What will two north poles do to each other?
What will two south poles do to each other?
What will a south and north pole do to each other?
Is mangentism an example of a contact or non contact force?
State some differences between permanent and induced magnets
- A permanent magnet always has a magnetic field.
- An induced magnet only becomes magnetic when placed in a magnetic field.
- Induced magnets always causes force of attraction.
- Permanent magnets can attract or repel
State the magnetic materials
What do we call the region around a magnet where a force acts on another magnetic material?
Which direction do magnetic field lines point in?
North to South
What do magnetic fields line show?
- The closer the fields lines the stronger the field.
- The arrows show the direction of force a north pole would feel.
Describe the magnetic field pattern around a bar magnet
Field lines loop from the North pole to the south pole
What is the relationship between the size of attraction/repulsion and distance from a magnet
As distance increase the force or field gets weaker
Where is the strength of a magnetic field the strongest?
The poles of a magnet
Why do magnetic field lines always need arrows on them?
The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point.
Describe how a magnetic compass works
A magnetic compass contains a small bar magnet.
The Earth has a magnetic field. T
he compass needle points in the direction of the Earth’s magnetic field.
Describe how a magnetic compass can be used to find the pattern around a bar magnet.
- Put the magnet in the middle of the piece of paper and draw around it.
- Draw nine dots around the north end of the magnet.
- Put a plotting compass on the paper so that its tail is over one of the dots.
- Draw a dot on the paper near to the point of the compass.
- Join the two dots and mark the line with an arrow.
- Then move the compass so that its tail is over your new dot.
- Make another dot next to the point.
- Repeat until you come to the edge of the paper or loop back round to the south pole.
When a current flows what is always created?
A magnetic field
Describe the magnetic field pattern around a straight wire carrying a current.
In circualr loops around the wire.
Describe the right hand rule for a straight wire carrying a current
Thumb points in the direction of the current, fingers curl around in the direction of the magnetic field
What factors affect the strength of the magnetic field produced by a current?
- Size of the current
- Distance from the wire
What is a solenoid?
When a current carrying wire is turned into a coil. this will increase the strength of the magnetic field.
Describe the magnetic field inside a solenoid
The magnetic field inside a solenoid is strong and uniform. This is shown by parallel staight lines that are close to each other.
Describe the magnetic field around a solenoid
The magnetic field around a solenoid has a similar shape to that of a bar magnet. It loops from North to South
How can a solenoid be turned into an electromagnet?
Adding an iron core, this again increases the strength of the magnetic field.
Describe the magnetic field of an electromagnet
Its the same as a bar magnet
How can the strength of an electromagnet be increased?
- Larger current
- More turns
- Using an iron core
What is the motor effect?
When a conductor carrying a current is placed in a permanet magnetic field. The magnet fields from the magnet and the current exert a force on each other.
Describe Fleming’s left hand rule.
Thumb, first finger and second finger are put at right angles to each other.
Thumb - direction of force/motion
First Finger - direction of magnetic field (always from North to south)
seCond finger - direction of current (always from positive to negative)
In the motor effect how can the size of the force be increased?
- Larger current
- Stronger permanent magnets
- Longer length of wire inside the magnetic field
- Keeping the conductor at a right angle to the magnetic field
In the motor effect how can the direction of the force be reversed?
- Reversing the direction of the current
- Reversing the poles of the permanent magnets
State the units for magnetic flux density.
Describe how a coil of wire inside a magnetic field can cause rotation eg an electric motor
The current goes into the coil of wire, creating an electromagnetic field.
This interacts with a the permanent magnetic field causing a force of attraction or repulsion.
The current then comes out of the coil changing direction so the force changes direction. This causes rotation.
How do loudspeakers/headphones use the motor effect to create sound waves.
A coil of wire is wrapped around a permanent magnet.
An alternating current passes through the wire creating an electromagnetic field.
The magnetic field from the current and permanent magnetic interact causing a force of attraction or repulsion.
As the current is a.c. the force changes between attraction and repulsion.
This causes the loud speaker cone to vibrate which creates sound waves.
What is magnetic induction?
If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor.
State the four conditions needed for magnetic induction
- A magnetic field
- A conductor
- A complete circuit
When will magnetic induction create a current?
If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.
State some factors that can increase the size of the induced potential difference
- Faster movement
- More turns of wire
- Stronger magnet
State some factors that can reverse the direction of the induced potential difference
- Reverse the direction of movement
- Change the pole of the magnet
Describe the role of a microphone
To convert sound waves into variations in current
Describe how microphones use induction/generator effect to turn sound waves into electricity
A microphone has a cone that is attached to a coil of wire wrapped around a permanent magnet.
As a sound wave hits the cone it causes the diaphram, cone and coil and wire to vibrate up and down.
A conductor is now moving through a magnetic field so an induced potential difference is created.
As the coil of wire is part of a complete circuit then an induced current is created.
What is an a.c. generator also known as?
Why does an alternator produce an a.c. output?
the two sides of the coil are always connected to the same side of the lamp. when the LHS of the coil moves past the top it reverses the direction of movement so voltage and therefore current reverses
Sketch the voltage time graph for an a.c. generator
How would a voltage - time graph for a generator change if the coil rotated quicker?
- Higher peaks - more voltage induced
- Higher frequency - peaks closer together due to faster rotation
What is a d.c. generator also known as?
Describe how an ac generator (alternator works)
An alternator is a coil or wire inside a permanent magnetic field.
The coil of wire is made to rotate inside the magnetic field.
As the coil of wire cuts through the magnetic field an induced potential difference is created.
As the coil is part of a complete circuit an induced current flows.
As the coil is constantly changing direction the induced potential difference and induced current is constantly changing direction as well.
Describe the differences between an ac generator (Alternator) and a d.c. generator (Dynamo)
A dynamo works in the same way as an alternator except that a a dynamo has a split ring commutator instead of two separate slip rings. As the coil spins the split ring commutator reconnects the coil the opposite way around every half turn. This means the potential difference doesn’t reverse its direction. The p.d. will vary from zero to a maximum twice each cycle but never changes polarity.
Why does a dynamo produce a d.c. output?
the two sides the coil are connected to split rings so the two sides of the coil are not connected to the same side. during a rotation every half turn the connections are reversed so when induced voltage is reversed, the current flowing is not reversed
Sketch a voltage - time graph for a d.c. generator
Voltage is always postive but increases and decreases.
Describe the structure of a transformer
An insulated primary coil and an insulated secondary coil wrapped around an iron core.
Why is iron used in transformers?
Iron can be easily magnetised and demagnetised.
Describe the role and design of a step up transformer.
A step up transformer increases the output voltage. it does this by having more turns of wire on the secondary side.
Describe the role and design of a step down transformer.
A step down transformer decreases the output voltage. It does this by having less turns of wire on the secondary side.
Explain how a transformer works
- a.c. flows in primary coil.
- This creates an alternating magnetic field in primary coil.
- Magnetic field from primary coil cuts through secondary coil via the iron core.
- This induces a potential difference and current in the secondary coil.
- Current in secondary coil is also a.c.
Do transformers work with a.c. or d.c.?
Only a.c. the magnetic field must be moving through the secondary coil to induce a potential difference
What assumption is made with the transformer equation?
Transformers are 100% efficient so the Power on the primary coil is equal to the power on the secondary coil