electromagnet's magnetic field can be turned on and off.
Easy
True or false?
13.A balloon is rubbed on hair and becomes negatively charged. What happens to the hair?
Medium
- AIt becomes positively charged
- BIt becomes negatively charged
- CIt remains neutral
- DIt becomes magnetic
14.Two charged objects are brought closer together. How does the electrostatic force between them change?
Medium
- AIt increases
- BIt decreases
- CIt stays the same
- DIt becomes zero
15.Which of the following correctly describes the magnetic field lines around a bar magnet?
Medium
- AThey go from the north pole to the south pole
- BThey go from the south pole to the north pole
- CThey are parallel to the magnet
- DThey are random
16.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Both wire ends connected to the battery | 3 | | Only 1 wire end connected to the battery | 0 | In the electromagnet experiment, what happened when only one wire end was connected to the battery?
Medium
- A0 paper clips were picked up
- B3 paper clips were picked up
- C5 paper clips were picked up
- DThe nail became magnetized
17.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Both wire ends connected to the battery | 3 | | Coil turns increased to 30 | 5 | | 2 batteries connected | 4 | | Thicker iron nail | 4 | Which of the following is NOT a factor that affects the strength of an electromagnet according to the experiment?
Medium
- AThe color of the wire
- BThe number of coils
- CThe number of batteries
- DThe thickness of the iron nail
18.The graphs below show how a coil's magnetic field strength changes as the current through it increases. Graph C shows the result for a coil with a certain number of turns of wire. If the same coil is rewound with MORE turns (keeping the same length), which graph would show the new relationship between current and magnetic field strength?
Medium
- AGraph A
- BGraph B
- CGraph C
- DGraph D
19.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Both wire ends connected to the battery | 3 | | Thicker iron nail | 4 | Which of the following correctly explains why a thicker iron nail increases the strength of an electromagnet?
Medium
- AIt provides more material for the magnetic field to align
- BIt allows more current to flow
- CIt increases the number of coils
- DIt makes the magnet heavier
20.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Plastic-coated paper clips | 0 | | Metallic paper clips | 3 | Which of the following is the best explanation for why the plastic-coated paper clips were not picked up by the electromagnet?
Medium
- APlastic is not a magnetic material
- BThe plastic coating made them too heavy
- CThe electromagnet was too weak
- DThe plastic coating blocked the magnetic field
21.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Both wire ends connected to the battery | 3 | | 2 batteries connected | 4 | | Coil turns decreased to 3 | 0 | | Thinner iron nail | 2 | | Electromagnet 20 cm from the paper clips | 0 | A student builds an electromagnet and picks up 3 paper clips. Which change would likely allow it to pick up more paper clips?
Medium
- AIncreasing the number of batteries
- BDecreasing the number of coils
- CUsing a thinner nail
- DMoving the electromagnet farther away
22.Electric and magnetic forces are both contact forces.
Medium
True or false?
23.Which of the following correctly describes the cause and effect relationship when the south pole of one magnet is brought near the south pole of another magnet?
Medium
- AThey repel each other
- BThey attract each other
- CThey have no effect
- DThey become demagnetized
24.Which of the following statements is TRUE about the electric force between two charged objects?
Medium
- AThe force increases as the distance between them increases
- BThe force decreases as the distance between them increases
- CThe force is independent of distance
- DThe force only acts when they are touching
25.Which of the following is the best explanation for why a child's hair stands up after going down a plastic slide?
Hard
- AThe hair and the slide have opposite charges, causing attraction
- BThe hair and the slide have the same charge, causing repulsion
- CThe hair is magnetic
- DThe slide is magnetic
26.A student wants to find out whether changing the amount of current affects how strong an electromagnet's magnetic field is, and how much magnetic force it can exert on objects. Which question should the student investigate?
Medium
