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HSC Physics · Module 6 flashcards
Electromagnetism
101 cards. Work out each answer first, then open the card to check it and see the working.
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A bar magnet dropped through a vertical copper tube falls more slowly than one dropped through a plastic tube. This is because …Show answer
Answer: induced currents in the copper produce fields that oppose the magnet's motion
The moving magnet changes the flux through the tube, inducing eddy currents. By Lenz's law these currents oppose the change that produced them, so they oppose the motion of the magnet.
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A charged particle moves parallel to a uniform magnetic field. The magnetic force on it is …Show answer
Answer: zero
F = qvB sinθ. With θ = 0° there is no force; the particle continues in a straight line.
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A proton moves in a circle in a uniform magnetic field. If the field strength is doubled and the speed is unchanged, the radius of the path …Show answer
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The magnetic flux through a flat coil is a maximum when …Show answer
Answer: the plane of the coil is perpendicular to the field
Φ = BA cosθ where θ is between the field and the normal to the coil. Flux is greatest when the field passes straight through the coil face, i.e. the plane is perpendicular to the field.
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A 200-turn coil has the flux through it reduced from 0.040 Wb to zero in 0.10 s. What is the magnitude of the induced emf?Show answer
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How much work is done moving a 2.0 μC charge through a potential difference of 50 V?Show answer
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The force on a current-carrying wire at 90° to a magnetic field is F. If the wire is turned so it makes 30° with the field, the force becomes …Show answer
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Electricity is transmitted at very high voltage mainly to …Show answer
Answer: reduce the current and so reduce I²R heating losses
For a given power P = VI, raising V lowers I. Power lost as heat is I²R, so lowering current cuts losses sharply.
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What is the acceleration of an electron in a uniform electric field of 1000 V m-1?Show answer
Answer: 1.8 × 1014 m s-2
a = qE/m = 1.602 × 10-19 × 1000/9.109 × 10-31 = 1.8 × 1014 m s-2.
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A horizontal wire carries current towards the north in a magnetic field directed vertically upward. The force on the wire is directed …Show answer
Answer: east
Right-hand rule: fingers along current (north), curl toward field (up), thumb gives force. North × up = east.
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The purpose of the split-ring commutator in a DC motor is to …Show answer
Answer: reverse the current in the coil every half turn so the torque keeps the same sense
Without reversal, the torque would flip direction every half turn and the coil would rock back and forth instead of rotating.
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As a DC motor speeds up, the current drawn from the supply …Show answer
Answer: decreases because the back emf increases
The rotating coil cuts flux and induces a back emf that opposes the supply. Net voltage across the coil, and so current, falls as speed rises. Current is greatest at start-up or when stalled.
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The frequency of the emf produced by an AC generator depends on …Show answer
Answer: the rate of rotation of the coil
One rotation gives one full cycle of emf, so frequency equals the rotation frequency. Turns, field and area change the amplitude, not the frequency.
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A positive charge is released from rest just beside the positive plate of a pair of parallel plates. It …Show answer
Answer: accelerates uniformly towards the negative plate, gaining kinetic energy qV
The field is uniform, so the force qE and acceleration are constant. Work done by the field over the full gap is qV.
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The force per unit length between two long parallel current-carrying wires is …Show answer
Answer: inversely proportional to the separation
F/L = (μ0/2π)(I1I2/r). The 1/r dependence comes from the field of a long straight wire.
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The potential difference across two parallel plates is kept constant while the plates are moved further apart. The electric field between them …Show answer
Answer: decreases
E = V/d. Larger d at fixed V gives a smaller field.
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An electron moving in a uniform magnetic field (no electric field) experiences a change in …Show answer
Answer: direction only
The magnetic force is always perpendicular to the velocity, so it does no work. Speed and kinetic energy are constant; only direction changes.
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The period of a charged particle moving in a circle in a uniform magnetic field …Show answer
Answer: is independent of speed
T = 2πr/v and r = mv/qB, so T = 2πm/(qB). Faster particles move in bigger circles and take the same time.
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A 2.0 m wire carries 10 A in a 0.050 T field at 60° to the field. What is the force on it?Show answer
Answer: 0.87 N
F = BIL sinθ = 0.050 × 10 × 2.0 × sin60° = 0.87 N.
