2023 A Level H2 Physics P3 Soln

2023 A Levels H2 Physics 9749/02 Paper 3 suggested solutions 1a 3 3 -2 2 3 3 -2 3 -2 3 2 4 -2 4 -1 -2 4 units of m kg m sunits of E m m m s kgunits of 4 kg m smm m m s kg mm kg m s L gMx wt E x Force area L gM wt E = = == =  ==  3 3Since the unit of is the same as unit o f ,4 the equation is homogenous. L gMx wt E 1b 3 3 3 10 4 3000.800 9.81 1000 2.10 4.56 1.004 100 1000 100 1.8918 10 Pa L gME wt x=  =     = 33 0.005 2 0.02 0.01 0.01330.800 300 2.10 4.56 1.00 0.0515 % uncertainty 0.0515 100% 5.2% E L M w t x E L M w t x      = + + + +    = + + + +       = =  = 101.89 10 Pa 5.2%E =   *Note: Value of E should be 3 s.f. since all the variables are 3 s.f. in the question. 2ai ( ) 22 22 1 2 5.0 2( 9.81)( 1.5) 7.4 m s v u as v v − =+ = + − − =

2aii 7.4 5.0 9.81 1.3 s v u at t t =+ − = − = 2bi 3a Electric current is the rate of flow of charge. 3b number of charge carriers N in the rod = number density n × volume = nAl charge in the rod Q = Nq = nA ql current I = Q t = nA q t l = nAvq since v = t l 3ci v nAq= I NNn nAA=  =ll subst this in first equation gives v Nq= Il = 23 19 2 00 3 00 1 45 10 1 60 10 .. .. −     = 2.59 × 10−4 m s−1

3cii electrons in tungsten wire move in random directions at high speed (due to temperature) while the drift velocity of the electrons is the average velocity in the direction of the net flow of the electrons along the wire (which is typically very slow) so the drift velocity will be less, in fact, much less than the mean speed of the electrons 4a The magnetic flux density of a magnetic field is the force per unit length, per unit current, on a long straight conductor placed at right angles to the magnetic field. 4b Since the reading on the balance decreases, there is a magnetic force on by the wire on the magnet, upwards. Hence, by Newton’s third law, there is a magnetic force of equal magnitude by the magnet on the wire, downwards. Since the force on the wire is downwards, and the magnetic field is from north to south pole, by Fleming’s Left Hand Rule the current in the wire is from X to Y. 4ci Average reading of the mass is when the current is off = 202.17 201.62 201.895 g2 + = Thus the change in the reading due to the force by the wire = 202.17 201.895 0.275 g−= The magnetic force of the wire on the magnet = 330.295 10 9.81 2.70 10 N−−  =  The magnetic flux density: 32.70 10 0.0141 T1.6 0.12 FB IL −= = = 4cii Since the force on the magnet is given by F BIL= it is independent of the resistivity of the material. As long as the current remains the same, the force on the magnet is the same and the reading is the same as in (b)(ii). 5a The gravitational potential at a point is the work done per unit mass by an external force in bringing a small test mass from infinity to that point without a change in kinetic energy. 5b By conservation of energy,