p7: magnetism and electromagnetism

where are the magnetic forces on a magnet the strongest?

at the poles

what happens when two magnets are brought closely together?

they exert a force on eachother

what does a permanent magnet produce?

its own magnetic field

what is an induced magnet?

a material that becomes a magnet when it is placed in a magnetic field

what type of force can an induced magnet cause?

always a force of attraction

what happens when an induced magnet is removed from a magnetic field?

an induced magnet loses most/ all of its magnetism quickly

what is a magnetic field?

the region around a magnet where a force acts on another magnet or on a magnetic material(iron, steel, cobalt and nickel)

what is the force between a magnet and a magnetic material?

always one of attraction

what does the strength of the magnetic field depend on?

the disance from the magent

where is the magnetic field the strongest?

the poles of the magnet

what is the direction of the magnetic field at any point given by?

the direction of the force that would act on another north pole place at that point

what is the direction of a magnetic field line?

from the north pole of a magnet to the south pole of the magnet

what does a magnetic compass contain?

a small bar magnet

what does a compass needle point to?

in the direction of the earth’s magnetic field

describe how to plot the magnetic field pattern using a compass.

place magnet on a paper. trace around the magnet, position a plotting compass nearby, mark the needle’s direction, then move the compass so its tail is on the mark, marking the new needle direction and repeat to draw a line north to south: repeat this for multiple starting points to map the field lines which show field strenght where they are closest

what happens when a current flows through a conducting wire?

a magnetic field is produced around the wire

what does the strength of the magnetic field rely on?

the current through the wire

the distance from the wire

how to increase the strength of magnetic field?

shaping a wire to form a solenoid

adding an iron core

what is an electromagnet?

a solenoid with an iron core

how does a solenoid increase magnetic effect?

by coiling the wire, causing the individual magnetic fields from each loop to align and add up

what is the motor effect?

when a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other

what are the factors that affect the size of the force on a conductor?

• the current

• length of the conductor

• the magnetic field strength

• the angle between the current and the field

explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.

1. a coil of wire is placed between the north and south poles of magnets. when current flows through the coil, it creates its own magnetic field, which interacts with the external magnetic field

2. due to this interaction, one side of the coil experiences an upwards force, while the other side experiences a downwards force

3. these opposing forces produce a turning affect on th coil causing it to rotate

4. as the coil reached the vertical position (where the force would be zero, the split ring commutator reverses the direction of the current in the coil

5. this current reversal ensure the forces on the sides of the coil always push in the same rotational direction, allowing for continuous 360 degree rotation

how do loudspeakers and headphones use the motor effect?

to convert variations in current in electrical circuits to the pressure variations in sound waves

explain how a moving-coil loudspeaker and headphones work.

1. an electrical audio signal, an alternating current, from a device flows into the coil

2. the current flowing through the coil creates its own magnetic field

3. this electromagnet’s field interacts with the fixed magnetic field from the permanent magnet, causing a force

4. as the audio signal constantly changes direction, the magnetic field of the coil reverses, pushing the cone out and pulling it in rapidly

5. the vibrating cone pushes and pulls the surrounding air, creating pressure waves that travel to your ear.

6. the speed of. thecurrent’s alteration determines the pitch while the current’s strength controls the volume

when is a potential difference induced?

when an electric conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor

what is the generator effect?

the principle where a voltage is induced, creating an electric current, when a conductor moves through a magnetic field, or when a magnetic field changed around a stationary conductor, a process known as electromagnetic induction

the factors that affect the size of the induced potential difference/induced current.

• moving the magnet/ wire faster

• using a stronger magnet

• increasing the number of turns in the coil

• increasing the coil’s area

• ensuring the conductor cute the magnetic field at a 90 degree angles

the factors that affect the direction of the induced potential difference/induced current

• the direction of relative motion between the conductor and the magnetic field

• the polarity of the magnet

• Lenz’s Law

explain how the generator effect is used in an alternator to generate ac.

as the coil rotates, the induced current naturally reverses direction every half turn. the slip rings maintain this reversal in the external circuit, producing a sinusoidal AC waveform that alternates positive and negative

explain how the generator effect is used in a dynamo to generate dc.

the commutator reverses the connection to the coil at the exact moment the induced current reverses. this keeps the current flowing in the external circuit, resulting in a pulsating DC

how do microphones use the generator effect?

to convert the pressure variations in sound waves into variations in current in electrical circuits

explain how a moving-coil microphone works.

• sound waves, which are variations in air pressure, strike a thin, flexible diaphragm

• the diaphragm vibrates back and forth in response to the changing pressure of the sound waves

• a coil of fine wire is attached to the diaphragm. as the diaphragm vibrates, the coil moves with it, oscillating back and forth through the fixed magnetic field created by a permanent magnet

• moving the wire coil through the magnetic field induces an electrical potential difference across the ends of the wire, a principle known as the generator effect

• because the coil is constantly changing direction, it induces an alternating current that changes in size and direction, matching the frequency and amplitude of the original sound wave

• this alternating electrical current is carried by wires to an amplifier or recording device, effectively converting acoustic energy to an electrical signal

what does a basic transformer consist of?

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

why is iron used?

it is easily magnetised

what does the ratio of the potential differences across the primary and secondary coils of a transformer V _p and V _s depends on?

the ratio of the number of turns on each coil, n _p and n _s .

what happens if transformers were 100% efficient

the electrical power output would equal the electrical power input.

if it is a step-up transformer…

V _s > V _p

if it is a step-down transformer…

V _s < V _p

explain how the effect of an alternating current in one coil in inducing a current in another is used in transformers

an alternating current (AC) in the primary coil creates a changing magnetic field in the iron core, which then induces a corresponding AC voltage and current in the secondary coil, with the voltage change determined by the ratio of turns between the coils.

explain how the ratio of the potential differences across the two coils depends on the ratio of the number of turns on each

if you double the turns in the secondary coil compared to the primary, you double the secondary voltage