ESAT Physics: Electricity and magnetism, set 1

• Answer all questions. No calculator.
• Each question has exactly one correct answer.

1. 11 mark

   A student rubs an acetate rod with a dry cloth. After rubbing, tests show the rod carries a positive charge. Which statement correctly explains why the rod becomes positively charged?

   1. A Electrons transfer from the rod to the cloth, leaving the rod with fewer electrons than protons and so a positive charge.
   2. B Protons transfer from the rod to the cloth, leaving the rod with fewer protons than electrons and so a positive charge.
   3. C Electrons transfer from the cloth to the rod, giving the rod more electrons than protons and so a positive charge.
   4. D The rod's protons repel each other more strongly once rubbing has taken place, producing an overall positive charge.
2. 21 mark

   Which one of the following correctly describes a standard circuit symbol?

   1. A An ammeter is drawn as a circle containing the letter V, and a voltmeter is drawn as a circle containing the letter A.
   2. B A resistor is drawn as a rectangle; a variable resistor is drawn as a rectangle with a diagonal arrow through it.
   3. C A diode is drawn as a triangle only, with no bar across its tip.
   4. D A switch in the open (off) position is drawn as a closed, unbroken line joining its two terminals.
3. 31 mark

   Two students discuss electrical circuits. Student 1 says: 'A direct current periodically reverses the direction in which it flows.' Student 2 says: 'Rubber is a good insulator because it has very few electrons that are free to move through the material.' Which student, if either, is correct?

   1. A Student 1 only
   2. B Both students
   3. C Student 2 only
   4. D Neither student
4. 41 mark

   A charge of 12 coulombs flows past a point in a wire in 4 seconds. What is the current in the wire?

   1. A 0.33 A
   2. B 8 A
   3. C 48 A
   4. D 3 A
5. 51 mark

   A student wants to measure the current through a resistor and the potential difference (voltage) across it. How should the student connect the meters in the circuit?

   1. A Connect the ammeter in parallel with the resistor, and the voltmeter in series with the resistor.
   2. B Connect the ammeter in series with the resistor, and the voltmeter in parallel across the resistor.
   3. C Connect both the ammeter and the voltmeter in series with the resistor.
   4. D Connect both the ammeter and the voltmeter in parallel with the resistor.
6. 61 mark

   A resistor has a potential difference of 6 V across it when a current of 2 A flows through it. What is the resistance of the resistor?

   1. A 3 ohms
   2. B 0.33 ohms
   3. C 4 ohms
   4. D 12 ohms
7. 71 mark

   A student plots a graph of potential difference (on the vertical axis) against current (on the horizontal axis) for two components: a fixed resistor held at constant temperature, and a filament lamp. Which statement correctly compares the two graphs?

   1. A The fixed resistor gives a straight line through the origin; the filament lamp's line curves so that its gradient decreases as the current increases, because the lamp's resistance falls as it heats up.
   2. B The filament lamp gives a straight line through the origin; the fixed resistor's line curves because its resistance changes as the current through it changes.
   3. C The fixed resistor gives a straight line through the origin; the filament lamp's line curves so that its gradient increases as the current increases, because the lamp's resistance rises as it heats up.
   4. D Both the fixed resistor and the filament lamp give a straight line through the origin, since both obey Ohm's law at every current.
8. 81 mark

   Which one of the following statements, comparing an NTC thermistor, a light-dependent resistor (LDR) and an ideal diode, is correct?

   1. A The resistance of a light-dependent resistor increases as the light intensity falling on it increases.
   2. B An ideal diode has a very low resistance to current flowing in either direction through it.
   3. C An NTC thermistor's resistance increases as its temperature increases, in the same way as a filament lamp.
   4. D The resistance of an NTC thermistor decreases as its temperature increases.
9. 91 mark

   Which one of the following statements about series and parallel circuits is correct?