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The rotation rate of an AC generator is doubled. The peak emf …Show answer
Answer: doubles
ε = nBAω. The flux changes twice as fast, so the peak emf doubles (and the frequency doubles too).
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An ideal transformer takes 120 W from the mains and delivers 12 V. What current flows in the secondary?Show answer
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A 3.0 μC charge is moved through a potential difference of 200 V. The work done is …Show answer
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An electron moving at 1.0 × 107 m s-1 passes between plates 5.0 cm long with a field of 2.0 × 103 V m-1. How far is it deflected while between the plates?Show answer
Answer: 4.4 mm
t = 0.050/1.0 × 107 = 5.0 × 10-9 s. a = qE/m = 3.5 × 1014 m s-2. y = ½at² = 0.5 × 3.5 × 1014 × 2.5 × 10-17 = 4.4 × 10-3 m.
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A motor coil has 100 turns, each side 0.10 m long, carrying 1.5 A in a 0.20 T field. The force on one side of the coil is …Show answer
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Two parallel wires carrying current in the same direction …Show answer
Answer: attract each other
Each wire lies in the other's field and the F = IL × B direction points towards the other wire for parallel currents.
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A 0.50 m conductor moves at 4.0 m s-1 at right angles to a 0.30 T field. The emf induced across it is …Show answer
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Electric field lines between two oppositely charged parallel plates (away from the edges) are …Show answer
Answer: straight, parallel and evenly spaced, from positive to negative
The field is uniform, so the lines are straight, equally spaced, and run from + to −.
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A proton and an alpha particle enter the same magnetic field with the same velocity. The ratio of the alpha particle's path radius to the proton's is …Show answer
Answer: 2 : 1
r = mv/(qB). The alpha has 4× the mass and 2× the charge, so r × 4/2 = 2.
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The magnetic force on a current-carrying wire is zero when …Show answer
Answer: the current is parallel to the field
F = BIL sinθ. sin0° = 0.
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Two long parallel wires each carry 1.0 A. The force per unit length between them is 2.0 × 10-5 N m-1. How far apart are they?Show answer
Answer: 1.0 cm
F/L = 2 × 10-7 I²/r, so r = 2 × 10-7 × 1/2.0 × 10-5 = 0.010 m.
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A transformer is connected to a steady DC supply. The secondary output is …Show answer
Answer: zero (apart from a brief pulse when switched on or off)
A transformer needs a changing flux to induce an emf. Steady DC gives constant flux, so no continuous output.
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The coil of an AC generator is rotated at a higher speed. The output has …Show answer
Answer: a higher peak emf and a higher frequency
Faster rotation means the flux changes faster (bigger emf) and more cycles per second (higher frequency).
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An electron experiences a force of 4.0 × 10-15 N in an electric field. The field strength is …Show answer
Answer: 2.5 × 104 N C-1
E = F/q = 4.0 × 10-15/1.602 × 10-19 = 2.5 × 104 N C-1.
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An electron is accelerated through 2.0 kV. The kinetic energy it gains is …Show answer
Answer: 3.2 × 10-16 J
KE = qV = 1.602 × 10-19 × 2000 = 3.2 × 10-16 J.
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In a velocity selector, E = 5.0 × 104 V m-1 and B = 0.10 T. Particles that pass undeflected have speed …Show answer
Answer: 5.0 × 105 m s-1
qE = qvB gives v = E/B = 5.0 × 104/0.10 = 5.0 × 105 m s-1.
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The torque on a current loop in a uniform magnetic field is zero when …Show answer
Answer: the plane of the loop is perpendicular to the field
When the plane is perpendicular to the field, the forces on opposite sides are in line with each other and produce no turning effect.
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Pulling a coil out of a magnetic field requires a force. This is a consequence of …Show answer
Answer: Lenz's law and the conservation of energy
The induced current opposes the motion. The work done against this opposing force is what provides the electrical energy — energy is conserved.
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A transmission line of resistance 5.0 Ω carries 10 A. The power lost as heat is …Show answer
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An electron starts from rest in a field of 2.0 × 104 V m-1 and travels 2.0 cm along the field. Its kinetic energy gain is …Show answer
Answer: 6.4 × 10-17 J
W = qEd = 1.602 × 10-19 × 2.0 × 104 × 0.020 = 6.4 × 10-17 J.
