Part 1: Multiple choice
One ampere is equal to:
A current of 3 A flows in a wire for 20 s. The charge that passes a cross-section is:
A charge of 240 C passes a point in 2 minutes. The current is:
The volt is equivalent to:
Work of 60 J is done to move 5 C between two points. The potential difference is:
To measure the current in a lamp, an ammeter must be connected:
A voltmeter measuring the voltage across a lamp is connected:
By convention, the electric current outside the source flows from:
Which of the following is needed for a current to flow in a wire?
The potential difference between the terminals of a battery in a working circuit is slightly less than its emf because of:
Four identical 1.5 V cells are connected in series. The emf is:
The same four 1.5 V cells connected in parallel give an emf of:
Which statement about connecting cells in parallel is correct?
Increasing the length of a wire of the same material and thickness:
Two wires of the same metal and the same length have cross-sectional areas A and 2A. The thicker wire has:
The filament of a bulb glows while the copper connecting wires do not, because the filament:
A rheostat is used to:
The unit of electric resistance is the:
Ohm's law states that, at constant temperature, the current through a conductor is:
The potential difference across a resistor is 24 V and the current is 3 A. Its resistance is:
The current through a 4 Ω resistor connected to a 12 V battery is:
In an experiment to verify Ohm's law, the factor that must be kept constant is:
If the potential difference across a fixed resistor is doubled, the current becomes:
Four identical 5 Ω resistors are connected in series. The equivalent resistance is:
Three identical 9 Ω resistors are connected in parallel. The equivalent resistance is:
The equivalent resistance of identical resistors connected in parallel is:
Three identical resistors in parallel have an equivalent resistance of 2 Ω. If they are connected in series, the equivalent resistance is:
Household appliances are connected in parallel so that:
Resistors in series have the same:
The unit watt is equivalent to:
One kilowatt-hour equals:
A 100 W lamp works for 5 hours. The energy consumed is:
A 2 kW heater runs for 3 hours. If a kW·h costs 1.5 pounds, the cost is:
Which of these is a common unit for measuring household electrical energy bills?
Which action is unsafe?
Oersted discovered that an electric current flowing in a wire:
Soft iron is used as the core of an electromagnet because it:
Which change does NOT increase the strength of an electromagnet?
An electric motor converts:
The principle of operation of an electric generator is the:
Part 2: Written questions and problems
1. [Give reasons] A compass needle deflects when placed beneath a wire carrying a current.
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The current produces a magnetic field around the wire, and this field exerts a force on the magnetised needle.
Oersted's observation.
2. [Give reasons] Soft iron is used as the core of an electromagnet.
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It magnetises easily when current flows and loses its magnetism when the current stops, so the electromagnet can be switched on and off.
Steel would stay magnetised.
3. [Give reasons] A bicycle lamp connected to a dynamo lights when the wheel turns.
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The wheel rotates the coil of the generator in a magnetic field, producing an emf that lights the lamp.
Mechanical energy is converted to electrical energy; faster turning gives a brighter lamp.
4. [Give reasons] Household appliances are connected in parallel.
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Each gets the full mains voltage and works independently; if one is switched off or damaged, the others keep working.
Each has its own path to the source.
5. [Give reasons] The filament of a bulb glows but the connecting wires do not.
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The filament is long, thin tungsten with a large resistance, so much more heat is produced in it by the same current than in the short, thick copper wires.
Heat produced increases with resistance for the same current.
6. [Give reasons] The potential difference between the terminals of a battery is usually less than its emf.
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Part of the emf is used to push the current through the battery's own internal resistance.
ε is the total work per coulomb inside and outside.
7. [Give reasons] Some devices, such as torches, use cells connected in series.
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In series the emfs add, giving a larger total emf sufficient to power the device.
ε(total) = n × ε(cell).
8. [Give reasons] Some power banks contain cells connected in parallel.
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The emf stays equal to one cell, but the cells share the load, so the phone can be charged several times without raising the output voltage.
Parallel connection increases the capacity.
9. [Give reasons] Turning the volume knob of a radio changes the loudness.
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The knob is a variable resistor; it changes the resistance and the current, hence the sound.
Rheostat principle.
10. [Give reasons] You must not connect several high-power appliances to one socket.
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The total current becomes very large and overheats the wires, which may cause a fire.
Overloading.
11. [Give reasons] A crane with an electromagnet is used to move iron scrap.
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It is magnetised when current flows to pick up the scrap, and releases it when the current is switched off; its strength can be controlled.
Temporary magnet.
12. [Give reasons] A coil in a motor keeps turning in the same direction.
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The couple of equal, opposite forces acting on the two sides of the coil gives a torque; the split-ring commutator reverses the current every half turn so that the torque always has the same sense.
Commutator action.
13. [Give reasons] Hands, wires and equipment must be dry before closing an electric circuit.
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Tap water conducts electricity, so wet hands can allow current to pass through the body (shock).
Safety precaution.
14. [Give reasons] LED lamps are preferred to CFL lamps.
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For the same light output LED lamps have lower power, so they consume less energy and cost less.
Example: 12 W LED vs 20 W CFL.
15. [Give reasons] The rotation of a motor can be reversed by reversing the battery connections.
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The direction of the current in the coil is reversed, so the magnetic forces on the coil sides reverse.
Force direction depends on current and field direction.
Part C — Define / compare
16. [Define] Define: electric current intensity; potential difference; electromotive force.
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Current intensity is the charge passing through a cross-section of a conductor per second (I = Q/t, ampere). Potential difference is the work done to move one coulomb between two points (V = W/Q, volt). Emf is the total work done by the source to move one coulomb through the whole circuit including the source.
Definitions of Lesson 1.
17. [Define] State Ohm's law and write its mathematical form.
