Physics Unit 7 Magnetism and Electromagnetism

Where is the magnetic force of a magnet strongest?

At the poles (North and South).

What occurs when two like magnetic poles are brought close together?

They repel each other.

What occurs when two unlike magnetic poles are brought close together?

They attract each other.

Magnetic attraction and repulsion are examples of what type of force?

Non-contact force.

Term: Induced Magnet

Definition: A material that becomes a magnet only when placed in a magnetic field.

What happens to an induced magnet when it is removed from a magnetic field?

It loses most or all of its magnetism quickly.

What is the force between a permanent magnet and a magnetic material?

It is always a force of attraction.

List the four main magnetic materials.

Iron, steel, cobalt, and nickel.

Term: Magnetic Field

Definition: The region around a magnet where a force acts on another magnet or a magnetic material.

How does the strength of a magnetic field change with distance from the magnet?

The strength decreases as the distance from the magnet increases.

In which direction do magnetic field lines always point?

From the North pole to the South pole.

What does a magnetic compass contain that allows it to function?

A small bar magnet (the needle).

How does a compass provide evidence that the Earth's core is magnetic?

The compass needle aligns with the Earth's magnetic field.

What is produced around a wire when an electric current flows through it?

A magnetic field.

Which rule is used to find the direction of the magnetic field around a straight current-carrying wire?

The right-hand grip rule.

Term: Solenoid

Definition: A coil of wire that creates a strong and uniform magnetic field inside when current flows through it.

Describe the magnetic field inside a solenoid.

It is strong and uniform.

How does the external magnetic field of a solenoid compare to a bar magnet?

The external field has a similar shape to that of a bar magnet.

How does adding an iron core affect the magnetic field of a solenoid?

It increases the strength of the magnetic field.

Term: Electromagnet

Definition: A solenoid containing an iron core.

Concept: The Motor Effect

The interaction where a current-carrying conductor in a magnetic field experiences a force.

Equation: Force on a conductor ($F$) at right angles to a magnetic field.

$F = B \times I \times l$

In the equation $F = B \times I \times l$, what does the symbol '$B$' represent?

Magnetic flux density.

What is the unit of measurement for magnetic flux density ($B$)?

Tesla ($T$).

In Fleming's Left-Hand Rule, what does the thumb represent?

The direction of the force (thrust or motion).

In Fleming's Left-Hand Rule, what does the first finger represent?

The direction of the magnetic field (North to South).

In Fleming's Left-Hand Rule, what does the second finger represent?

The direction of the current (positive to negative).

What must be the orientation of a wire relative to magnetic field lines to use $F = B \times I \times l$?

The wire must be at right angles (perpendicular) to the magnetic field.

How is the rotation of an electric motor achieved?

Current-carrying coils in a magnetic field experience opposite forces on each side, causing rotation.

How does a moving-coil loudspeaker produce sound waves?

Variations in current create a varying magnetic field that interacts with a permanent magnet, vibrating the cone.

In a loudspeaker, what happens to the force when the direction of the current reverses?

The direction of the force reverses.

What is the 'Generator Effect'?

The induction of a potential difference across a conductor moving relative to a magnetic field.

Under what condition will an induced potential difference produce an induced current?

The conductor must be part of a complete circuit.

What effect does increasing the speed of movement have on an induced potential difference?

It increases the size of the induced potential difference.

What effect does using a stronger magnet have on the generator effect?

It increases the size of the induced potential difference.

How does an induced current affect the magnetic field that produced it?

The induced current generates a magnetic field that opposes the original change in movement or field.

Which device uses the generator effect and slip rings to produce alternating current ($a.c.$)?

An alternator.

Which device uses the generator effect and a split-ring commutator to produce direct current ($d.c.$)?

A dynamo.

How does a moving-coil microphone work?

Sound waves vibrate a diaphragm, moving a coil in a magnetic field and inducing a varying current.

What are the two basic components of a transformer?

A primary coil and a secondary coil wound on an iron core.

Why is iron used for the core of a transformer?

Iron is easily magnetised and demagnetised.

What is the function of a step-up transformer?

To increase potential difference and decrease current.

What is the function of a step-down transformer?

To decrease potential difference to a safe level for domestic use.

Equation: Relationship between potential differences and number of turns in a transformer.

$\frac{V{p}}{V{s}} = \frac{n{p}}{n{s}}$

Equation: Power input and output for a $100\%$ efficient transformer.

$V{p} \times I{p} = V{s} \times I{s}$

In the National Grid, why is electricity transferred at very high potential differences?

To keep current low, which reduces thermal energy transfer to the surroundings (increasing efficiency).