   1. A In a series circuit the current is the same at every point; in a parallel circuit the current splits between the branches.
   2. B In a series circuit the current splits between the components; in a parallel circuit the current is the same in every branch.
   3. C In a series circuit the potential difference is the same across every component; in a parallel circuit the potential difference splits between the branches.
   4. D In a parallel circuit the potential difference across each branch is different, and depends on the resistance of that branch alone.
10. 101 mark

    A 4 ohm resistor and a 6 ohm resistor are available. Which statement correctly describes the total resistance obtained by (i) connecting them in series, and (ii) connecting them in parallel instead?

    1. A In series the total resistance is 10 ohms; in parallel the total resistance is more than 6 ohms.
    2. B In series the total resistance is 24 ohms; in parallel the total resistance is less than 4 ohms.
    3. C In series the total resistance is 10 ohms; in parallel the total resistance is less than 4 ohms.
    4. D In series the total resistance is 2 ohms; in parallel the total resistance is less than 4 ohms.
11. 111 mark

    A device transfers 24 joules of energy for every 4 coulombs of charge that pass through it, and a current of 2 A flows through it. What are (i) the potential difference across the device, and (ii) the power it dissipates?

    1. A 6 V and 3 W
    2. B 6 V and 12 W
    3. C 96 V and 192 W
    4. D 0.17 V and 0.33 W
12. 121 mark

    A device operates at a potential difference of 5 V and draws a current of 2 A. How much energy is transferred by the device in 10 seconds?

    1. A 17 J
    2. B 20 J
    3. C 50 J
    4. D 100 J
13. 131 mark

    Which one of the following statements about magnets and magnetic materials is correct?

    1. A Iron is described as a 'hard' magnetic material because, once magnetised, it retains its magnetism well and is difficult to demagnetise.
    2. B A bar magnet's field lines outside the magnet run from its south pole to its north pole.
    3. C Steel is described as a 'hard' magnetic material because, once magnetised, it retains its magnetism well and is difficult to demagnetise.
    4. D Two like poles, such as two north poles, brought close together will attract one another.
14. 141 mark

    Two students describe magnetic effects of electric currents. Student 1 says: 'Increasing the current in a solenoid increases the strength of the magnetic field it produces.' Student 2 says: 'A current-carrying wire placed parallel to a uniform magnetic field experiences the maximum possible force on it.' Which student, if either, is correct?

    1. A Student 1 only
    2. B Student 2 only
    3. C Both students
    4. D Neither student
15. 151 mark

    Two students describe electromagnetic induction and transformers. Student 1 says: 'A voltage is induced in a wire when it moves so as to cut through magnetic field lines.' Student 2 says: 'A transformer with more turns on its secondary coil than on its primary coil is called a step-down transformer.' Which student, if either, is correct?

    1. A Student 2 only
    2. B Student 1 only
    3. C Both students
    4. D Neither student

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: A

   1. Charging by friction happens because electrons, not protons, transfer between two materials that are rubbed together.
   2. The rod is found to be positively charged, which means it has ended up with fewer electrons than protons.
   3. The only way friction charging can leave an object with fewer electrons is for that object to lose electrons to the other material, so the rod must have lost electrons to the cloth.
   4. Therefore the answer is A.
   • Why not B: This wrongly has protons moving between the two materials. Protons are held in the nucleus and cannot be transferred by friction; only electrons move during charging.
   • Why not C: This has the direction of electron transfer backwards. Gaining extra electrons would leave the rod with more negative charge than positive, so this describes negative charging, not positive.
   • Why not D: This invents a change in the number or arrangement of protons. Friction charging never changes the number of protons in an object; it only moves electrons between the two materials in contact.

