6.3 Elecromagnetism Flashcards
What is a magnetic field?
The reigion around a permanent magnet or a moving charge in which another body with magnetic properties will feel a force.
Region where a force will act at a distance.
What are magnetic feild lines?
These show the shape of the field and the direction. The direction of the feild line at a point shows the direction in which a compass would point.
What is a solenoid?
A long coil of current carrying wire.
What is a magnetic field caused by?
Either a permanent magnet or by moving charges (an electric current).
What will a magnetic field act on?
A magnetic material or a charged particle.
What are magnetic fields represented by?
Field lines.
What are the shape and direction of magnetic feild lines dependent on?
The nature of the magnetic field .
What is always true about magnetic field lines?
The always point from north to south, and the closer they are drawn, the stronger the field is.
Describe magnetic field lines.
Magnetic field lines have no beginning or end, they always form closed loops. The direction is from a north pole to a south pole, but they do not begin at a orth pole or end at the south pole.
What would the field lines look like on this magnet?
What would the field lines look like on these magnets?
How can you determine the direction of the magnetic field lines when an electric current flows through a straight conductor?
Using the right-hand rule. By aligning the thumb on your right hand parallel to the direction of the conventional current in the wire, the curling of the fingures will indicate the direction of the field lines.
What does the magnetic field lines look like of a straight current- carrying wire shaped into a coil?
Describe what the magnetic field lines would look like after shaping a straight current-carrying wire into a single coil?
On one side of the coil, the magnetic field lines are in a clockwise direction, whereas on the other side they are in an anticlockwise direction. The vector addition of the fiekds around each wire leads to an increase in the strength of the magnetic fields within the coil.
Describe what a solenoid is?
A long wire wrapped around a hollow cylinder and current is passed through.
What does the north-south polarity of a solenoid depend on?
Direction of the current.
How can you work out which side the north pole of a solenoid is?
Using a variation of the right hand rule- if curing your fingers indicates the direction of the current flowaround the solenoid, your thumb will point towards north.
What does the magnetic field associated with a solenoid look like?
Describe the magnetic field inside and outside a solenoid.
Inside the field is strongest and very uniform. Outside, the firls is similar to the magnetic field produced by a bar magnet and that to earths magnetic field.
What happens if a cylinder of iron is placed inside a solenoid?
The magnetic field in increased considerably.
What does the earths magnetic field look like?
The earth’s magnetic field changes direction over a period of many thousands of years, why might this be significant?
The means the earths magnetic North pole will switch to a south pole.
What does Flemming’s left-hand rule show?
Shows the direction of the force on a conductor carrying a current in a magnetic field.
What does each fingre represent in Flemming’s left-hand rule?
Thumb- Force
First fingre- Feild
Second fingre- Current
What will be produced whhen a current flows through a straight wire?
A magnetic field will be present around the wire.
What will occur if a current-carrying wire is then placed perpendicuar in the uniform magnetic field between north and south poles?
The magnetic fields from the wire and the magnet interact which results of either an upwards or downwards force on the wire.
Whatis the motor effect?
When the magnetic field from a mangenet and a current-carrying wire interact and it results in an upards force on the wire.
What s the catapult field?
The resultant field of the magnet and a current carrying wire is called the catapult field because of it’s shape.
What does each finger represent in Flemmings left-hand rule?
- Thumb-Motion of wire/ direction of the wire.
- First finer- Direction of the field between poles of magnet.
- Second finger- Direction of current flowing through the wire.
What quantities is the force on a current-carrying wire directly proportional to?
- Current flowing though the wire
- Length of the wire that is in the region of the magnet’s field.
- Magnetix flux density of the magnetic magnetic field.
- The sine of the angle between the feild lines.
What does equation does the quantities which are dirrectly proportional to the force on the current-carrying wire lead to?
F = BILsin
θ
What does F=BILsin
θ become if the wire and the field lines are at right angles to each other?
F-BIL
Because sin90 = 1
Provided the wire is perpendicular to the field, what is the equation for magnetic flux density?
B= F/IL
What is magnetic flux?
(Φ) Is the product of magnetic flux dentist, B, and the area A at right angles to the flux.
What is the equation for magnetic flux?
Φ=BAcosθ
What is the unit for magnetic flux?
Webers (Wb)
What is the magnetic flux density?
(B) is a measure of the strength of the magnetic field.
What is the equation for magnetic flux density?
F=BILsinθ
θ is the angle between the wire and the field lines.
What are the units for magnetic flux density?
Tesla, T
Define one weber?
The magnetic flux when a magnetic flux density of one tesla passes though an area of one square metre.