How does the number of turns on the secondary coil compare to the primary in a step-down transformer?

The secondary coil has fewer turns than the primary coil ($n{s} < n{p}$).

How does the number of turns on the secondary coil compare to the primary in a step-up transformer?

The secondary coil has more turns than the primary coil ($n{s} > n{p}$).

What induces a potential difference in the secondary coil of a transformer?

An alternating magnetic field created by the primary coil passes through the iron core.

What happens to the potential difference in a generator if the rotation speed is doubled?

The potential difference doubles (and the frequency doubles).

Term: Split-ring commutator

Definition: A device in a dynamo that reverses the connection to the external circuit every half-turn to maintain $d.c.$.

How can you increase the moment of a force without increasing the force itself?

Increase the perpendicular distance between the force and the pivot.

Process: Creating an electromagnet.

Wrap a coil of wire (solenoid) around an iron core and pass an electric current through the wire.

Why do some materials lose magnetism faster than others?

Materials like 'soft' iron demagnetise easily, whereas 'hard' steel retains magnetism longer.

What is the purpose of the National Grid?

A system of cables and transformers that links power stations to consumers.

In a $d.c.$ motor, what is the role of the commutator?

It swaps the contacts every half-turn to keep the motor rotating in the same direction.

How does a transformer core transfer energy between coils?

Through a changing (alternating) magnetic field, not by conducting electricity.

What type of current is required for a transformer to work?

Alternating current ($a.c.$).

Why does a transformer not work with a $d.c.$ supply?

A $d.c.$ supply produces a constant magnetic field, so no potential difference is induced in the secondary coil.

In the context of the National Grid, what is 'efficiency' improved by?

Reducing power loss in transmission cables by using very high potential differences.

What determines the direction of the force in the motor effect?

The direction of the current and the direction of the magnetic field.

In a generator graph of $p.d.$ against time, what does one complete cycle represent?

One full revolution of the coil or magnet.

What happens to the $p.d.$ when a coil in a generator is parallel to the magnetic field lines?

The $p.d.$ is at its maximum (the coil is cutting field lines at the fastest rate).

What happens to the $p.d.$ when a coil in a generator is perpendicular to the magnetic field lines?

The $p.d.$ is zero (no field lines are being cut).

How do headphones utilize the motor effect?

Varying electrical signals in a coil interact with a magnet to vibrate a diaphragm, producing sound.

Define 'Magnetic Flux Density'.

A measure of the number of magnetic field lines passing through a specific area (field strength).

How can the direction of rotation of an electric motor be reversed?

Reverse the direction of the current or reverse the direction of the magnetic field.

List three factors that increase the force on a wire in the motor effect.

Increased current, stronger magnetic field, and increased length of wire in the field.

What effect is used by a moving-coil microphone?

The generator effect.

What effect is used by a moving-coil loudspeaker?

The motor effect.

Why is the potential difference from the National Grid decreased before reaching homes?

To make it safe for consumer use in domestic appliances.

Cloze Deletion: In an alternator, _ are used to maintain a connection with the rotating coil without reversing the current.

slip rings

Formula: Current drawn from the primary coil ($I_{p}$) of a $100\%$ efficient transformer.

$I{p} = \frac{V{s} \times I{s}}{V{p}}$

What happens to the induced current if the magnet is held stationary inside a coil?

No current is induced because there is no change in the magnetic field around the conductor.

How does the Earth's magnetic field interact with a current-carrying power cable?

It exerts a very small force on the cable (Motor Effect).

What is the assumption usually made when calculating the length of a cable using $F = BIl$ in Earth's field?

The cable is at right angles to the Earth's magnetic field.

How does the 'Generator Effect' differ from the 'Motor Effect'?

The generator effect uses motion to create electricity; the motor effect uses electricity to create motion.

In a dynamo, what is the effect of having more turns on the rotating coil?

It increases the maximum potential difference generated.

Why are transformers necessary for an efficient National Grid?

They allow for high-voltage transmission, which minimizes energy wasted as heat in cables.

If a transformer has a primary voltage of $230~V$, $200$ primary turns, and $1200$ secondary turns, what is the output voltage?

$1380~V$

Calculate the current in a $6.9~W$ secondary coil if the output voltage is $12~V$.

$0.575~A$

Describe the field lines of a strong magnetic field.

The field lines are very close together.

In a solenoid, where is the magnetic field 'uniform'?

Inside the center of the coil.

What happens to the magnetic field strength of a wire if the current is halved?

The magnetic field strength is halved.

Which pole of a compass needle points towards the Earth's magnetic North?

The North-seeking pole (it is attracted to the magnetic South pole located at the geographic North).