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PHYSICS > Electric and Magnetic Fields > Flashcards

Electric and Magnetic Fields Flashcards

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1
Q

Explain the role of electric and magnetic fields in a particle detector

A

Electric fields can be used to accelerate particles. F = EQ = ma….. a = EQ/m

Magnetic fields produce circular motion/ centripetal force. Direction of curvature indicates sign of charge. Momentum can be found from the radius. mv^2/r = Bqv…. r = mv/Bq.

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2
Q

Explain how current is induced in a coil.

A

There is a magnetic field in the iron core. Flux linkage is changing due to A.C input. Magnetic field is perpendicular to the current. The change in magnetic flux induces an emf.

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3
Q

Describe how a metal disc can be made to emit electrons

A

A cathode is heated causing thermionic emission.

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4
Q

What is a Magnetic Field

A

A region of space that will cause a magnetic pole to feel a force.

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5
Q

Define magnetic flux density

A

The ratio of magnetic flux to the area it is passing through. It is measured in T, Tesla.
Magnetic Flux = Magnetic flux density (B) x Area x Sinx

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6
Q

What is another word for magnetic flux density

A

Magnetic field strength

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7
Q

Define Flux Linkage

A

The amount of magnetic flux interacting with a coil of wire. Measured in Wb, Webers.
Flux Linkage = BAN = Magnetic Flux x Number of Coils.

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8
Q

What is the Motor Effect?

A

A wire carrying a current within a magnetic field will experience a force.

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9
Q

What is the equation for a magnetic force on a current carrying conductor?

A

F = BILSinx

Can be remembered as Fred = BIL

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10
Q

What effect does a magnetic field have on a charged particle?

A

The magnetic force acts perpendicular to the velocity of the particle, causing circular motion/ it acts as centripetal force.
The magnetic field does no work on the particle so KE and speed of charged particle in magnetic field remain constant.

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11
Q

How is a mass spectrometer used?

A

Separates chemicals depending of their charge/mass ratio, which allows them to be identified.

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12
Q

Explain electromagnetic induction

A

A coil and wire move in relation to each other and otion causes flux linkage to change, inducing an emf. The greater the speed, the greater the emf induced.

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13
Q

State Faradays Law

A

induced emf is proportional to the rate of change of flux linkage.

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14
Q

State Faradays and Lenz Equation and explain why there is a minus.

A

emf = -change in flux linkage/ time taken

The minus comes from Lenzs law which states the direction of an induced emf opposes the change that created it.

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15
Q

State Faradays and Lenz Equation and explain why there is a minus.

A

emf = -change in flux linkage/ time taken
The minus comes from Lenzs law which states the direction of an induced emf opposes the change that created it. Due to conservation of energy.

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16
Q

What is the peak voltage in the UK?

Define peak voltage

A

Peak voltage is the maximum value of alternating current.
Vrms = Vo/√2
230 x √2 = Vo = 325V

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17
Q

Define Electric Field

A

A charged particle in an electric field will cause charged particles to experience a force.
Force = Electric Field Strength x Charge. F = EQ.

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18
Q

Define Electric Potential

A

Energy per unit Charge at that point. V = E/Q

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19
Q

What are Equipotentials?

A
  • Positions in a field with zero potential difference between them.
  • Field is always perpendicular to equipotential lines.
  • Closer the equipotentials, the faster the electric potential is changing, therefore stronger electric field.
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20
Q

Explain Milikan’s Oil Drop experiment.

A
  • Oil is squirted into chamber, friction gives droplets an electrostatic charge.
  • Some fall through the anode under gravity and enter the uniform field created between charged plates.
  • If the field is turned off they continue to fall at terminal velocity.
21
Q

What were the conclusions of Milikan’s Oil Drop experiment?

A

The charge on the droplets were always a multiple of 1.59x10^19 C.

22
Q

Explain mathematics of Milkans experiment

A 
mg = 6πηrv
weight = electric force = EQ = VQ/d
6πηrv = VQ/d
Q  = 6πηrvd/V
23
Q

Explain what a radial field is.

A

Electric Field strength gets weaker the further away you move from the sphere, electric potential is changing less quickly.

24
Q

State the equation for radial electric field strength

A

Coulombs Law: F = k Qq/r^2

k = 8.99 x 10^9 Nm^2C^-2

25
Q

What is meant by a uniform electric field?