What is a velocity selector?
A devise using perpendicular magnetic and electric fields to select charged particles travelling at a specific velocity, which leave the region of crossed field undeflected.
How can you derrive the equation F=BQv?
F=BIL
I=Q/t
L=vt
F=B.Q/t.vt
F=BQv
Using Flemming’s left hand rule, what must you remember finding the direction of a negatively charged particle?
Direction is for conventional current: if the negatively charged, the conventional current direction is in the opposite direction to the particles motion.
What direction is the force always on charged particle? What effect does this cause?
The force is always on a right angle to it’s motion, producing a centripetal force and the charge will move in a circle in the plane perpendicular to the magnetic field.
Where will there be no net force on a charged particcle in a magnetic field? |What does this cause?
There will ne no net force acting in the direction of its motion and it will move at a constant speed.
What equation do you get if you equate centripetal force to F=BQv?
r=mv/BQ
What about a charged partical can be changed that will increase the radius of its movment?
Its velocity or mass.
What affect will increasing the charge on a charged particle have of its radius of it’s movement?
It will decrease it’s radius
What is a mass spectrometer used for?
Measure the mass of ions.
What is an ion?
An atom or molecule with a net electric charge due to the loss or gain of one or more electrons.
What is the maggnetic field used for in mass spectometers?
To delect the ions into circular paths.
Using a mass spectrometer how can the mass of an ion be found?
From r=mv/BQ the radius path can be taken by the ion once it enters the magnetic field will be dirdctly proportionl to the mass.
What equation gives the force on a charged particle moving at velocity, v, in a uniform magnetic feild?
F=BQv
What equation gives the force acting on a charged particle in an electric field is given by F=QE (E is the electric field strength).
How can we arrange an electric force in the opposite direction to the magnetic force?
If we arrange a uniform magnetic field at right angles to the uniform electric field between 2 charged plates.
We acn adjust the strength of the 2 fields until the 2 forces arre equal, leading to BQv=QE.
What can BQv=QE be rearranged to?
v=E/B
Using the equation v=E/B, which prticles will travel at speed v?
Those that don’t experience any defelctions and pass through the velocity selector.
What is a velocity selector used to selct for a mass selector?
Used to select particles of a paricular speed, so that the mass of these ions can be found.
How does a mass spectrometer work?
You can adjust the magnetic field strength and the elecrtic field be be equal (BQv=QE) so the a charged particle moving through it will pass through the to otherside. This leads to the equation v=E/B- meraning that the only particles passing through have a particular spped. Additionally r is directly proportional to m.
What is elecromagnetic induction?
The process of inducing an e.m.f. in a conductor when there is a change in magnetic flux linkage across the conductor.
What is induced emf?
Induced emf is the emf produced by elecromagnetic induction.
What is magnetic flux linkage?
Magnetic flux linkage for a coil equals the product of the magnetic flux through the coil and the number of turns on the coil. =NΦ, weber turns.
What does induced emf cause ina conductor?
Induced current
Who discovered magnetic induction?
Micheal Faraday
How can you demostrate elecromagnetic induction?
With a magnet, a coil of wire and a galvenometre/ microameter- the magnet must be moved towards or away from the coil, when it’s moved, the electrons move, causing a current.
When demonstrating electromagnetic induction, what must be done to produce elecrical output?
Work must be done.
How many watts of power does a simple demonstration of elecromagnetic induction produce?
A few microwatts
When would you use Flemmings left-hand rule?
When dealing with the direction of the motion produced in a current-carrying wire or coil that has been placed in a uniform magnetic field.
When would you use Flemming’s right-hand rule?
For describing what happens when we move a coil or wire through a magnetic field in order to generate elecrical current.
What does each fingure in Flemming’s right hand rule represent?
- Thumb- direction of motion relative to the field.
- First finger- direction of the field
- Second finger-direction of induced current.
What did Faraday conclude in order that an elecric current will be induced in a wire?
There must be a change in the magnetic flux in the reigion around the circuit- this can be done by relative motion between a magnet and a wire by cvhanging a magnetic field.
What is Faraday’s law of electromagnetic induction?
It states that the magnitude of the induced e.m.f. is equal to the rate of change of flux linkage.
What is Lens law?
It states that the direction of nay induced e.m.f. or induced current is in a direction that opposes the flux change that causes it.
What is a search coil?
It’s a small, flat coil used to determine the strength of a magnetic field.
What did Faraday’s investigation show?
Showed that the faster the relative motion between a wire and a magnetic field/ the faster the flux within a loop is changed, due to a changing magnetic field, the greater the induced e.m.f.
