Electricity & magnetism Banker

1. (a) A transformer has 2000 turns on its primary coil, which is connected to the 230 V mains. It gives an output of 11.5 V. How many turns are on the secondary coil?A 10B 100C 400D 40 000 [1]

The north pole of a bar magnet is pushed quickly into a coil of wire connected to a centre-zero galvanometer. The pointer deflects to the right.

1. (a) State what is observed on the galvanometer when the magnet is held still inside the coil. [1]
2. (b) The magnet is pulled back out of the coil. Describe what the galvanometer shows. [1]
3. (c) State three ways of increasing the size of the induced e.m.f. [3]

A power station supplies 66 MW to a distant city through cables with a total resistance of 0.50 Ω.

1. (a) Calculate the current in the cables when the power is transmitted at 132 kV. [2]
2. (b) Calculate the power wasted as heat in the cables at 132 kV. [2]
3. (c) Calculate the power that would be wasted if the same power were transmitted at 11 kV. [2]
4. (d) Use your answers to explain why electrical energy is transmitted at high voltage. [1]

A simple d.c. motor has a rectangular coil that can rotate between the poles of a magnet. It is connected to a battery through a split-ring commutator and brushes.

1. (a) Explain why the coil rotates when there is a current in it. [2]
2. (b) State the purpose of the split-ring commutator. [2]
3. (c) State two ways of making the motor turn faster. [2]

1. (a) A horizontal wire carries a current from south to north. It is in a uniform magnetic field that points vertically downwards. In which direction is the force on the wire?A eastB westC vertically upwardsD north [1]

A simple a.c. generator has a coil rotating at a steady rate in a uniform magnetic field. The ends of the coil are connected to slip rings.

1. (a) Describe how the e.m.f. induced in the coil varies during one complete revolution. [3]
2. (b) Explain why the e.m.f. is largest when the plane of the coil is parallel to the magnetic field. [1]
3. (c) The coil is rotated twice as fast. State the effect on the peak e.m.f. and on the frequency of the output. [2]
4. (d) State the purpose of the slip rings. [1]

A transformer connected to the 230 V a.c. mains supplies a 12 V, 36 W lamp in a model railway set. The lamp works normally.

1. (a) Calculate the current in the lamp. [1]
2. (b) Calculate the current in the primary coil. Assume the transformer is 100% efficient. [2]
3. (c) The primary coil has 1150 turns. Calculate the number of turns on the secondary coil. [2]
4. (d) Explain why the transformer would not work if connected to a 230 V d.c. supply. [2]

1. (a) A 6.0 Ω resistor and a 3.0 Ω resistor are connected in parallel. What is their combined resistance?A 0.50 ΩB 2.0 ΩC 4.5 ΩD 9.0 Ω [1]

A 9.0 V battery is connected in series with a 4.0 Ω resistor and an 8.0 Ω resistor.

1. (a) Calculate the combined resistance of the two resistors. [1]
2. (b) Calculate the current in the circuit. [2]
3. (c) Calculate the potential difference across each resistor. [2]
4. (d) State how the current in the 4.0 Ω resistor compares with the current in the 8.0 Ω resistor. [1]

A 12 V battery is connected to a 2.0 Ω resistor in series with two lamps that are in parallel with each other. One lamp has resistance 12 Ω and the other 6.0 Ω. Treat the lamp resistances as constant.

1. (a) Calculate the combined resistance of the two lamps in parallel. [2]
2. (b) Calculate the current from the battery. [2]
3. (c) Calculate the current in each lamp. [2]
4. (d) The 12 Ω lamp breaks. State and explain what happens to the brightness of the 6.0 Ω lamp. [2]

A garden light in Islamabad is controlled by a potential divider. A 2.0 kΩ fixed resistor and a light-dependent resistor (LDR) are connected in series across a 6.0 V supply. The output is the p.d. across the LDR.

