Topic 7 - Electromagnetism

When a wire moves, the ammeters shows a value. What effect does this demonstrate?

The generator effect

Why was a current detected when the wire was moved upwards?

• the wire cuts through the magnetic field

• a potential difference was induced across the wire

• since it was part of a complete circuit with current

Explain why sound waves have an effect on the electric circuit of a microphone?

• the pressure variations in the sound waves cause the diaphragm to vibrate
• the diaphragm causes the coil/ write to wibrate
• the coil repeatedly changing direction induces an alternating current in the circuit

Why is iron used in an electromagnet?

It is easily magnetised and demagnetised

With a split ring commutator, explain why a direct current is induced in the coil?

• the coil cuts the magnetic fields lines (as it moves through the magnetic field)
• a potetial difference is induced (across the coil)
• there is a complete circuit so a current is induced in the coil
• each half-revolution, the two halves of the commutator swtich brushes/ the two ends of the coil swap form one brush to the other
• so the direction of the induced current does not reverse every half rotation

A lamp is disonnected from a dynamo. Explain why the dynamo becomes much easier to turn?

• after disconnection, there is no (induced) current
• so no magnetic field is produced around/ by the coil
• too oppose the movement of the coil

Parts of a tranformer

primary coil, secondary coil, iron core

Explain why there is an alternating current in the output when the transformer is connected to a circuit

• the alternating current causes a changing magnetic field around the primary coil
• creates a magnetic field that changes direction in the core
• this induces an alternating potential difference across the secondary coil (causing an alternating current)

What is one assumpting made when calculating the length of a cbale with mag.f.density and a force to a wire?

• the wire/ force is at right angles to the magnetic field
• (the current is constant)
• (the field is uniform)

The variations in pressure in sound waves in a microphone between variations in ___?

current/ pd

What is an induced magnet?

a material that becomes a magnet when it is placed in a magnetic field (+ loses most/ all of its magentism quickly when removed)

What is the name of the effect that causes the coil to move in a loudspeaker?

Motor effect

How can Fleming’s left hand rule be used to predict the direction in which the copper rod will move?

• hold thumb, first finger and second finger at right angles to each other
• the 2nd finger shows the current pointing out of the paper
• 1st finger represents the field pointing downwards
• thumb points in direction of the force
• so the rod moves from the left to right

How to change the equipment so that the force would increase?

• decrease the resistance of the variable resistor
• use a stronger magnet

Explain how a moving-coil loudspeaker produces a sound wave

• current creates a magnetic field around the coil
• which interacts with the permanent magnet field (cuts the lines)
• producing a resultant force causing the coil to move
• when the direction of the current reverse, the direction of the force reverses (since magnetic field reverses)

In an experiment which changes the frequency and the loudness, why can the results not be used to make a valid conclusion?

2 variables were changed at the same time; it is not possible to know the effect of each variable

Why does the door unlock when the switch is closed?

• there is a current in the solenoid/ circuit
• creating a magnetic field
• attracting the bolt

How to increase the force at which the bolt is attracted?

• increase the current
• add more turns to the solenoid
• use a spring with a lower spring constant

The transformer in a portable power supply can be adjusted to have different numbers of turns on the secondary coil - suggest why

• to vary the output potential difference
• so that you don’t need a different generator for each type of device

Explain why an alternating current is induced inthe coil with an alternator

• the coil moves through the magnetic field // coil cuts magnetic field lines
• potential difference is induced across the coil
• there is a complete circuit, so a current is induced in the coil
• every half turn the pd reverses direction
• so every half turn, the current changes direction

What is the purpose of slip rings?

provides a continuous connection between the coil and transformer // stops the wires from twisting together

Explain why the handle of the alternator becomes easier to turn when disconnected from the power supply

• there is no induced current
• so no magnetic field produced around the coil
• to oppose the movement of the coil

what is the normal audible soung range?

20Hz - 20kHz

What is the function of a microphone?

to convert the pressure variations in sound waves into variations in current/ pd

Explain how a moving-coil microphone works

• sound waves cause the diaphragm to vibrate
• the diaphragm causes the coil/ wire to vibrate
• the coil/ wire moves through the mangetic field// the coil cutes magnetic field lines
• a potential different is induced across the ends of the coil/ wire

Explain why the iron rods move apart when the switch is closed

• the rods become induced magnets
• with the same polarity at end at (so like poles repel)

Which seismic waves are longitudinal/ transverse?

P-waves are longitudinal; S-waves are transverse

Explain why the study of seismic waves provides evidence for the structure of the Earth’s core

• S-waves can’t travel through a liquid
• S-waves don’t travel through the outer core

Why does the movement of the bar magnet induce a pd across the coil?

magnetic field around the coil changes// the magnetic field lines cut by the coil

Why does the induced pd across the coil alternate?

the magnet changes direction

A seismometer can’t detect small vibrations - suggest 2 changes to the design that would make it more sensitive?

• stronger magnetic field
• more turns on the coil
• turns pushed closer together
• spring with a lower spring constant

Why would there by an anomaly?

• too few turns on the secondary coil
• the pd across the primary was reduced

How can you tell from the graph the transformer goes from stepdown to stepup if the student set the primary coil at 2V?

the pd across the secondary goes across 2V

How does the amount of infrared radiation emitted by the nails change when the power supply is switched on?

it increased until the nails reach a constant temperature

What does the cross in a circle mean?

the current is going into the plane of the paper

When there is a current in the coil with a split-ring commutator, why does the coil rotate continuously?

• the sides of the coil experience a force in the opposite direction
• the forces cause moments that act in the same direction // the moment cause the coil to rotate
• each half-revolution the 2 halves of the commutator swap from one carbon brush to the other
• each half-revolution the commutator reverses the current in the coil/ keeping the forces in the same direction

What is the difference between the motor and generator effect?

• the motor effect converts electrical energy into mechanical energy
• the generator effect (electromagnetic induction) uses motion within a magnetic field to induce a current

What is the difference between a longitudianal and transverse wave?

• L - the oscillations are parallel to the direction of the energy transfer
• T - the oscillations are perpendicular to the direction of energy transfer

Describe one piece of evidence that shows when a sound wave travels through the air it is the wave and not the air that travels

a vibrating drum skin does not move the air away to create a vacuum

Explain how the loudspeaker converts current in an electrical circuit to a sound wave

• the current in the electrical circuit is varying
• the current passes through the bone
• the coil experiences a force
• reversing the current reverses the force
• the size of the current affects the size of the force
• the varying current causes the coil to vibrate
• the vibrating coil causes the cone to vibrate
• the vibrating cone causes the air molecules to move
• the movement of the air molecules produces pressure variations in the air needed for a sound wave
• the air molecules bunch together forming compressions and spread apart forming rarefactions