Physics 6.3

Electromagnetism

What is a magnetic field?

A region of space in which moving charged particles are subject to a magnetic force

What produces magnetic fields?

Moving charged particles

What creates a magnetic force?

The interaction of 2 magnetic fields

How is a magnetic field generated around a current carrying wire?

Electrons in wire are moving charges which generate a circular magnetic field around the wire

How is a magnetic field demonstrated around a current carrying wire? no

Demonstrated by moving a compass around wire, compass points in direction of field

What is a permenant magnet?

An object made from a magnetised material that creates its own persistent magnetic field

How is the magnetic field of a bar magnet demonstrated?

• Iron filings: a magnetic field is induced in the iron so that the induced north pole points in the direction of the magnetic field at that point

• Magnetic field of bar magnet interacts with induced field in each filing causing a magnetic force

• Causes filings to line up with the field

Describe electromagnets

• use field generated by current carrying wire to create a large magnetic field that can be varied by varying the current

• electromagnets made from coils of wire act as bar magnets when an electric current passes through them

• Wrapping coil around a core or increasing number of turns causes a greater magnetic field

What is a magnetically soft material?

Can be magnetised but don't stay magnetised when removed from external field

What is a magnetically hard material?

Stay magnetised when removed from external field

How is the magnetic field of the earth produced?

Produced from electrical currents moving through the core

Where are the poles of the earth's magnetic field?

Magnetic north pole is in the southern hemisphere and vice versa

Why is the earth's magnetic field important?

Protects earth's organisms from harmful solar radiation output from the sun

What is a magnetic field line?

The path a North Pole would take when placed in a magnetic field: north to south

How is strength of a magnetic field represented on a diagram?

By how close together the field lines are: closer means stronger

Describe the field lines of a current carrying wire

Concentric circles centred on the wire

How does the right hand grip rule apply to a carrying wire?

• Thumb: direction of current

• Fingers: direction of magnetic field

How does the right hand grip rule apply to a solenoid?

• Fingers: direction of current in wires

• Thumb: direction of field inside solenoid - points to north pole

What does a dot inside a circle represent?

Field line coming out of page

What does a cross inside a circle represent?

Field line pointed into the page

What is the motor effect?

When a current carrying conductor is placed within a magnetic field, it experiences a force perpendicular to the flow of current and the field lines

What is Fleming’s left hand rule?

• Used when a current carrying wire is placed into a magnetic field and experiences a force

• Thumb: direction of force (motion)

• 1st finger: direction of magnetic field

• 2nd finger: direction of current

What equation links the force on a current carrying wire to magnetic flux density?

F=BILsinø

What is magnetic flux density?

• magnetic field strength

• Amount of magnetic flux per unit area

What are the units of magnetic flux density?

Tesla (T)

What second equation links force and magnetic flux density?

F= BNQv

What equation gives the force on an individual charged particle?

F=BQv

How do charged particles move in a magnetic field?

In a circular orbit

Why do particles move in a circular orbit in a magnetic field?

• Force is always perpendicular to the particle's velocity so particles are forced in a circular orbit

• Particles undergo centripetal acceleration

What acts as the centripetal force on particles in a magnetic field?

The magnetic force

What equation can be derived for the radius of the orbit of particles in a magnetic field?

r = mv/BQ

What is a mass spectrometer?

Measures relative abundances of atoms and molecules in a substance

What does a velocity selector do?

Isolates particles of a specific velocity

How does a velocity selector work?

• Electric plates above and below: electric force acts up

• Magnetic field passes through sideways: magnetic force acts down

• For particles to pass undeflected, electric and magnetic fields must balance so BQv=EQ therefore v=E/B

• If velocity not correct, particle will be deflected

What is magnetic flux?

Product of magnetic flux density and the area perpendicular to field lines

What is the unit for magnetic flux?

Weber (Wb)

What is the formula for magnetic flux?

Flux =BAcosø

What is magnetic flux linkage?

The magnetic flux of an entire coil of wire

What is the equation for magnetic flux linkage?

Flux x N where N= number of coils of wire

What are the units of magnetic flux linkage?

Weber (Wb)

What is Lenz’s law?

Induced emf is always in a direction so as to oppose the change that caused it

Why is Lenz's law necessary in terms of energy?

If induced emf was in a direction that aided the change that caused it, it would create electrical energy and break the conservation of energy principle

What is faraday's law?

The induced emf in a circuit is directly proportional to the rate of change of magnetic flux linkage

What formula links faraday's law and Lenz's law?

Emf = (-N x change in flux linkage) / change in t

What is the structure of an A.C. generator?

Rectangular coil which spins in a uniform magnetic field

How does an A.C. generator work?

Flux linkage in the coil changes continuously, inducing an alternating current in the coil

How can peak emf of an A.C. generator be increased? (4)

• Increase speed of rotation

• Increase magnetic flux density of the field

• Increase cross-sectional area of the coil

• Increase number of turns on the coil

What is the purpose of a transformer?

Changes the output voltage of a system

Which type of transformer increases output voltage of a system?

Step up transformer

Which type of transformer decreases output voltage of a system?

Step down transformer

What is the structure of a transformer?

• 2 coils wrapped around the sides of a laminated iron ring

• Step up: more turns on secondary coil

• Step down: more turns on primary coil

How do transformers work?

• Alternating current is run through the primary coil

• Induces an alternating magnetic field in the iron core

• This induces an alternating emf in the secondary coil

What formula links number of turns with pd of each coil?

Vs/Vp = Ns/Np

What formula links pd and current of both coils?

IpVp = IsVs

What role do transformers play in the national grid?

Step up transformers increase voltage and decrease current before electricity travels long distances, to reduce energy lost as heat to resistance in the wires