Unit notes and practice questions

Grade 10 Physics unit 5: Magnetism

Everything here is for unit 5 of the Grade 10 Physics textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.

6
Cards: 10
Practice questions on unit 5: 132–155
The textbook pages for unit 5

What unit 5 covers

1.Magnet
2.Magnetic Field
3.The compass and the earth's magnetic field
4.Magnetic field of a current carrying conductor
5.Magnetic force on a moving charge
6.Magnetic force on a current carrying conductor
7.Magnetic force between two parallel current carrying con
8.Applications of magnetic forces and fields

Short notes for unit 5

The cards filed under this chapter on the Grade 10 Physics page. Open one at a time.

Forces between parallel currentsRule

When you use it

Use this rule to predict whether two parallel current-carrying wires will attract or repel each other.

Watch out

Parallel currents in the same direction attract and opposite directions repel. This is the opposite behaviour of electrostatic charges.

Drafted from Grade 10 Physics, pages 132-155, then checked twice before it went up

Magnetic field of a straight wireFormula

B = \frac{\mu_0 I}{2\pi r}

B = \frac{\mu_0 I}{2\pi r}

$B$: magnetic field strength $\text{T}$
$\mu_0$: permeability of free space $\text{T}\,\text{m}\,\text{A}^{-1}$
$I$: current $\text{A}$
$r$: radial distance from wire $\text{m}$

When you use it

Use this formula to calculate the magnetic field strength at a distance r from a long straight wire carrying electric current.

Watch out

Convert distance r from centimetres to metres before calculating.

Drafted from Grade 10 Physics, pages 132-155, then checked twice before it went up

Magnetic force on a current-carrying wireFormula

F = I l B \sin\theta

F = I l B \sin\theta

$F$: magnetic force $\text{N}$
$I$: current $\text{A}$
$l$: length of wire in field $\text{m}$
$B$: magnetic field strength $\text{T}$
$\theta$: angle between current and magnetic field $\text{rad}$

When you use it

Use this formula to calculate the deflecting magnetic force on a straight conductor carrying current in an external magnetic field.

Watch out

The angle theta is measured between the direction of current flow and the magnetic field lines.

Drafted from Grade 10 Physics, pages 132-155, then checked twice before it went up

Magnetic force on a moving chargeFormula

F = q v B \sin\theta

F = q v B \sin\theta

$F$: magnetic force $\text{N}$
$q$: charge $\text{C}$
$v$: speed of charge $\text{m}\,\text{s}^{-1}$
$B$: magnetic field strength $\text{T}$
$\theta$: angle between velocity and magnetic field $\text{rad}$

When you use it

Use this formula to find the magnetic force exerted on a charged particle moving through an external magnetic field.

Watch out

The force is zero when the charge moves parallel to the magnetic field direction because sin 0 is zero.

Drafted from Grade 10 Physics, pages 132-155, then checked twice before it went up

Right hand thumb rule for a straight wireRuleWhich way does the field circle the wire? · What shape are the field lines? · Which current does the thumb follow? · +3

Question	What to do	What it tells you
Which way does the field circle the wire?	Grip the wire with your right hand, thumb lying along the conventional current.	Your curled fingers point the way the field circles the wire.
What shape are the field lines?	Look at a plane cut at right angles to the wire.	Closed circles centred on the wire, all lying in that plane.
Which current does the thumb follow?	Conventional current: from the positive terminal to the negative terminal outside the source.	That is the opposite of the electron flow.
Which way does the field point at one chosen place?	Draw the circle through that point and take the tangent to it.	The field is at right angles to the wire and to the line from the wire to that point.
How strong is the field?	Use $B = \dfrac{\mu_0 I}{2\pi r}$ , where $r$ is the perpendicular distance from the wire's axis and $\mu_0 = 4\pi \times 10^{-7}\,\text{T}\cdot\text{m/A}$ .	Double the current and the field doubles. Double the distance and the field halves.
When does that formula stop working?	Check the shape of the conductor before you use it.	It belongs to a long straight wire only. A circular loop and a solenoid each have their own formula.

When you use it

Use to find which way the magnetic field circles a straight wire that is carrying current. Grip the wire with your right hand and lay your thumb along the conventional current: your curled fingers then point the way the field goes round.

Watch out

The commonest slip is running the rule backwards, curling the fingers along the wire and sweeping the thumb around it. That reverses every answer. The thumb lies along the wire pointing the way the conventional current flows, and the fingers do the circling. The second trap is the distance. The field falls as 1/r, so a point twice as far from the wire feels half the field and not a quarter, and a student who carries the inverse square habit over from Coulomb's law gets it wrong every time.

Magnetic domains and why poles attract or repelRuleA plain piece of iron · The same iron being magnetized · A saturated magnet · +4

Situation	What the domains are doing	What you observe
A plain piece of iron	The domains point in random directions, so their effects cancel.	The material shows no magnetism outside itself.
The same iron being magnetized	Domains already pointing the right way grow, and the rest swing round to join them.	Their effects add up and the material becomes a strong magnet.
A saturated magnet	Every domain is already lined up.	The magnet cannot be made any stronger.
A magnet heated strongly, hammered or dropped	The domains are knocked back out of line.	The magnet loses its strength.
An iron nail held near one pole	The magnet nearby lines up the nail's own domains first.	The nail is attracted by either pole, so attraction proves nothing.
Two magnets brought together	Each magnet sits in the field the other one's aligned domains produce.	Like poles repel and unlike poles attract.
A magnet broken in two	Each piece still holds its own aligned domains.	Every piece has both a north pole and a south pole.

