Magnetic Fields Flashcards
What is a magnetic field?
A region in which a force acts on a magnetic material.
What are the key features of a magnetic field around a bar magnet?
Field lines go from north to south pole.
Field strongest at poles, gets weaker with distance.
Denser field lines -> stronger field.
Describe the key features of the magnetic field around a current carrying straight piece of wire
Concentric circles around the wire that get further apart with distance from wire as field gets weaker.
Explain how to predict the direction of the magnetic field around a current carrying straight piece of wire
Use right hand thumb rule:
Thumb = conventional current (+ to -).
Curled fingers = direction of circular field lines.
Describe the key features of the magnetic field around a current carrying solenoid
Outside solenoid - same as a bar magnet.
Inside solenoid – straight parallel lines.
Explain how to predict the direction of the magnetic field around a current carrying solenoid.
Use right hand grip rule:
Curled fingers = conventional current (+ to -).
Thumb = points towards north pole.
Why does a current carrying wire experience a force when placed in an external magnetic field?
The field around the wire and the external magnetic field are added together.
The resultant field produces a force on the wire.
What does each finger represent in Fleming’s left-hand rule?
Thumb = F - force on wire
First finger = B - field (N to S)
Second finger = I – conventional current (+ to -)
In what position will the force on a current carrying wire be: a. a maximum? b. a minimum (zero)?
a. When the current is perpendicular to the field.
b. When the current is parallel to the field.
What happens if an AC is passed through the wire in a magnetic field?
Current constantly changes direction.
So, force constantly changes direction according to Flemings’ left-hand rule.
Causes wire to vibrate.
Define magnetic flux density (B)
The force on 1 m of wire carrying a current of 1 A perpendicular to the magnetic field.
Give the units for magnetic flux density (B) and define this unit. State whether it is scalar or vector.
Units = tesla
The strength of a magnetic field that produces a force of 1 N on a 1 m wire carrying a current of 1 A flowing perpendicular to the magnetic field. (1 T = 1 N A-1 m-1)
Vector quantity.
Give the equation for the force on a current carrying wire in a magnetic field (when I and B are perpendicular).
F = BIL
Give the equation for calculating the force on a charged particle moving perpendicular to a magnetic field.
F = BQv
Give two conditions that would not result in a force on a charged particle in a magnetic field.
It is not moving (i.e. stationary).
It is moving parallel to the magnetic field.
How can Fleming’s left-hand rule be applied to charged particles moving magnetic fields?
Thumb = F - force on charged particle
First finger = B - field (N to S)
Second finger = I – velocity direction of positive charges (swap the direction for negative charges)
Why do charged particles move in circular motion in a magnetic field?
The force is always perpendicular to the velocity.
This provides a centripetal force towards the centre of the circle.
Derive the equation for the radius of the circle for a charged particle in circular motion in a magnetic field
Centripetal force = force on charged particle in B field.
Derive the equation for the time period for a charged particle in circular motion in a magnetic field.
Time period = circumference / velocity
What does a cyclotron do?
It accelerates charged particles (e.g. for use in radiotherapy).
Describe the structure of a cyclotron.
Two hollow semi-circular electrodes (‘dees’).
Magnetic field applied perpendicular to plane of electrodes.
Alternating electric field (p.d.) applied between electrodes.
Explain the motion of the particles when they are inside the semi-circular electrodes.
- Move in circular motion.
Force from magnetic field is always perpendicular to velocity -> provides a centripetal force.
Radius of the circle increases each time due to increased velocity as 𝑟 = 𝑚𝑣 / 𝐵𝑄.
Explain the motion of the particles when they are moving across the gap between the semi-circular electrodes.
Accelerated across the gap by the applied p.d..
Applied p.d. is alternating as it has to repeatedly change direction.
Explain why the alternating p.d. has a fixed frequency.
The time period of the circular motion is independent of velocity as 𝑇 = 2𝜋𝑚 / 𝐵𝑄.
Particle spends the same amount of time in each electrode.
Define magnetic flux φ in words and with an equation. Give the units and state whether it is scalar or vector.
The product of the magnetic flux density (B) and the area (A) (when B is normal to A).
φ = BA - Units = webers (Wb)
Scalar quantity.
Define magnetic flux linkage Nφ in words and with an equation. Give the units and state whether it is scalar or vector.
The product of the magnetic flux (BA) and the number of turns (N) (when B is normal to A).
Nφ = BAN
Units = weber turns (Wb turns)
Scalar quantity.
Give the equation for magnetic flux linkage when magnetic flux is not perpendicular to area. Define the angle θ.
Nφ = BAN cosθ
Where θ is the angle between the magnetic field and the normal to the plane of the coil.
When is magnetic flux linkage a maximum and a minimum?
Maximum when θ = 0, cos θ = 1.
Minimum when θ = 90, cos θ = 0
What is electromagnetic induction?
Relative motion between a conducting rod and a magnetic field.
Conducting rod cuts the magnetic field lines.
Causes a change in magnetic flux.
Induces an emf in the conducting rod.
Which induced a current if there is a complete circuit.
State Faraday’s law of electromagnetic induction and give the equation.
The induced emf is directly proportional to the rate of change of flux linkage.