2. Question 2Answer: B

   1. The specification requires recognising the basic circuit symbols, including resistor, variable resistor, ammeter, voltmeter and switch.
   2. A fixed resistor is a plain rectangle; adding a diagonal arrow across it is the standard way of showing that the resistance can be varied.
   3. An ammeter's circle contains 'A' (for amps) and a voltmeter's circle contains 'V' (for volts); the letters are not interchangeable, and an open switch needs a gap in the line while a diode needs the bar across its triangle.
   4. Therefore the answer is B.
   • Why not A: This swaps the letters used in the ammeter and voltmeter symbols. An ammeter is a circle containing 'A', and a voltmeter is a circle containing 'V', not the other way round.
   • Why not C: This omits the short bar across the point of the triangle, which is precisely what shows the diode only allows current to flow in one direction; without the bar, the symbol has lost the feature that makes it a diode rather than some other component.
   • Why not D: This describes a closed switch, not an open one. An open switch is drawn with a gap in the line, showing that no current can flow across it.

3. Question 3Answer: C

   1. Direct current (dc) flows in one direction only, while alternating current (ac) periodically reverses direction, so Student 1 has described ac while naming it dc.
   2. A conductor contains many free electrons that can move and carry charge; an insulator, such as rubber, contains very few free electrons, which is why it conducts so poorly.
   3. Student 2's statement matches this correctly, so Student 2 is correct.
   4. Therefore the answer is C.
   • Why not A: This accepts Student 1's claim, but a current that periodically reverses direction is alternating current (ac). Direct current (dc) flows in one direction only, so Student 1 has the definitions swapped.
   • Why not B: This wrongly accepts Student 1 alongside Student 2. Student 1's description of dc is actually a description of ac, so only one of the two students is correct, not both.
   • Why not D: This wrongly rejects Student 2's statement, which correctly identifies why insulators such as rubber do not conduct: they have no electrons free to move and carry charge.

4. Question 4Answer: D

   1. The relationship between current, charge and time is I = Q/t.
   2. Substituting the given values gives I = 12/4.
   3. 12 divided by 4 is 3, so the current is 3 A.
   4. Therefore the answer is D.
   • Why not A: This inverts the formula, calculating time / charge (4/12) instead of charge / time (12/4).
   • Why not B: This subtracts instead of dividing (12 - 4 = 8), rather than applying I = Q/t.
   • Why not C: This multiplies charge by time (12 x 4 = 48) instead of dividing charge by time.

5. Question 5Answer: B

   1. An ammeter measures the current flowing through a component, so it must be placed in series with that component so all the same current passes through both.
   2. A voltmeter measures the potential difference across a component, so it must be placed in parallel with that component, connected across its two ends.
   3. Connecting either meter the wrong way disturbs the circuit it is meant to measure.
   4. Therefore the answer is B.
   • Why not A: This swaps the correct arrangement round: it is the voltmeter that must go in parallel (across the component) and the ammeter that must go in series (in the main current path), not the other way round.
   • Why not C: This puts the voltmeter in series, which would place a component of very high resistance directly in the current path, drastically reducing the current the student is trying to measure.
   • Why not D: This puts the ammeter in parallel, which would give the ammeter's very low resistance an alternative, easier path than the resistor, so most of the current would bypass the resistor entirely and the reading would not represent the resistor's current.

6. Question 6Answer: A

   1. The relationship between resistance, voltage and current is R = V/I.
   2. Substituting the given values gives R = 6/2.
   3. 6 divided by 2 is 3, so the resistance is 3 ohms.
   4. Therefore the answer is A.
   • Why not B: This inverts the formula, calculating current / voltage (2/6) instead of voltage / current (6/2).
   • Why not C: This subtracts the two values (6 - 2 = 4) instead of dividing voltage by current.
   • Why not D: This multiplies voltage by current (6 x 2 = 12) instead of dividing, as though calculating power rather than resistance.