Using Faraday’s law, how do you work out the size of induced e.m.f?
Size of induced e.m.f.= Change in flux/ Time taken
What is the difference between Faraday’s law and Len’s law?
**Len’s law states dirrection of the current and the e.m.f. **while Faraday’s law doesn’t.
If a magnet generates an induced current, what pole on a coil of wire will be facing the north pole on the magnet?
The north pole- Len’s law.
If you change the flux, what will be produced in a wire?
A current is induced that will oppose the change.
What would it mean, if a induced current didn’t oppose a change in flux?
If a system did not oppose the change then this would unltimately lead to a violation of the principle of the conservation of energy.
Describe how Len’s law describes how energy is conserved in a system, using an example?
If an approahing north pole of the magnet caused the induced current to generate a magnetic field that atrrected the magnet, the magnet would accelerate and increase it;’s kinetic energy. The extra kinetic energy would have been created out of nowhere, without any work done on the magnet, hence violating the principle of conservation of energy.
What equation is created by combing Faraday’s and Len’s law’s?
ϵ= -Δ(NΦ)/Δt
Why does the equation of Faraday’s law and Len’s law have a minus sign?
It comes from Len’s law, it indicates that the induced emf is in the opposite direction to the change that causes it.
Explain how induction hobs make use of Len’s law?
Induction cooktops have coild beneath the surface of the cookpot, which produce a changing magnetic field when there is an alternating current in the coils. When a metal sauspan is placed on the cooktop the changing magnetic fiels induces a small, localisd emf in the metal, resulting in eddy currents. The resistance of the metal causes the electrial energy generated to be dissipated as heat in the sauspan.
What is a generator?
A devise used to generate electricity, in which the work done is to run the coil within a magnetic field is transferred to electrical energy. The rotation of a coil within a magnetic field produces a constantly changing flux linkage through the coil. This turn produces a constantly changing flux linkage through the coil. This is turn produces a constantly changing emf in the coil.
What is an alternating current?
An electrical current that reverses its direction with a constant frequency.
What is an example of alternating current?
The UK mains electricity supply is a.c. with a frequency of 50Hz.
What occurs if there is relative movement between a conductor and a magnetic field?
The change in magnetic flux causes an induced emf in the conducor.
What does a simple generator consist of?
A multi-turn coil that rotates in a uniform magnetic field.
In a generator, what does the rotating coil produce?
It produced a constantly changing flux linkage through the coil, which produces a constantly changing induced emf.
What is the induced emf proportional to in a generator?
The rate of change in magnetic flux linkage.
How is alternating current produced?
- A multi-turn coil rotates in a magnetic field.
- This produces a constantly changing flux linkage through the coil.
- This produces a constatly changing induced emf.
What does the dirrection of an induced current depend on?
Depends on the direction of movement of the conductor in relation to the direction of the magnetic field.
What happens to the direction of the induced current if the dirrection of the conductor is reversed?
The induced current in the circuit will reverse.
What are the units for drift velocity?
mms^-1
What will the magnetic field be like around a wire when generating a direct current?
The magnetic field will not be constantly changing.
What will the motion of electrons be like in a wire with an alternating current?
The conduction electrons move backwards and forwards about there mean positions within the wire.
Describe the magnetic field produced associated with an alternating current?
Since the current is constantly changing direction, the magnetic field associated with the current will also be changing.
What does the graph of the variatio of current with time in an alternating current look like?
What does a simple ac generator consist of?
- A permanent magnet
- A rectangular coil or armature
- Slip rings
- Brushes.
Explain how the parts of an ac generator work together.
- The rectangular coil is forced to rotate within the uniform magnetic field of the perment magnet.
- The 2 slip rings are connected to the 2 ends of the coil, and rotate with the coil.
- The brushes, which are made from carbon and copper,press against the slip rings and allow electrical contact from the slip rings to an external circuit.
What will be produced when a rectangular coil of the a.c. genrator rotates at constant frequency?
An alternating emf will be produced across it’s ends and an alternting current will flow in the external current.
In an ac generator, what position will the coil be so that magnetic flux linkage is at it’s maximum?
When the rectangular coil is in it’s vertical position.
In an ac generator, where will there be no induced emf?
When tyhe coil is in it’s vertical position- as the coil passes through this position since the rate of change of flux linkage is zero.
Describe what is produced when a coil rotates from being verticle to 90° in an ac generator?
The magnetic flux linking the coil falls from it’s maximum value to zero and emf is induced across the coil.