Magnitude of induced emf: ε = NΔφ / Δt
What can you calculate from a graph of magnetic flux linkage Nφ against time t?
Gradient = magnitude of induced emf.
What can you calculate from a graph of emf ε against time t?
Area under graph = change in flux linkage.
State Lenz’s law of electromagnetic induction
The induced emf is always in such a direction as to oppose the change of flux that caused it.
(This agrees with the principle of conservation of energy.)
Use Lenz’s law to explain the acceleration of a magnet entering a vertical coil (North pole first) as: a. enters the coil. b. leaves the coil.
a. Induced emf gives north pole at top of coil -> repels magnet, opposing its motion, accelerates less than g.
b. Induced emf gives north pole at bottom of coil -> attracts magnet, opposing its motion, accelerates less than g.
Sketch a graph to show the emf induced in the vertical coil (when a magnet enters it North pole first) and explain its shape.
Induced emf in opposite direction as it enters compared to as it leaves (Lenz’s law).
Greater emf when leaving due to falling faster and greater rate of change of flux linkage (Faraday’s law).
Positive and negative areas must be equal -> represents change in flux linkage.
Describe the phase difference between magnetic flux linkage and emf induced for a coil rotating in a magnetic field.
Out of phase by 90 degrees.
Sketch the oscilloscope trace for D.C. and A.C. when the time base is switched on.
DC - horizontal line
AC - sinosoidal curve
Sketch the oscilloscope trace for D.C. and A.C. when the time base is switched off.
DC - single point
AC - vertical line
Sketch an A.C. trace with the time base switched on and label:
a. time period T
b. peak voltage V0
c. peak-to-peak voltage (2V0)
a. Distance between two peaks/ troughs
b. Amplitude of the wave
c. 2 x amplitude of the wave
What does the y-gain setting of an oscilloscope control?
Volts per (vertical) division or volts per cm.
What does the time base setting of an oscilloscope control?
Time per (horizontal) division or time per cm.
Time is usually in milliseconds (ms).
How is an oscilloscope set up?
Connect the source to the y-input.
The trace can be vertically centred with the y-shift and horizontally centred with the x-shift.
What are root mean square (rms) current and voltage?
The rms voltage/current for A.C. give the equivalent D.C. voltage/current that produce the same power (and same heating effect).
(It is the square root of the mean of the squares of all the values of the voltage/current in one cycle.)
How can the power of A.C. be calculated?
Power = IrmsVrms = (Irms)2R = (Vrms)2 / R
What does the 230 V for UK mains electricity represent?
The rms voltage.
What is the frequency of UK mains electricity?
50 Hz
Describe the structure of a transformer.
Made up of a (electrically insulated) primary and secondary coil.
Wrapped around a laminated iron core.
Describe the function of a transformer.
Make use of electromagnetic induction to change the p.d. of an alternating current.
For a step-up transformer, give the key facts about number of turns on the coils, p.d. and current.
More turns on secondary coil than primary coil.
Increases p.d..
Decreases current.
For a step-down transformer, give the key facts about number of turns on the coils, p.d. and current.
More turns on primary coil than secondary coil.
Decreases p.d..
Increases current.
Why are step-up transformers used in the national grid?
Used between power stations and transmission cables.
For a given power, as P=IV, increasing the p.d. decreases the current.
Less heating effect in cables
Less energy wastefully dissipated to surroundings.
Increases efficiency.
Why are step-down transformers used in the national grid?
Used between transmission cables and domestic use.
Lowers p.d. so safer for domestic use.
Describe the function of the primary coil.
Alternating p.d. applied across primary coil -> causes alternating current to flow.
This produces an alternating magnetic field in the iron core causing it to magnetise and demagnetise quickly.
Describe the function of the iron core.
Increases the magnetic flux linkage from the primary to the secondary coil by focusing the direction of the magnetic flux.
Describe the function of the secondary coil.
Has an alternating magnetic field surrounding it.
Secondary coil therefore cuts the field lines.
This induces an emf proportional to the rate of change of flux linkage (Faraday’s law) – will depend on the p.d. in the primary coil and the turns ratio between primary and secondary coils.
This induces an alternating current in the coil.
Why do transformers only work with A.C.?
Only A.C. will produce an alternating magentic field.
How are the coils of a transfomer designed to increase efficiency?
Made from thick copper wire.
Copper has a lower resistivity.
Larger diameter reduces resistance.
Reduces power lost due to heating in the coils.
How does the core and positioning of coils increase efficiency?
Iron core used with coils as close as possible.
Increases magnetic flux linkage from primary to secondary coil as direction of magnetic flux is focused.
Why does a core made from iron increase efficiency?
Iron is magnetically soft.
Reduces energy required to magnetise and demagnetise the core.
Reduces power lost due to heating in the core.
Why do eddy currents occur?
The iron core is being cut by the changing magnetic flux.
This induces an emf in the core causing looping currents to flow as the iron conducts.
Why do eddy currents decrease efficiency?
Eddy currents create a magnetic field that acts against the field that induced them (Lenz’s law).
This reduces the overall field strength.
Eddy currents also cause power loss due to heating in the core.
Why does laminating the iron core increase efficiency?
Laminating the core involves having laminations of iron separated by laminations of insulator (high resistivity material).
Reduces the effect of effect current.