7. Question 7Answer: C

   1. A fixed resistor held at constant temperature has a constant resistance, so V = IR is a straight line through the origin with a constant gradient equal to that resistance.
   2. A filament lamp heats up as more current flows through it, and its resistance rises as it gets hotter.
   3. Since resistance is the ratio of voltage to current, a rising resistance means the graph's gradient increases as the current increases, so the filament lamp's line curves upward more and more steeply.
   4. Therefore the answer is C.
   • Why not A: This has the filament lamp's resistance changing the wrong way: a filament's resistance rises, not falls, as it heats up under increasing current, so the graph's gradient increases rather than decreases.
   • Why not B: This swaps the two components round: it is the fixed resistor, held at constant temperature, that keeps a constant resistance and gives the straight line, while the filament lamp is the one whose resistance (and therefore whose graph gradient) changes with current.
   • Why not D: This wrongly claims the filament lamp obeys Ohm's law. A fixed resistor at constant temperature does give a straight-line, constant-resistance graph, but a filament lamp's resistance changes with current, so it is a non-ohmic component and cannot give a straight line through the origin.

8. Question 8Answer: D

   1. 'NTC' stands for negative temperature coefficient, meaning the thermistor's resistance falls as its temperature rises.
   2. A light-dependent resistor's resistance falls as light intensity rises, since more light generates more charge carriers able to conduct.
   3. An ideal diode conducts with almost no resistance in one direction and blocks current almost completely in the other.
   4. Therefore the answer is D.
   • Why not A: This has the LDR's behaviour backwards: its resistance decreases, not increases, as light intensity increases, since more light frees more charge carriers in the material.
   • Why not B: This describes a component with no preferred direction, but an ideal diode has a very low resistance in one direction only (letting current flow) and a very high (in the ideal case, infinite) resistance in the reverse direction, blocking current.
   • Why not C: This wrongly makes an NTC thermistor behave like a filament lamp. 'NTC' stands for negative temperature coefficient, meaning its resistance falls as temperature rises, which is the opposite of a filament lamp's rising resistance.

9. Question 9Answer: A

   1. In a series circuit there is only one path for charge to flow, so the same current passes through every component in turn.
   2. In a parallel circuit there are multiple paths, so the total current entering the branches splits between them.
   3. The supply voltage in a series circuit splits between the components, but in a parallel circuit every branch shares the same potential difference, since each branch connects across the same two points.
   4. Therefore the answer is A.
   • Why not B: This swaps the series and parallel current rules round: it is the series circuit where the same current flows through every component, and the parallel circuit where the current splits between branches.
   • Why not C: This swaps the series and parallel voltage rules round: in a series circuit the supply voltage splits between the components, while in a parallel circuit each branch has the same potential difference across it.
   • Why not D: This wrongly claims parallel branches have different potential differences depending on their resistance; every branch connected across the same two points in a parallel circuit shares the same potential difference, regardless of its own resistance.

10. Question 10Answer: C

    1. Resistors in series simply add, so the total resistance is 4 + 6 = 10 ohms.
    2. The total resistance of resistors in parallel is always less than the smallest of the individual resistances, here 4 ohms.
    3. Only 10 ohms in series and 'less than 4 ohms' in parallel are both true together.
    4. Therefore the answer is C.
    • Why not A: This gets the series total right but claims parallel resistance is more than 6 ohms; the total resistance of resistors in parallel is always less than the smallest individual resistance, so it must be below 4 ohms, not above 6.
    • Why not B: This computes the series total by multiplying the two resistances (4 x 6 = 24) instead of adding them; multiplying the two values is the numerator used when finding a parallel combination, not the series total.
    • Why not D: This computes the series total by subtracting one resistance from the other (6 - 4 = 2) instead of adding them; resistors in series add directly, giving 4 + 6 = 10 ohms.

11. Question 11Answer: B

    1. The potential difference across a device is the energy transferred per unit charge, V = E/Q.
    2. Substituting the given values gives V = 24/4 = 6 V.
    3. Power is the product of current and voltage, P = IV, so P = 2 x 6 = 12 W.
    4. Therefore the answer is B.
    • Why not A: This gets the voltage right (24/4 = 6 V) but then divides voltage by current (6/2 = 3) instead of multiplying, so it fails to apply P = IV correctly.
    • Why not C: This multiplies energy by charge (24 x 4 = 96) instead of dividing, giving a voltage far too large, and then compounds the error by using this wrong voltage in P = IV.
    • Why not D: This inverts the voltage formula, calculating charge / energy (4/24) instead of energy / charge (24/4), and then carries this much-too-small value into the power calculation.