1. (a) Calculate the output voltage across the LDR at night, when its resistance is 10 kΩ. [2]
2. (b) Calculate the output voltage across the LDR in daylight, when its resistance is 500 Ω. [2]
3. (c) The lamp switches on when the output voltage is above 3.0 V. Explain how the circuit switches the lamp on automatically at dusk. [2]

A diode is connected in series with a small lamp and a 6.0 V battery, so that the diode conducts.

1. (a) The lamp lights. The battery is then reversed. State and explain what happens to the lamp. [2]
2. (b) The battery is replaced by an alternating (a.c.) supply. Describe the current in the lamp. [2]
3. (c) State the name given to this use of a diode. [1]

A poultry farmer near Faisalabad uses an automatic fan. A thermistor and a 4.0 kΩ fixed resistor are connected in series across a 9.0 V supply. A relay coil is connected across the fixed resistor; the relay contacts switch a separate mains circuit containing the fan. Assume the relay coil takes negligible current.

1. (a) Calculate the p.d. across the fixed resistor when the thermistor resistance is 14 kΩ (at 20 °C) and when it is 2.0 kΩ (at 35 °C). [3]
2. (b) The relay switches on when the p.d. across its coil is 5.0 V. Calculate the resistance of the thermistor at which the fan switches on. [2]
3. (c) Explain why a relay is used rather than connecting the fan directly into the thermistor circuit. [2]

A 6.0 V supply is connected to a 10 Ω resistor in series with a parallel combination of a 20 Ω resistor and a 30 Ω resistor.

1. (a) Calculate the current from the supply. [3]
2. (b) Calculate the p.d. across the 30 Ω resistor. [1]
3. (c) The 30 Ω resistor is removed. Calculate the new p.d. across the 20 Ω resistor and state whether the current from the supply increases or decreases. [3]

A mobile phone is charged with a current of 1.5 A for 2.0 hours.

1. (a) State what is meant by electric current. [1]
2. (b) Calculate the charge that flows into the phone. [2]
3. (c) State the unit of charge in terms of base units. [1]

1. (a) A piece of nichrome wire has a resistance of 4.0 Ω. A second nichrome wire is twice as long and has twice the diameter. What is the resistance of the second wire?A 1.0 ΩB 2.0 ΩC 4.0 ΩD 8.0 Ω [1]

A torch battery of e.m.f. 1.5 V is connected in series with a lamp and a small resistor. Assume the battery itself has no resistance.

1. (a) Define the electromotive force (e.m.f.) of a source. [2]
2. (b) Calculate the energy transferred by the battery when 400 C of charge flows round the circuit. [2]
3. (c) The p.d. across the lamp is 1.2 V. Calculate the p.d. across the resistor and the energy transferred in the resistor when 400 C flows. [2]

The current in a 6 V filament lamp is measured at different values of potential difference.p.d. / V: 0, 1.0, 2.0, 4.0, 6.0current / A: 0, 0.20, 0.32, 0.46, 0.55

1. (a) Calculate the resistance of the lamp at 2.0 V and at 6.0 V. [2]
2. (b) Describe how the resistance of the filament changes as the p.d. increases, and explain why. [3]
3. (c) Describe how the I–V graph for a fixed resistor at constant temperature would differ from the graph for this lamp. [1]

A 1.5 kW air conditioner in a house in Hyderabad runs for 8 hours a day during a 30-day month.

1. (a) State what is meant by one kilowatt-hour (kW h). [1]
2. (b) Calculate the energy used by the air conditioner in 30 days, in kW h. [2]
3. (c) The electricity company charges Rs 62 per kW h. Calculate the cost of running the air conditioner for the 30 days. [1]
4. (d) Calculate the energy used in 30 days in joules. [1]

1. (a) A 12 V motor in a toy car takes a current of 0.50 A for 2.0 minutes. How much electrical energy is transferred?A 6.0 JB 12 JC 24 JD 720 J [1]

A small electric motor in a school lift model is connected to a 12 V supply and takes a current of 2.5 A. In 8.0 s it raises a 6.0 kg load through a height of 2.4 m.