A

An area where a force acts on a charged particle. The force is the same at all points/electric field strength is constant/ field lines are equispaced.

26
Q

Describe how a uniform field can be demonstrated in a laboratory.

A

A potential difference is applied to two parallel plates. Seeds in a tray of glycerol/ charged oil drops/

27
Q

Define Capacitance and State the units.

A

The amount of charge stored per unit voltage across the capacitor. Measured in F, Farads.

28
Q

Define Capacitance and State the units.

A

The amount of charge stored per unit voltage across the capacitor. Measured in F, Farads. C = Q/V

29
Q

What is the point of an insulator in a capacitor?

A

The insulator stops electrons form crossing the gap between plates, electrons build up

30
Q

What is the point of an insulator in a capacitor?

A

The insulator stops electrons form crossing the gap between plates, electrons build up on the plate connected to the negative terminal.
Attraction between opposite charges across the gap create an electric field, which increases until pd across gap = pd of power supply.

31
Q

What does capacitance depend on?

A

Size of plates, their separation and type of insulator between them.

32
Q

Define time constant

A

T = RC = Resistance x Capacitance.

How many seconds it takes for current to fall to 37% of starting value.

33
Q

In practice the time for discharge is longer than the calculated time. Why?

A

Time is longer because the rate of discharge decreases/ Current decreases over time

34
Q

Capacitors in parallel have:

A

Same pd. Ctotal = C1 +C2

35
Q

Capacitors in series:

A

1/Ct = 1/C + 1/C

36
Q

What happens when distance between a capacitor doubles?

A

Due to E=V/d, Electric field strength will 1/2. Capacitance decreases.

37
Q

Explain the action of a step-down transformer

A

AC in primary coil. Produces a changing B field which is carried through core. Changing B field over secondary induces emf. Rate of change of flux linkage is less through secondary because less turns.

38
Q

Explain how the electron gun creates a beam of electrons

A

Vacuum in electron gun. Filament is heated causing thermionic emission

39
Q

Explain the role of a magnetic field in a particle detector

A

to produce circular motion, so that momentum/energy/charge/ velocity/mass can be investigated

40
Q

Explain what happens to the capacitor when the switch is closed

A

p.d. across capacitor becomes (equal to) p.d. of cell. Negative charge on one plate and positive charge on the other. movement of electrons from one plate and to the other (around the circuit)

41
Q

Explain why it is difficult to contain anti-hydrogen atoms compared with anti-protons

A

Atoms are not charged

Magnetic / electric fields have no effect

42
Q

Use Faraday’s law to explain why the lamp flashes once when the switch S is opened

A

Current in coil generates a magnetic field. Current drops causing a change in flux in a short time, generating a large emf.

43
Q

Explain why the ions travel in a circular path.

A

The ions experience a force perpendicular to their velocity. The resultant force on the ions causes an acceleration at right angles to their velocity. There is a magnetic force acting towards the centre of
the path.

44
Q

State and explain how the path of the ions in just the magnetic field would be different from the path in just the electric field.

A

Magnetic path is arc of a circle, as magnetic force is perpendicular to the velocity of ion. Electric force constant direction.

45
Q

In the LHC, a magnetic field allows charged particles to move at a constant speed in
a horizontal circular path of the required radius. By reference to the force acting on the charged particles, explain how this is
achieved.

A

Magnetic field is perpendicular to velocity. Magnetic force acts as centripetal force. No work is done, so speed is constant, therefore r is constant. r = p/BQ

46
Q

Explain why an alternating potential difference of constant frequency is applied to the dees.

A

Time spent in each dee in constant. t =πm/BQ. P.d must reverse polarity while proton is in dee. Time for half a.c cycle is equal to time spent in d. Particle is travelling in opposite direction, so direction of field must reverse.

47
Q

Explain why even though the wire is not straight the force F on a single turn of the coil can be calculated using. F=BIL

A

The field is always at 90 degrees to the current/ coil. So sinx is 1 and F=BIL

48
Q

Explain why the electrons follow a circular path.

A

Magnetic force on electrons acts at right angles to B and v. Hence force is always towards the centre of the circle, providing a centripetal force on the electron/ centripetal acceleration that maintains circular motion.

49
Q

Explain how the capacitor discharges.

A

Electrons transfer from negatively charged plate to positively charged plate through the resistor. Hence charge on capacitor decreases exponentially until charge on capacitor equals 0.

PHYSICS (12 decks)

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