- AHow does changing the magnetic field acting on an electromagnet affect the current flowing through it?
- BHow does changing the current through an electromagnet affect its magnetic field strength and the magnetic force it exerts on objects?
- CHow does changing the magnetic force on an object affect the magnetic field produced by the current?
- DHow do the current and the magnetic force on an object change the magnetic field at the same time?
27.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | 1 battery connected | 3 | | 2 batteries connected | 4 | A student claims that increasing the number of batteries in an electromagnet will always increase its strength. Based on the data, is this claim supported?
Hard
- AYes, because 2 batteries picked up more paper clips than 1
- BNo, because the data only shows one comparison
- CYes, because more batteries always mean more current
- DNo, because the nail thickness also matters
28.Which of the following correctly compares electric force and magnetic force?
Hard
- AElectric force can attract or repel, magnetic force can also attract or repel
- BElectric force only attracts, magnetic force only repels
- CElectric force acts at a distance, magnetic force requires contact
- DElectric force is always stronger than magnetic force
29.In the floating paperclip trick, the paperclip rises because the magnet exerts a force on it. What type of force is this?
Challenge
- AMagnetic force
- BElectric force
- CGravitational force
- DFrictional force
30.Match each description to the correct term.
Easy
- A region in space where an object with certain properties will experience a force
- The two opposite ends of a magnet
- A fundamental property of matter that comes in two types and is carried by protons and electrons
- Field
- Poles
- Electric charge
31.Arrange the steps in the correct order to create an electromagnet.
Medium
- Wrap a copper wire around an iron nail
- Connect the wire to a 9-volt battery
- Place the electromagnet near paper clips
- Electricity flows through the wire and the nail becomes magnetic
32.Complete the sentence: The two poles of a magnet are called the ___ pole and the ___ pole.
Easy
The two poles of a magnet are called the ____ pole and the ____ pole.
33.Complete the sentence: When an object has more electrons than protons, it is ___ charged.
Easy
When an object has more electrons than protons, it is ____ charged.
34.Complete the sentence: The force of attraction or repulsion between charged particles is called ___.
Easy
The force of attraction or repulsion between charged particles is called ____.
35.Complete the sentence: The buildup of electric charges on an object is called ___.
Easy
The buildup of electric charges on an object is called ____.
36.Complete the sentence: The region in space where an object with certain properties will experience a force is called a ___.
Easy
The region in space where an object with certain properties will experience a force is called a ____.
37.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Both wire ends connected to the battery | 3 | | Coil turns increased to 30 | 5 | In the electromagnet experiment, when the coil amount was increased from 25 to 30, how many paper clips were picked up?
Easy
38.Complete the sentence about magnetic materials.
Easy
The three magnetic materials are iron, nickel, and ____.
39.A student wraps a coil of wire around an iron nail and connects it to a battery to make an electromagnet, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Thicker iron nail | 4 | | Thinner iron nail | 2 | According to the data, what is the difference in the number of paper clips picked up between using a thicker iron nail and a thinner iron nail?
Medium
40.A student wraps a coil of wire around an iron nail and connects it to a battery, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Both wire ends connected to the battery | 3 | | Coil turns increased to 30 | 5 | Using evidence from the electromagnet data, explain how the number of coils affects the strength of an electromagnet.
Hard
41.Explain why a child's hair stands up after going down a plastic slide.
Hard
42.A student wraps a coil of wire around an iron nail and connects it to a battery, then tests changes one at a time. | What was changed | Paper clips picked up | |---|---| | Electromagnet 2 cm from the paper clips | 3 | | Electromagnet 20 cm from the paper clips | 0 | Using evidence from the data, explain how distance affects the strength of an electromagnet.
Hard