12. Question 12Answer: D

    1. The energy transferred by a device is given by E = VIt, using potential difference, current and time together.
    2. Substituting the given values gives E = 5 x 2 x 10.
    3. 5 x 2 = 10, and 10 x 10 = 100, so the energy transferred is 100 J.
    4. Therefore the answer is D.
    • Why not A: This adds the three quantities together (5 + 2 + 10 = 17) instead of multiplying them, so it is not using the formula E = VIt at all.
    • Why not B: This uses only current and time (2 x 10 = 20), leaving the potential difference out of the calculation entirely.
    • Why not C: This uses only potential difference and time (5 x 10 = 50), leaving the current out of the calculation entirely.

13. Question 13Answer: C

    1. A 'hard' magnetic material, such as steel, magnetises less easily than a 'soft' one but retains its magnetism well once magnetised, making it useful for permanent magnets.
    2. A 'soft' magnetic material, such as iron, magnetises very easily but also loses its magnetism easily, making it useful for temporary or induced magnetism.
    3. Like poles repel and unlike poles attract, and the external field lines of a bar magnet run from north to south.
    4. Therefore the answer is C.
    • Why not A: This wrongly assigns 'hard' magnetic behaviour to iron. Iron is the classic 'soft' magnetic material: it magnetises easily but loses its magnetism again just as easily, whereas steel is the 'hard' material that retains it.
    • Why not B: This has the direction of the external field lines reversed; a bar magnet's field lines outside the magnet run from its north pole to its south pole (they then run from south back to north inside the magnet, completing the loop).
    • Why not D: This has the rule for magnetic poles backwards; like poles (two norths, or two souths) repel one another, while unlike poles (a north and a south) attract.

14. Question 14Answer: A

    1. The magnetic field strength around a current-carrying wire or solenoid increases with the current flowing through it, among other factors, so Student 1 is correct.
    2. The force on a current-carrying wire in a magnetic field (the motor effect) is greatest when the wire is at right angles to the field, and falls to zero when the wire is parallel to the field, so Student 2's claim is the wrong way round.
    3. Only Student 1's statement is correct.
    4. Therefore the answer is A.
    • Why not B: This accepts Student 2's claim, but a wire feels the maximum force when it is at right angles to the field, not when it is parallel to it; a wire placed parallel to the field lines experiences no force at all.
    • Why not C: This wrongly accepts Student 2 as well as Student 1; Student 2 has the orientation for maximum force backwards.
    • Why not D: This wrongly rejects Student 1's correct statement; increasing the current is one of the factors, alongside the number of turns and the presence of a core, that increases a solenoid's magnetic field strength.

15. Question 15Answer: B

    1. A voltage is induced in a conductor whenever it cuts through magnetic field lines, or whenever the magnetic field linking it changes; this is electromagnetic induction, so Student 1 is correct.
    2. A transformer with more turns on its secondary coil than its primary coil increases the voltage from primary to secondary, which makes it a step-up transformer, not a step-down transformer, so Student 2 is incorrect.
    3. Only Student 1's statement is correct.
    4. Therefore the answer is B.
    • Why not A: This accepts Student 2's claim, but a transformer with more turns on its secondary than its primary coil increases the voltage and is therefore a step-up transformer, not a step-down transformer.
    • Why not C: This wrongly accepts Student 2 alongside Student 1; Student 2 has the step-up and step-down definitions swapped round.
    • Why not D: This wrongly rejects Student 1's correct statement; a voltage is induced whenever a conductor cuts through magnetic field lines (or the field through it changes), which is the basic principle of electromagnetic induction.