5054 Physics
Random Multiple-Choice Questions
Random Multiple-Choice Questions
[I recommend that you answer the questions in this link first before looking at the explanations below.]
1 What material can a temporary magnet be made from?
A plastic B soft iron C steel D wood
Answer: B
The Physics Behind
- There are materials called hard magnetic materials which gained magnetism and tend to retain this magnetic property for a long period of time. An example of a hard magnetic material is steel.
- The other type of material, soft magnetic material, gains magnetism quickly but also lose it very quickly. An example is a soft iron.
Which compass correctly shows the direction of the magnetic field due to the magnet?
Answer: D
The Physics Behind
- Magnetic field of a magnet goes from the N pole to S pole.
- This can be shown using a compass, that is, it points the direction of the magnetic field at the point where the compass is placed.
- Among the options, D is the only one correctly showing how a compass will behave at a point.
What happens to the plastic rod and what is the charge on the cloth?
Answer: C
The Physics Behind
- Charges are conserved. This is to say that when something loses charge, that charge must have moved to another thing.
- The movement of charges from one thing to another is essentially because of either giving off electrons or absorbing electrons
- When electrons are given off, the object becomes positively-charged.
- When electrons are absorbed, the object becomes negatively-charged.
- Since the rod becomes positively charged, we know therefore that it has given off some of its electrons to the cloth.
- Since the electrons were absorbed by the cloth, we know that this makes it negatively-charged.
4 The circuit diagram shows cells connected with a resistor in a series circuit.
Each cell has an electromotive force (e.m.f.) of 1.5 V.
What is the total e.m.f. in the circuit?
A 1.5 V
B 4.5 V
C 6.0 V
D 10.5 V
Answer: D
The Physics Behind
- Electromotive force is the energy per unit charge in moving the charge through the entire circuit.
- This means that, in the case of the above series circuit, the total energy must come from the combined cells.
- Therefore, seven times 1.5 V is the answer.
5 An electric current can produce a heating effect and a magnetic effect.
Which row shows the effect that a relay uses and one application of a relay?
Answer: C
The Physics Behind
- A relay is a switch based on the principle of an electromagnet.
- It usually used in situations when we want one circuit to control another, especially if the current and power are larger in the second circuit.
6 A negatively-charged rod is brought near to an uncharged metal sphere on an insulating stand.
Which diagram shows the charge distribution on the sphere?
Answer: D
The Physics Behind
- A neutral object has equal number of positive charges and negative charges in it.
- Unlike charges repel while like charges attract.
- When a charged object is placed close to a neutral object, the charged object
- attracts charges having opposite its charge, and
- repels charges having the same charge.
7 In the circuit shown, ammeter X reads 0.5 A.
What does ammeter Y read?
A 0
B 0.5 A
C 3.5 A
D 4.0 A
Answer: B
The Physics Behind
- Ammeter Y measures the total current in the circuit.
- Total current equals the electromotive force [or total voltage] divided by the total resistance.
- The total resistance is 8.0 ohm while the e.m.f. is 4.0 V. Therefore, the reading will be 0.5 A
8 In the circuit below, one of the lamps breaks, causing all the other lamps to go out.
Which lamp breaks?
Answer: D
The Physics Behind
- When any of the lamps breaks, it acts as if it is a switch which is off.
- However, only when lamp D is off that the conventional current goes from positive terminal to the negative terminal of the source.
9 A pupil measures the potential difference across a device and the current in it.
Which calculation gives the resistance of the device?
A current + potential difference
B current ÷ potential difference
C potential difference ÷ current
D potential difference × current
Answer: C
The Physics Behind
- From Ohm's Law we can see that resistance R is equal to the voltage V divided by current I.
R = V / I
- The answer is C.
10 In which circuit does the ammeter read the total current through both resistors?
Answer: C
The Physics Behind
- The current passing through each resistor combines as they pass through a node.
- If an ammeter is to measure the total current, then it has to be connected in series in a wire directly connected to or after the node.
11 A certain electrical appliance is powered from a mains supply. The appliance normally uses a current of 3 A, but the current briefly rises to 4 A at the instant the appliance is switched on. The cable to the appliance is designed for currents up to 6 A.
The fuses available to protect the cable are rated at 1 A, 3 A, 5 A and 13 A.
Which fuse should be used?
A 1 A B 3 A C 5 A D 13 A
Answer: C
The Physics Behind
- The tolerance of fuse in an appliance is rated a little above the normal current passing through an appliance.
- The following cannot be an answer:
- Option A because it is lower than the normal current passing through the appliance. It will be busted even the current is fine for the appliance.
- Option B because it is the same as the normal current passing through the appliance. it will break the circuit in the appliance even when the current is normal.
- Option D because it will be useless as the cable to the appliance for current lower than it.