Apuntes de repaso

## Electric and Magnetic Forces ### Key Concepts - **Electromagnetic force**: One of the four fundamental forces, responsible for interactions between charged particles. It holds atoms together and causes chemical bonds. - **Electric forces**: Occur between charged objects. Like charges repel, opposite charges attract. Strength depends on amount of charge and distance. - **Magnetic forces**: Occur between magnets or moving charges. Like poles repel, opposite poles attract. Strength depends on magnet strength and distance. - **Static electricity**: Buildup of electric charge on an object. Can cause attraction or repulsion without contact. - **Electromagnetism**: Electricity and magnetism are related. A moving electric charge creates a magnetic field; a changing magnetic field can induce electric current. ### Factors Affecting Force Strength - **Amount of charge**: More charge → stronger electric force. - **Distance**: Closer objects experience stronger forces (both electric and magnetic). - **Magnet strength**: Stronger magnets produce stronger magnetic forces. - **Material**: Some materials (e.g., iron) are more affected by magnets. ### Attraction and Repulsion - **Electric**: Like charges repel, opposite charges attract. - **Magnetic**: Like poles (N-N or S-S) repel, opposite poles (N-S) attract. ### Real-World Examples - **Static electricity**: Balloon sticking to wall after rubbing. - **Magnets**: Compass needle aligning with Earth's magnetic field. - **Electromagnets**: Used in junkyards to lift scrap metal. ### Important Notes - Electromagnetic force has unlimited range but weakens with distance. - All forces (friction, contact) are ultimately due to electromagnetic interactions between atoms. - Electromagnetic waves (light, radio) are oscillations of electric and magnetic fields.

Electromagnetic force: A fundamental force that acts between charged particles; responsible for electricity, magnetism, and light.
Electric force: Force between charged objects; can be attractive or repulsive depending on charge types.
Magnetic force: Force between magnets or moving charges; can be attractive or repulsive depending on poles.
Static electricity: An imbalance of electric charge on an object, often caused by friction.
Attraction: Force pulling objects together; occurs between opposite charges or opposite magnetic poles.
Repulsion: Force pushing objects apart; occurs between like charges or like magnetic poles.
Electromagnetic field: A physical field produced by charged objects; combines electric and magnetic fields.

Tarjetas

What are the two types of electric charge?

Positive and negative. Like charges repel, opposite charges attract.

How does distance affect the strength of an electric force?

The force gets weaker as distance increases. It follows an inverse square law.

What happens when two magnets have like poles facing each other?

They repel each other. Like poles (N-N or S-S) push apart.

What happens when opposite poles of two magnets are brought together?

They attract each other. Opposite poles (N-S) pull together.

What is static electricity?

An imbalance of electric charge on the surface of an object, often caused by friction.

How can you increase the strength of an electromagnet?

Increase the number of wire coils, increase the electric current, or use a core with higher magnetic permeability (e.g., iron).

What is the relationship between electricity and magnetism?

Moving electric charges (current) create a magnetic field; changing magnetic fields can induce electric current.

What is an electromagnetic force?

A fundamental force that acts between charged particles, responsible for electricity, magnetism, and light.

What are the four fundamental forces of nature?

Gravitational, electromagnetic, strong nuclear, and weak nuclear.

What causes the force you feel when you push a book across a table?

Electromagnetic forces between atoms in your hand and the book, and between the book and the table.

What is an electromagnet?

A magnet created by running electric current through a coil of wire, often wrapped around an iron core.

How does the number of coils affect an electromagnet's strength?

More coils increase the magnetic field strength, making the electromagnet stronger.

What is a magnetic field?

A region around a magnet or current-carrying wire where magnetic forces can be detected.

What is an electric field?

A region around a charged object where electric forces can be exerted on other charges.

What happens to the force between two charges if the distance is doubled?

The force becomes one-fourth as strong (inverse square law).

What is the role of electrons in static electricity?

Electrons can transfer from one object to another, creating a charge imbalance.

What is a permanent magnet?

A material that retains its magnetic properties without an external current, like a bar magnet.

What is the difference between a permanent magnet and an electromagnet?

A permanent magnet always has a magnetic field; an electromagnet only has a field when current flows.

How does a compass work?

The compass needle is a small magnet that aligns with Earth's magnetic field, pointing north.

What is the law of electric charges?

Like charges repel; opposite charges attract.

What is an electric current?

The flow of electric charge, usually electrons, through a conductor.

What is the effect of inserting an iron core into a solenoid?

It greatly increases the magnetic field strength because iron is ferromagnetic.

What is the relationship between current and magnetic field strength in an electromagnet?

Increasing the current increases the magnetic field strength.

What is electromagnetic induction?

The process of generating electric current by changing the magnetic field around a conductor.

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