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A charged particle enters a magnetic field with a velocity component parallel to the field and a component perpendicular to it. Its path is …Show answer
Answer: a helix
The perpendicular component gives circular motion; the parallel component is unaffected. Combining them gives a helix along the field direction.
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Two long parallel wires 5.0 cm apart each carry 10 A. The force per metre between them is …Show answer
Answer: 4.0 × 10-4 N m-1
F/L = 2 × 10-7 × 100/0.050 = 4.0 × 10-4 N m-1.
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A 100-turn coil of area 0.10 m² is in a field that increases from 0.20 T to 0.80 T in 0.50 s. The induced emf is …Show answer
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In an ideal step-up transformer, compared with the primary, the secondary has …Show answer
Answer: higher voltage and lower current
Power is conserved: VpIp = VsIs. Stepping up V steps down I.
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An electron moves to the right across a page in a magnetic field directed into the page. The magnetic force on it is directed …Show answer
Answer: towards the bottom of the page
For a positive charge, v (right) × B (into page) gives a force towards the top. The electron is negative, so the force is towards the bottom of the page.
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A 5.0 μC charge sits in an electric field of 2000 N C-1. The force on it is …Show answer
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As a motor coil rotates in a uniform field with constant current, the torque …Show answer
Answer: is greatest when the plane of the coil is parallel to the field and zero when perpendicular
τ = nIAB cosθ, θ being the angle between the coil plane and the field. The lever arm of the forces on the sides shrinks to zero when the coil is perpendicular to the field.
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The number of turns in a generator coil is doubled, with everything else unchanged. The peak emf …Show answer
Answer: doubles
ε = nBAω. Emf is proportional to the number of turns.
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An ideal transformer draws 0.50 A at 240 V. The secondary delivers 12 V. The secondary current is …Show answer
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An oil drop of mass 1.0 × 10-15 kg is held stationary between plates by a field of 6.1 × 104 V m-1. The charge on the drop is about …Show answer
Answer: 1.6 × 10-19 C (one electron)
qE = mg, so q = mg/E = 1.0 × 10-15 × 9.8/6.1 × 104 = 1.6 × 10-19 C.
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A magnetic field does no work on a moving charged particle because …Show answer
Answer: the force is always perpendicular to the velocity
Work = F·s. A force perpendicular to the displacement transfers no energy, so the particle's speed is unchanged.
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A 0.40 m wire carries 2.0 A at 90° to a 0.50 T field. The force is …Show answer
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When a DC motor is running, the current in its coil is less than when it is stationary because …Show answer
Answer: the back emf induced in the rotating coil opposes the supply voltage
The rotating coil is also a generator. Its induced emf opposes the supply, reducing the net voltage driving current.
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One tesla is equivalent to …Show answer
Answer: one weber per square metre
Φ = BA, so B = Φ/A has units Wb m-2.
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A copper disc spins between the poles of a magnet and slows quickly when the magnet is switched on. This braking is due to …Show answer
Answer: eddy currents induced in the disc that oppose its motion
Flux through each part of the disc changes as it spins, inducing eddy currents. By Lenz's law their effect opposes the rotation.
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An electron is accelerated from rest through 100 V. Its final speed is about …Show answer
Answer: 5.9 × 106 m s-1
½mv² = qV, v = √(2qV/m) = √(2 × 1.602 × 10-19 × 100/9.109 × 10-31) = 5.9 × 106 m s-1.
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The direction of the magnetic field around a straight current-carrying wire is found using …Show answer
Answer: the right-hand grip rule (thumb along current, fingers curl with field)
Point the right thumb along the conventional current; the curled fingers show the direction of the circular field lines.
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An electron moving at 3.0 × 106 m s-1 enters a 0.010 T field at right angles. The radius of its path is …Show answer
Answer: 1.7 × 10-3 m
r = mv/(qB) = 9.109 × 10-31 × 3.0 × 106/(1.602 × 10-19 × 0.010) = 1.7 × 10-3 m.
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Two wires 2.0 cm apart carry 4.0 A and 6.0 A in opposite directions. The force per metre is …Show answer
Answer: 2.4 × 10-4 N m-1, repulsive
F/L = 2 × 10-7 × 24/0.020 = 2.4 × 10-4 N m-1. Opposite currents repel.
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Laminating a transformer core reduces …Show answer
Answer: eddy current losses
Insulated thin sheets interrupt the paths of the circulating currents the changing flux would otherwise drive through a solid core.