This happens because the rate of change of flux linkage reached maximum value so the emf generates at this moment is a maximum.
What is a transformer?
A device that can either increase or decrease trhe size of an alternating voltage with little loss of power.
Define efficiency.
The ratio of useful energy to total input energy, usually expressed as percentage, (useful output energy/ total inout energy) x100
What does a simple transformer consist of?
Consists of 2 coils of insulated copper wire on a core of easily insulated iron (soft iron).
One coil is connected to an alternating voltage supply- primary coil, and the secomd is connected to the output- secomndary coil.
Explain what the transformer effect is?
The transformer effect enables a constant output of elternating current from the secondary coil.
An alternating current of 50hz in the primary coil causes the iron core to be magnetised and then remagnitised in the opposite dirrection 50 times per second. This rapidily chaging magnetic flux leads to an equally fast change in magnetic flux through the secondary coil
Why are transformers necessary?
When electrical energy is transferred at a high current, moer elecrical energy is dissapated by heating in the wire’s resistance than would be at a lower current. It follows that the efficiency of the energy transfer decreases significantly as the current increases.
Describe the relationship between power and current in respencts to the equation P= I^2R?
The power dissapated in a resistor is calculated using P=I^2R, is the power lost by heating is proportional to the square of the current.
How are transformers used to transmit elecrical energy over long distances with maximum efficency?
Transformers are used to increase or ‘step up’ the voltage output from power stations to very high voltages. Then transformers are used to decrease, ‘step down’ the voltage which are used in the homes.
What voltage is a step down transformer aproximately to?
230V
What do transformers opperate by?
What do transformers opperate by?
Elecromagnetic induction
Transformers operate by elecromagnetic induction. What do they require to work?
A constantly changing magnetic flux
A transformer requires a constantly changing magnetic flux. How is this produced?
With an ac source.
What sort od current is produced and trasmitted from power stations to our homes?
Alternating current
How can you set up an expaeriment investigatong the action of a transformer?
MAke a transformer using 2 coils of insulated copper wire and an iron rod. Connect the wires from the primary coilto a 1.5V cell connected in series with a switch. Connect the ends of the secondary coil to a 2.5V lamp. Open and close the switch. Explain what you see.
Explain how to carry out an experiment investigating the action of a transformer.
- Make a transformer using 2 1 metre lengths of insulated copper wire, a double iron C core and a clip.
- Wind 20 coils round each C core and clip the 2 C cores together.
- Connect the wires from the primary coil to a low-voltage ac power supple set at 2V.
- Use a miltimeter setg to ac to measure the primary and secondary voltages.
- Keeping the muner of turns on the prmary coil constant, investigate how the readings change as the number of coils on the secondary is reduced. Tabulate you readings.
- For ideal transformers, Ns/Np=Vs=Vp
- Plot a graph to verify if this relationship is true for the transformer you have made.
- Work out uncertainties.
What should you note when changing the number of coils on the secondaerty coil when investigation the action of a transformer?
When you change the number of turns on the wire on the coil, the total length should remain constant to that its resistance does not change.
If there was just one turn in the secondary coil, what will the emf be equal to according to Faraday’s law?
The emf will be equal to the rate of change of flux through the coil.
If there was 100 turns in the secondary coil, what will the emf be equal to?
The emf will be 100 times larger, as each turn has the same flux running through it because all the turns will be in series.
What relationship does the number of coils on the secondary coil and the size of the induced emf have?
The more coils on the secondary coil, the larger the induced emf.
Number of turns on secondary/ number of turns on primary= emf across secondary/ emf across primary
On a step up tansformer,what is the ratio of turns on the primar and secondary coil?
Ns/Np>1
On a step down tansformer,what is the ratio of turns on the primar and secondary coil?
Ns/Np<1
What is the equation for power acrodd a transfomner assuming no energy is lost?
Power input=power output
What equation relates number of turns on a coil, voltage and current of each coil?
Ns/Np=Vs/Vp=Ip/Is
What do we assume if a transformer is 100% efficient?
There is no resistance of the coils.
What ate the currents called induced in the iron core of a transformer?
eddy currents
How is energy dissapated in transformers?
Energy is dissapated by heating in the wire’s resistance and energy is also dissapated by currents induced in the iron core (eddy currents).
How can we minimise eddy currents to maxumise efficiency of a transformer?
Transformer cores are often laminated or made up of layers of iron glued togther, rather than a solid piece of iron.
How do transformers cope with varying outputs?
A transformer can cope with varyiung outputs without any need for manual adjustment. It automatically adjusts to differnt demands for power.