When you use it

Use when a question asks how a piece of iron becomes a magnet, why a magnet loses its strength, or whether two objects will attract or repel. Domains say where a permanent magnet's field comes from, and the push or pull is each magnet sitting in the field of the other.

Watch out

Attraction on its own does not prove that something is a magnet. A magnet attracts an unmagnetized iron bar from either pole, because it lines up the bar's own domains first. Repulsion is the only certain test that both objects are magnets. The same trap catches the compass: the magnetic pole sitting near the geographic North is a magnetic south pole, and that is exactly why the needle's north end swings towards it.

Practice questions on unit 5

10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.

Question 1

What happens if a bar magnet is broken into two pieces?

A.One piece has only a north pole and the other piece has only a south pole.
B.Each piece becomes a complete magnet with both a north and a south pole.
C.Both pieces lose all of their magnetic properties permanently.
D.The pieces turn into unmagnetized pieces of iron.

Show the answer and why

Answer: B Each piece becomes a complete magnet with both a north and a south pole.

Why

Magnetic poles always exist in pairs. When a magnet is broken, each piece still has both a north pole and a south pole.

Question 2

Which type of magnet loses its magnetic property when the external magnetic field is removed?

A.Temporary magnet
B.Permanent magnet
C.Natural bar magnet
D.Hard magnet

Show the answer and why

Answer: A Temporary magnet

Why

Temporary magnets are materials that lose their magnetic property when the magnetic field is removed, whereas permanent magnets retain their magnetism.

Question 3

Where is the magnetic field around a bar magnet the strongest?

A.Exactly at the center of the magnet
B.Equally strong at every point around the magnet
C.Near the poles of the magnet
D.Far away from the magnet

Show the answer and why

Answer: C Near the poles of the magnet

Why

The magnetic field is strongest near the magnetic poles and weaker in the region between the poles.

Question 4

Two long parallel wires placed near Hawassa carry electric current in the same direction. What magnetic force do they exert on each other?

A.They repel each other.
B.They exert zero force on each other.
C.They push each other in opposite directions along their length.
D.They attract each other.

Show the answer and why

Answer: D They attract each other.

Why

When two parallel conductors carry currents in the same direction, the magnetic force between them is attractive.

Question 5

Why does the north-seeking pole of a compass needle point towards the Earth's geographic north?

A.Like magnetic poles always attract each other.
B.The Earth's magnetic south pole is located near the geographic north pole.
C.The Earth's geographic north has a large positive electric charge.
D.The needle is repelled by the Earth's geographic south pole.

Show the answer and why

Answer: B The Earth's magnetic south pole is located near the geographic north pole.

Why

Because opposite poles attract, the north pole of a compass needle is attracted to the Earth's magnetic south pole, which is located near the geographic north pole.

Question 6

The Earth's geographic north pole is separated from the direction of the magnetic north pole by an angle of approximately:

A.11.5 degrees
B.23.5 degrees
C.45.0 degrees
D.90.0 degrees

Show the answer and why

Answer: A 11.5 degrees

Why

The geographic north pole, through which the Earth's rotation axis passes, is about 11.5 degrees away from the magnetic north pole.

Question 7

On what two factors does the magnetic field (B) around a straight current-carrying wire depend?

A.Mass of the wire and air pressure
B.Length of the wire and voltage only
C.Current in the wire and distance from the wire
D.Temperature and cross-sectional shape of the wire

Show the answer and why

Answer: C Current in the wire and distance from the wire

Why

The magnetic field B produced by a long straight wire depends directly on the current I and inversely on the distance r from the wire (B = μ₀I / (2πr)).

Question 8

In a laboratory in Addis Ababa, a straight wire of length 0.5 m carries a current of 8.0 A perpendicular to a magnetic field. If the wire experiences a magnetic force of 0.40 N, what is the magnetic field strength?

A.1.60 T
B.0.25 T
C.0.80 T
D.0.10 T

Show the answer and why

Answer: D 0.10 T

Why

Using F = ILB, we solve for B: B = F / (IL) = 0.40 N / (8.0 A × 0.5 m) = 0.40 / 4.0 = 0.10 T.

Question 9

According to the right-hand rule for the force on a current-carrying wire in a magnetic field, what does your thumb point toward?

A.The direction of the conventional current in the wire
B.The direction of the magnetic field
C.The direction of the magnetic force
D.The direction of the geographic north pole

Show the answer and why

Answer: A The direction of the conventional current in the wire

Why

By the right-hand rule, your fingers point in the direction of the magnetic field B, your thumb points in the direction of the conventional current, and your palm pushes in the direction of the magnetic force.

Question 10

A wire 625 m long is placed perpendicular to a 0.40 T magnetic field. A magnetic force of 1.8 N acts on the wire. What is the current flowing in the wire?