Year 13 mock mark scheme Flashcards
(13 cards)
A cell is connected across a resistor. After a while the internal resistance of the cell increases. What is the change in current in the circuit and the change in the terminal potential difference across the cell?
Current decreases
Potential difference decreases
An object is falling at terminal velocity. Which of the following is NOT a valid conclusion from this statement?
A: the acceleration of the object is zero.
B: there is a resistive force acting on the object
C: there is a resultant force acting on the object
D: the object has weight
C: there is a resultant force acting on the object
A light dependent resistor is connected across a cell of negligable internal resistance. The light intensity in increased. Which of the following statements about the current is correct?
A: it decreases because there is an increase in the number of conduction electrons
B: it increases because there is an increase in the number of conduction electrons
C: it decreases because the amplitude of lattice vibrations decreases
D: it increases because the amplitude of lattice vibrations increases
B: it increases because there is an increase in the number of conduction e-s
Which of the following are the base units for impulse? A: kgm/s B: kgms-2 C: N m D: N s
A: kgms-1
A wire carries an alternating current of peak value 3A. Which of the following is the root-mean- square value of this current? A: 1.5A B: 2.1A C: 4.2A D: 9.0A
B: 2.1A
V rms = V0 / root 2
3 / root 2 = 2.1A
Electric and magnetic fields can be used in particle accelerators. Which row in the table correctly describes the use of electric and magnetic fields in the particle accelerator indicated?
A: cyclatron, B field not used, E field used to accelerate particles
B: cyclotron, B field used to accelerate particles, E field used to accelerate particles
C: linac, B field used to accelerate particles, E field not used
D: linac, B field used to accelerate particles, E field used to accelerate particles
B: cyclotron, B field used to accelerate particles, E field used to accelerate particles
11 A “metre bridge” is a circuit which can be used to measure an unknown resistance accurately. The metre bridge includes a metre length of nichrome wire.
a) calculate the resistance of a 1m length of the nichrome wire.
Resistivity of nichrome = 1.12 x 10 ^-6 ohm metres
Diameter of wire = 4.00 x 10-4m
R = roe x L/A
A = pi r2
(1.12 x 10 ^-6 x 1) ÷ pi (0.2 x 10 -3)2 = 8.91 ohms
11 b) This metre length of wire is connected to a 1.5 V cell of negligible internal resistance and a switch.
i) explain how the potential along this wire varies with distance from A when the switch is closed.
Resistance increases with length of wire
Potential difference proportional to length of wire
11 b) ii) Show that the potential difference between A and a point 75cm along the wire from A is about 1.1V
Ratio of lengths = ratio of resistances (because proportional)
75/100 x 1.5 = 1.125
(100 = whole length of wire)
11 c) A metal slider S can be moved along the nichrome and pressed firmly against it to make an electrical connection. The circuit includes a resistor of resistance 3.3ohms and a very sensitive ammeter and a resistor of unknown resistance R.
When the switch is closed and S is 75cm along the nichrome wire, the ammeter reads 0A because the p.d. across the ammeter is 0.
Calculate R.
I = V / R I = 1.125 / 3.3 = 0.34 A R = 0.375 / 0.34 = 1.1 ohms
- A simple electric motor consists of a coil that is free to rotate in a magnetic field. A studenr connects the motor to an ammeter and a battery. The graph shows how the current I in the coil varies with time t. The switch is closed at time T.
Explain why the current rises to a maximum then decreases. Your answer should include a reference to Faraday and Lenz’s laws.
Current as switch closed
Coil rotates
Movement of coil cuts (magnetic) flux (linkage) / field
Which induces an emf (according to Faraday’s Law)
Opposes original emf / current according to Lenz’s Law
Current reduces a current opposes change
The faster the coil rotates, the larger this (back) emf / effect the smaller the current
Beam engines contributed to powering the Industrial Revolution in Britain in the 18th century. A beam engine consisted of a beam which could rock to and fro around a well-oiled pivot. Attached to the beam there are two rods, one connected to a piston in a steam cylinder and the other connected to a pump.
a) The beam has a constant thickness and a mass of 3.05 x 10 ^4kg. The length of the beam is 11m. The pivot P is positions 6m from the steam cylinder end of the besam. In its resting position the steam cylinder rod is supported by the base of the steam cylinder with the beam at an angle of 20 degrees to the horizontal
The steam cylinder rod exerts a force T on the beam. The force exerted on the beam by the pump rod can be neglected.
Calcualte the force T.
anticlockwise moments = clockwise moments
moment = F x cos θ
T x 6 x cos 20
W = mg
3.05 x 10 ^4 x 9.81 = 2.99 x 10 ^5 N
T x 6 x cos 20 = (3.05 x 10 ^4 x 9.81) x 0.5 x cos 20
T = 24.9kN
13 b) The engine, which ran continuously, could lift a mass of 2500kg of water through 12m each minute.
The engine used 1250kg of coal a day. 1kg of coal can release 22.3 MJ of energy. The beam engine was said to have an efficiency of 10%.
Deduce whether this claim for efficiency was correct.
delta E grav = m g delta h
delta E grav = 2500 x 9.81 x 12 = 294 kJ / minute
294 kJ x 60 x 24 = 424 MJ per day
energy input = 1250 x 22.3 MJ = 2.7875 x 10 ^10
efficiency = 424 x ^6 / 27900 x 10 ^6 = 1.5%
therefore, claim is not correct