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The main structural difference between an AC generator and a DC generator is that the DC generator uses …Show answer
Answer: a split-ring commutator instead of slip rings
Slip rings pass the alternating emf out unchanged. A split ring reverses the connection each half turn, so the output stays one polarity.
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A charged particle enters a uniform electric field at right angles to the field lines. While in the field, its …Show answer
Answer: velocity parallel to the plates is constant and it accelerates towards one plate
The force is along the field only, so the perpendicular velocity component is unchanged while the parallel component grows uniformly — projectile-like motion.
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An alpha particle (q = 3.2 × 10-19 C, m = 6.64 × 10-27 kg) moving at 1.0 × 106 m s-1 enters a 0.20 T field at right angles. Its path radius is …Show answer
Answer: 0.10 m
r = mv/(qB) = 6.64 × 10-27 × 1.0 × 106/(3.2 × 10-19 × 0.20) = 0.10 m.
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A wire carries current into the page in a magnetic field directed towards the top of the page. The force on the wire is …Show answer
Answer: to the right
F = IL × B. (into page) × (up) = to the right by the right-hand rule.
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In a simple DC motor the coil keeps rotating in one direction because …Show answer
Answer: the commutator reverses the coil current every half turn
Reversing the current as the coil passes the vertical keeps the torque in the same rotational sense.
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The flux into the face of a coil is increasing. By Lenz's law the induced current produces a magnetic field that …Show answer
Answer: points out of the coil, opposing the increase
The induced effect always opposes the change in flux that caused it.
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A transformer with turns ratio Np:Ns = 20:1 is supplied with 240 V. Its output is …Show answer
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A 2.0 μC charge experiences a force of 0.040 N in an electric field. The field strength is …Show answer
Answer: 2.0 × 104 N C-1
E = F/q = 0.040/2.0 × 10-6 = 2.0 × 104 N C-1.
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An electron with kinetic energy 3.2 × 10-18 J is brought to rest by a retarding potential difference of …Show answer
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A charge of 1.6 × 10-19 C moving at 5.0 × 105 m s-1 at right angles to a magnetic field experiences a force of 2.0 × 10-14 N. The field strength is …Show answer
Answer: 0.25 T
B = F/(qv) = 2.0 × 10-14/(1.6 × 10-19 × 5.0 × 105) = 0.25 T.
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Two parallel wires 1.0 cm apart each carry 20 A. The force per metre between them is …Show answer
Answer: 8.0 × 10-3 N m-1
F/L = 2 × 10-7 × 400/0.010 = 8.0 × 10-3 N m-1.
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A generator coil rotates at constant speed in a uniform field. The induced emf is zero at the instant when …Show answer
Answer: the flux through the coil is maximum
Emf depends on the rate of change of flux. When flux is at its maximum (coil perpendicular to field) it is momentarily not changing.
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A transformer has 1000 primary turns at 240 V and 6000 secondary turns. The output voltage is …Show answer
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A charge is moved perpendicular to the field lines in a uniform electric field. The work done by the field is …Show answer
Answer: zero
Moving along an equipotential (perpendicular to the field) involves no component of force along the displacement.
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An electron and a proton are placed in the same uniform electric field. Compared with the proton, the electron experiences …Show answer
Answer: the same force and a much larger acceleration
F = qE is equal in magnitude (same |q|). a = F/m, and the electron's mass is about 1/1836 of the proton's, so it accelerates far more.
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A 0.20 m wire carrying 5.0 A lies perpendicular to a 0.10 T field. The force is …Show answer
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The separation between two parallel current-carrying wires is doubled. The force per unit length …Show answer
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A magnet is moved towards a coil faster than before. The induced emf …Show answer
Answer: is larger
Faraday's law: emf is proportional to the rate of change of flux. Faster motion means faster flux change.
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In an electricity distribution network, transformers are used to …Show answer
Answer: step up voltage at the power station and step down near consumers
High voltage for transmission minimises I²R losses; it is stepped down for safe domestic use.
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Two plates 0.50 cm apart have 100 V across them. The field strength is …Show answer
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A charged particle moves in a circle in a magnetic field. If its speed is doubled, the radius …Show answer
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A positively charged particle enters the region between two horizontal plates, the upper plate negative. The particle is deflected …Show answer
Answer: upward towards the negative plate
A positive charge is attracted to the negative plate.