1. (a) Calculate the electrical energy supplied to the motor. [2]
2. (b) Calculate the gain in gravitational potential energy of the load. Take g = 9.8 N/kg. [1]
3. (c) Calculate the efficiency of the motor system. [2]
4. (d) Suggest one way in which energy is wasted. [1]

A student tests three bars, P, Q and R, by bringing each end close to the north pole and then the south pole of a bar magnet.P: both ends are attracted by both poles.Q: one end is repelled by the north pole and attracted by the south pole.R: no force is felt with either pole.

1. (a) Identify which bar is a permanent magnet, which is an unmagnetised magnetic material, and which is non-magnetic. Give a reason for each. [3]
2. (b) Explain why attraction alone cannot prove that a bar is a magnet. [1]

1. (a) An electromagnet in a scrapyard crane must lift steel and then drop it as soon as the current is switched off. Which material should be used for its core, and why?A steel, because it keeps its magnetismB soft iron, because it is easily magnetised and demagnetisedC copper, because it is a good conductorD aluminium, because it is light [1]

Two identical bar magnets lie in line on a bench with their north poles facing each other, 10 cm apart.

1. (a) Describe the magnetic field pattern in the region between the two north poles. [2]
2. (b) State what is meant by the direction of a magnetic field at a point. [1]
3. (c) A small plotting compass is placed exactly halfway between the two north poles. Ignoring the Earth's field, state and explain how the compass behaves. [2]
4. (d) State how a field-line diagram shows where the magnetic field is strongest. [1]

1. (a) A polythene rod is rubbed with a woollen cloth. The rod becomes negatively charged. What has happened?A Electrons have moved from the cloth to the rod.B Protons have moved from the rod to the cloth.C Electrons have moved from the rod to the cloth.D Negative charge has been created by friction. [1]

1. (a) State what is meant by an electric field. [1]
2. (b) State how the direction of an electric field is defined. [1]
3. (c) Describe the electric field pattern around an isolated, positively charged metal sphere. [2]
4. (d) Two parallel metal plates are connected to a high-voltage supply; the top plate is positive and the bottom plate is negative. Describe the field between the plates, away from the edges. [2]

An aircraft lands at Lahore airport after a flight through dry air and must be refuelled.

1. (a) Explain how the aircraft becomes charged while flying through dry air. [2]
2. (b) Explain why a conducting cable connects the aircraft to the ground before refuelling begins. [2]
3. (c) A charge of 4.5 µC flows to earth in 3.0 ms. Calculate the average current. [2]

A hairdryer is rated 1.8 kW, 230 V.

1. (a) Calculate the normal current in the hairdryer. [2]
2. (b) Fuses rated at 3 A, 5 A and 13 A are available. State which fuse should be used and explain why. [2]
3. (c) The hairdryer has a plastic case and is double insulated. Explain why it does not need an earth wire. [2]

A security lamp outside a shop in Rawalpindi must switch on only when it is dark AND movement is detected. A light sensor gives output 1 when it is dark; a motion sensor gives output 1 when movement is detected.

1. (a) Name the single logic gate that should be used. [1]
2. (b) Write the truth table for this gate, with inputs A (light sensor) and B (motion sensor) and output Q (lamp). [2]
3. (c) The owner now wants the lamp to come on when it is dark OR when a test switch C is pressed (C gives 1 when pressed), but the motion sensor is no longer used. Name the gate needed. [1]

Input A is connected to a NOT gate. The output of the NOT gate and input B are the two inputs of an OR gate, whose output is Q.

1. (a) Complete the truth table for this circuit for all four combinations of A and B. [3]
2. (b) State the only combination of inputs that makes Q = 0. [1]
3. (c) An extra NOT gate is added after the OR gate. State the new output for A = 1, B = 0, and describe in words when the new output is 1. [2]