- Option C is the answer as it is a little more than the normal current passing through the appliance.
12 The diagram shows a battery connected to two identical resistors. Three ammeters M1, M2 and M3 are connected in the circuit.
Meter M1 reads 1.0 A.
What are the readings on M2 and on M3?
Answer: D
The Physics Behind
- In a series circuit, currents passing through any of the components have the same magnitude.
- Since the circuit above is a series circuit, all ammeters will read same current.
13 A circuit is set up to measure the resistance of a resistor R. The meter readings are 2.0 A and 3.0 V.
What is the resistance of the resistor R?
A 0.67 Ω B 1.5 Ω C 5.0 Ω D 6.0 Ω
Answer: B
The Physics Behind
- Resistance is potential difference divided by current.
- Therefore, the above resistance can be calculated as 3.0 V / 2.0 A.
- The answer is B.
14 A copper wire X has resistance R. Another copper wire Y has twice the length and half the cross- sectional area of X.
Answer: D
The Physics Behind
- Resistance is directly proportional to length l.
- Resistance is inversely proportional to area A.
- Since wire Y has twice the length and half the area, then its resistance is 4 times that of X.
15 An electrical appliance is plugged into a socket in the wall. The plug contains a fuse.
What is the main purpose of the fuse?
A to earth the appliance
B to earth the plug
C to protect the user from electric shock
D to protect the wiring from overheating
Answer: D
The Physics Behind
- The fuse is in the live wire of appliances.
- If there is a fault like the live wire includes the casing of the appliance, a large current will flow through the live wire which breaks the fuse.
- When the fuse breaks, there is no more current flows through the device.
- This removes the risk of getting electric shock.
- However, the fuse in question [in the above] is the fuse in the plug which is in the wall.
- In this case, its function is to break when the current passing through is too high.
- When the current is too high, heating will happen and could cause fire.
- So the answer is D instead of the purpose of fuse installed in appliances given in C.
16 After some building work in a house, a bare [uninsulated] live wire is left protruding from a wall.
What is the greatest hazard?
A a fire
B a fuse will blow
C an electric shock
D no current will flow
Answer: C
The Physics Behind
- Charges flow [current] between two points with potential difference.
- A bare live wire has a potential.
- The ground [Earth] has zero potential.
- A person has his/her feet at zero potential. If that person touches the bare live wire, there is a potential difference across his/her body. Thus, current passes through him/her and gets electric shock.
17 A water heater is connected to a 230 V supply and there is a current of 26 A in the heater.
It takes 20 minutes to heat the water to the required temperature.
How much energy is supplied by the heater?
A 6.0 × 103 J
B 1.0 × 104 J
C 1.2 × 105 J
D 7.2 × 106 J
Answer: D
The Physics Behind
- Power is the rate of change in energy.
- In electricity, power can be derived to be the product of current and potential differencce.
- Combining these 2 definitions/equations,
P = E / t and P = IV
E / t = IV
E = IVt
E = (26 A) (230 V) (20 x 60 s)
E = 7.2 × 106 J
18 Using the circuit shown, the current I is found for various voltages V. The temperature of the resistor does not change.
Which graph shows the results obtained?
Answer: A
The Physics Behind
- The circuit looks like one used to investigate Ohm's Law.
- Ohm's Law states that current is directly proportional to voltage.
- This relationship, when graphed, gives a straight line passing through the origin.
19 There is an electric current in a straight wire in the direction into the page. This produces a magnetic field around the wire.
All the field lines are circles but only one field line is shown.
Answer: D
The Physics Behind
- A current-carrying wire generates a magnetic field around it.
- The magnetic field is presented using field lines where
- the direction of field is given by the direction of field lines [note: use right hand rule i.e. thumb pointing direction of conventional current, 4 fingers give direction of the generated magnetic field],
- the magnitude of magnetic field is given by the spacing between field lines [note: the more lines there are in a specific area, the greater rthe magnitude].
- So the answer is D.
20 The diagram shows a wire placed between two magnetic poles of equal strength.
A current passes through the wire in the direction shown. The current causes a downward force on the wire.
What is the arrangement of the magnetic poles?
Answer: A
The Physics Behind
- Since a current-carrying wire generates its own magnetic field, it should be affected by permanent magnets placed close to it.
- The effect is determined using Fleming's Left Hand Rule [a.k.a. motor effect] where
- the force on the current-carrying wire is in the direction given by the thumb,
- the current through the wire is in the direction given by the middle finger, and
- the direction of magnetic field of the external magnet is in the direction of the index finger.
- In the diagram,
- the current [middle finger] points out of the page/screen,
- the force [thumb] point downwards,
- the direction of field [index finger] therefore is to the left.
- A field going to the left implies that the pole of the magnet at left is an S pole while the one at the right is an N pole.
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