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At constant temperature, the current through a conductor is directly proportional to the potential difference across it; I = V/R.
The graph is a straight line through the origin.
18. [Define] What is meant by (a) the electric power of a fan is 75 W? (b) the equivalent resistance of a group of resistors is 1 Ω?
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- (a) The fan consumes 75 J of electrical energy every second.
- (b) A single 1 Ω resistor could replace the whole group with the same effect on the circuit.
Definition meanings.
19. [Compare] Compare an ammeter and a voltmeter (quantity measured, unit, connection).
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Ammeter: current intensity; ampere; in series. Voltmeter: potential difference; volt; in parallel.
Different roles.
20. [Compare] Compare series and parallel connection of identical resistors in terms of current, potential difference and equivalent resistance.
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Series: same current through each; voltage divides equally; R′ = nR. Parallel: current divides equally; same voltage across each; R′ = R/n.
See table in lesson.
21. [Compare] Compare a permanent magnet and an electromagnet (control of strength; one use).
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A permanent magnet's strength cannot be controlled (compass needle); an electromagnet's strength can be controlled by turns, current and core and it can be switched off (crane).
Temporary vs permanent.
22. [Compare] Compare an electric motor and an electric generator (input energy, output energy, principle).
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- Motor: electrical in, mechanical out, force/torque on a current-carrying coil in a magnetic field.
- Generator: mechanical in, electrical out, emf produced when a coil cuts magnetic field lines.
Reverse processes.
23. [Compare] Compare direct current and alternating current.
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- Direct current has constant intensity and flows in one direction (cells and batteries).
- Alternating current changes in value and direction with time (generators in power stations; used in homes).
Source types.
24. [Compare] Compare a fixed resistor and a variable resistor (rheostat).
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A fixed resistor has constant resistance; a rheostat's resistance is adjustable by a slider, used to control the current.
Example: radio volume control.
25. [Compare] State two advantages of the alternating current produced in power stations.
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It can be transmitted over long distances with relatively little energy loss, and it can be converted to direct current when needed.
Reason for its use in homes.
Part D — Problems
26. [Problem] A current of 0.8 A flows for 5 minutes. Find the charge passed.
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240 C
t = 300 s; Q = 0.8 × 300 = 240 C.
27. [Problem] 450 J of work is done to move 90 C through a lamp. Find the potential difference across it.
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5 V
V = W/Q = 450/90 = 5.
28. [Problem] A 30 Ω resistor is connected across a 9 V battery. Find the current.
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0.3 A
I = V/R = 9/30.
29. [Problem] In an experiment to verify Ohm's law the readings were: V = 2, 4, 6 V and I = 0.4, 0.8, 1.2 A. Find the resistance of the resistor and show that Ohm's law holds.
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R = 5 Ω; V/I is constant (5 Ω) in every reading.
2/0.4 = 4/0.8 = 6/1.2 = 5.
30. [Problem] Three identical 3 Ω resistors are connected in series across a 9 V battery. Find the equivalent resistance, the current, and the voltage across each resistor.
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9 Ω; 1 A; 3 V.
R′ = 9, I = 9/9 = 1 A, V = 1 × 3 = 3 V (3 × 3 = 9 V total).
31. [Problem] Four identical 8 Ω resistors are connected in parallel across a 16 V source. Find the equivalent resistance, the total current and the current in each resistor.
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2 Ω; 8 A; 2 A each.
R′ = 8/4 = 2; I = 16/2 = 8; each branch 16/8 = 2 A.
32. [Problem] Five identical 1.5 V cells are connected in series to a lamp of resistance 15 Ω. Find the emf and the current (ignore internal resistance).
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7.5 V; 0.5 A.
ε = 5 × 1.5 = 7.5; I = 7.5/15 = 0.5.
33. [Problem] Two identical 6 Ω resistors are connected to a 12 V battery, first in series then in parallel. Find the total current in each case.
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Series: 1 A; parallel: 4 A.
Series R′ = 12 Ω, I = 1; parallel R′ = 3 Ω, I = 4.
34. [Problem] An electric iron of power 1200 W is used 2 hours a day for 30 days. If the price is 1.55 pounds per kW·h, find the monthly energy and cost.
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72 kW·h; 111.6 pounds.
1.2 kW × 60 h = 72 kW·h; 72 × 1.55 = 111.6.
35. [Problem] A lamp transforms 3600 J of electrical energy in 60 s. Find its power.
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60 W
P = E/t = 3600/60.
36. [Problem] A 10 W LED lamp replaces a 60 W filament lamp. Each is used 5 h a day for 30 days at 1.5 pounds per kW·h. Find the saving over the month.
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11.25 pounds
Hours = 150. LED: 0.01 × 150 = 1.5 kW·h = 2.25 pounds. Filament: 0.06 × 150 = 9 kW·h = 13.5 pounds. Difference = 11.25.
37. [Problem] Appliance A has power 500 W and works 4 hours. How long must appliance B of power 2000 W work to consume the same energy?
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1 hour
Energy = 0.5 × 4 = 2 kW·h; time = 2/2 = 1 h.
38. [Problem] Convert 7.2 × 10⁶ J into kW·h.
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2 kW·h
7.2 × 10⁶ ÷ 3.6 × 10⁶ = 2.
39. [Problem] A fan of power 80 W works 10 hours daily for 30 days. If a kW·h costs 1.55 pounds, find the monthly cost.
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37.2 pounds
0.08 × 300 = 24 kW·h; 24 × 1.55 = 37.2.
40. [Problem] A uniform wire of length 20 m has a resistance of 4 Ω. What is the resistance of 50 m of the same wire (same thickness and temperature)?
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10 Ω
R is proportional to length: 4 × 50/20 = 10 Ω.