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A coil of 200 turns and area 0.020 m² carries 0.50 A in a 0.30 T field. Its maximum torque is …Show answer
Answer: 0.60 N m
τ = nIAB = 200 × 0.50 × 0.020 × 0.30 = 0.60 N m.
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A wire loop is pulled out of a region of magnetic field directed into the page. The induced current in the loop flows …Show answer
Answer: clockwise, producing a field into the page inside the loop
Flux into the page is decreasing. Lenz's law: the induced current replaces it with a field into the page, which requires a clockwise current.
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A transformer is supplied with 50 Hz AC. Its output frequency is …Show answer
Answer: 50 Hz
The secondary emf follows the rate of change of the primary flux, so the frequency is unchanged. Only voltage and current change.
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A 2.0 C charge is moved through a potential difference of 12 V. The energy transferred is …Show answer
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An electron accelerated from rest through 1.0 kV reaches a speed of about …Show answer
Answer: 1.9 × 107 m s-1
v = √(2qV/m) = √(2 × 1.602 × 10-16/9.109 × 10-31) = 1.9 × 107 m s-1.
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The magnetic force on a straight wire carrying current I at angle θ to field B is zero when θ is …Show answer
Answer: 0°
F = BIL sinθ; sin0° = sin180° = 0. Current parallel or antiparallel to the field gives no force.
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A generator coil has 100 turns of area 0.010 m², rotating at 50 rad s-1 in a 0.20 T field. Its peak emf is …Show answer
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Lenz's law is a consequence of …Show answer
Answer: the conservation of energy
If induced currents aided the change instead of opposing it, a small push would create ever-growing motion and current — energy from nothing.
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An electron is released at the negative plate of a pair of plates 1.0 cm apart with a field of 1.0 × 105 V m-1. Its kinetic energy on reaching the positive plate is …Show answer
Answer: 1.6 × 10-16 J
V = Ed = 1.0 × 105 × 0.010 = 1000 V. KE = qV = 1.6 × 10-16 J.
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The force on a current-carrying wire in a magnetic field is directed …Show answer
Answer: perpendicular to both the current and the field
F = IL × B is perpendicular to the plane containing L and B.
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A proton (m = 1.67 × 10-27 kg) moving at 4.0 × 106 m s-1 enters a 0.50 T field at right angles. The radius of its path is …Show answer
Answer: 8.4 cm
r = mv/(qB) = 1.67 × 10-27 × 4.0 × 106/(1.6 × 10-19 × 0.50) = 0.084 m.
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Two parallel wires 2.0 m long and 10 cm apart carry 3.0 A and 4.0 A. The total force between them is …Show answer
Answer: 4.8 × 10-5 N
F/L = 2 × 10-7 × 12/0.10 = 2.4 × 10-5 N m-1. F = 2.4 × 10-5 × 2.0 = 4.8 × 10-5 N.
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A 500-turn coil has its flux reduced from 2.0 × 10-3 Wb to zero in 0.010 s. The induced emf is …Show answer
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A transformer is 95% efficient. With 1000 W input, the output power is …Show answer
Answer: 950 W
0.95 × 1000 = 950 W; the other 50 W becomes heat.
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An electron is placed in an electric field that points east. The force on the electron is directed …Show answer
Answer: west
Field direction is defined by the force on a positive charge. A negative charge feels the opposite direction.
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An electron moves from the negative plate to the positive plate of a pair of charged plates. Its electrical potential energy …Show answer
Answer: decreases
The field does positive work on it; potential energy converts to kinetic energy.
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A charged particle moves in a circle of radius r in a magnetic field. If its speed is doubled and the field is also doubled, the radius …Show answer
Answer: is unchanged
r = mv/(qB). Doubling both v and B leaves r the same.
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A 0.50 m wire carrying 4.0 A is perpendicular to a 0.25 T field. The force is …Show answer
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The torque on a motor coil is maximum when the plane of the coil is …Show answer
Answer: parallel to the field
When parallel, the forces on the two sides act at the full width of the coil, giving the largest lever arm.
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A magnet is dropped through a coil whose ends are connected to each other, and then through an identical coil whose ends are open. The magnet …Show answer
Answer: falls more slowly through the connected coil
An emf is induced in both, but only the closed coil carries a current and so only it exerts an opposing force (Lenz's law).
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