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Physics > Electricty and Magnetism > Flashcards

Electricty and Magnetism Flashcards

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1
Q

State the unit for charge and the amount of charge carried by an electron.

A
  • Charge is measured in Coulombs.
  • The amount of charge carried by an electron is 1.6×10^-19
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2
Q

State the law of electrostatics.

A

Unlike charges attract and like charges repel.

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3
Q

What is an electric field?

A

An electric field is region in which an electric charge experiences a force.

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4
Q

State what happens when silk cloth and glass rod are rubbed against each other.

A
  • Negative charges are transferred from the glass rod to the silk cloth.
  • Glass rod becomes positively charged and silk cloth becomes negatively charged
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5
Q

Define current and its SI unit.

A
  • Current is the rate of flow of electric charge.
  • Amperes, A
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6
Q

Formula for current.

A

I (current) = Q (coloumbs) ÷ t (seconds)

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7
Q

Define potential difference and its SI unit.

A
  • Potential difference defined as thework doneto driveunit chargethrough thecomponent.
  • Volts, V
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8
Q

Formula for potential difference.

A

V = W (work done) ÷ Q (coloumbs)

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9
Q

Define electromotive force and its SI unit.

A
  • The electromotive force is thework doneby the source in drivingunit chargearound a complete circuit.
  • Volts, V
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10
Q

Formula for electromotive force.

A

V = W (work done) ÷ Q (coloumbs)

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11
Q

Define resistance and its SI unit.

A
  • The resistance defined as the ratio of the potential difference across a component to the current flowing through the component
  • Ohms, Ω
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12
Q

Formula for resistance.

A

R = V (volts) ÷ I (current)

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13
Q

State the properties of wire that increases resistance

A
  • Longer length of wire
  • Higher resistivity
  • Smaller cross-sectional area
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14
Q

Formula for resistance of a wire.

A

R= (L (length) × ρ (resistivity)) ÷ A (cross-sectional area)

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15
Q

What equipment to measure current in a circuit?

A

Ammeter or Digital multi-meter

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16
Q

What equipment to measure e.m.f and p.d in a circuit?

A

Voltmeter

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17
Q

What is the net e.m.f of cells in series circuits.

A

The net e.m.f is the sum of their individual e.m.f.

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18
Q

What is the net e.m.f of cells in parallel circuits.

A

The net e.m.f is the same as the individual e.m.f.

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19
Q

What is e.m.f of cell that is placed opposing electron flow?

A

It has negative e.m.f.

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20
Q

State ohm’s law.

A

Ohm’s Law states that the current through a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions (such as temperature) remain constant.

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21
Q

State the properties of an I-V graph of a ohmic conductor.

A
  • The graph is a straight line. (constant gradient)
  • The graph cuts through the origin.
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22
Q

State the properties of an I-V graph of a non-ohmic conductor.

A
  • Graph is ** a curve and not a straight line**
  • The graph cuts through the origin.
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23
Q

How does an increase in temperature of wire lead to an increase in resistance?

A

When temperature increases, metal ions in the conductor will move more vigorously. The moving electrons collide with the metal ions more frequently, slowing down their drift and lowering the current, hence increasing the resistance

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24
Q

Define transducer

A

Atransduceris an electrical device that converts energy from one form to another.

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25
Q

Define semiconductor diode

A

A semiconductor diode is a device that allows current to flow in one direction only.

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26
Q

What are heating elements usually made out of and why.

A
  • These appliances have heating elements that are made of nichrome, an alloy of nickel and chromium.
  • Nichrome is used because it has a high resistivity and can withstand high temperature.
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27
Q

Define Power

A

Power is the rate of energy transfer

28
Q

How does fire hazards occur?

A

Electrical fires are caused byoverheatingof electrical components when an excessivelylarge currentpasses through the electrical circuit which increases temperature.

29
Q

How does electrocution occur?

A

An electric shock occurs when part of a person’s body is incontactwith ahigh-voltage sourcesuch as an exposed“live”electrical component or wire

30
Q

How can a damp environment cause electrocution?

A

Wet skin has lower resistance than dry skin. If a person accidentally touches a ‘live’ component with wet skin, a larger current can flow through his/her body and cause electrocution.

31
Q

State the colour, voltage and current of each wire in the plug.

A
  • Live (Brown) 240V, Has current
  • Neutral (Blue) 0V, Has current
  • Earth (Green and Yellow) 0V, has no current
32
Q

How does the fuse function as a safety device?

A

If excessive current flows through it, the fuse blows (a metal wire inside the fuse melts to open the circuit), thus stopping the current flow.

33
Q

How does the circuit breaker as a safety device?

A

Circuit breakers are safety devices that operate automatically to cut off the electrical supply when excessive current flows through them. In the event of an overload or short circuit or when current exceeds circuit breaker’s rating, a circuit breaker will trip to cut off the flow of current and disconnect appliance from the live wire.

34
Q

What is the law of magnetism.

A

Like poles repel, unlike poles attract.

35
Q

What is the north pole of a magnet?

A

The end of a magnet that points to the Earth’s Geographical North Pole when fully suspended in air.

36
Q

Define magnetic field

A

Magnetic field is a region in which the force of magnetism acts.

37
Q

How can permanent magnets be demagnetised?

A

Magnets can be demagnetized by hammering or dropping repeatedly, heating at high temperature, passing a.c current through coil of wire.

38
Q

What is soft magnetic material and its uses?

A
  • gain and lose its magnetism easily (e.g iron)
  • Used as core of electromagnet
39
Q

What is hard magnetic material and its uses?

A
  • harder to magnetise and demagnetise (e.g steel)
  • Good for making permanent magnets
40
Q

Why are fuse arrange in series not parallel?

A

So the current flowing through fuse is equal to current flowing throught the whole circuit

41
Q

Why are a.c current used for transmission of electricity

A

Only a.c. is used for electricity transmission because it can be easily converted to a high voltage (and hence low current for the same power transfer) using step-up transformers

42
Q

Why is electricity not transmitted with a low voltage?

A

A high current flowing through transmission cables will cause high transfer of energy to the internal store (P = I^2R). Therefore efficiency decreases

43
Q

What is a.c current

A

A.C current is an electric current that changes its direction of flow with time

44
Q

Explain how a circuit breaker works

A

When the current exceeds its rated value, the solenoid becomes a stronger electromagnet and exerts a large enough magnetic force to attract the soft iron latch, releasing the spring which pushes the safety bar outwards. The interrupt point is now open, creating an open circuit and the circuit is broken (switched off).

45
Q

How does split-ring commutator ensure that the rotation continues?

A
  • When the current is flowing from A to B, a downward force is exerted on side nearest to N- Pole.
  • As the coil rotates to the vertical position, current no longer flows in the coilas the split ring is not connected to the carbon brushes therefore no magnetic force is acting on sides of coil,
  • Coil continues to move past vertical position due to momentum and inertia.
  • When the coil moves past the vertical position, the split ring is connected to the carbon brushes again but the direction of current changes and now is flowing from B to A.
  • This cause an upward force to always be exerted near the north pole causing the coil to continously rotate
46
Q

How can turning effect of d.c motor be increased?

A
  • Inserting a soft iron core into the coil (is a soft magnetic material, thus it can strengthen the magnetic field around the coil and also respond quickly to changes in magnetic field
  • Increasing the number of turns in the coil
  • Increasing the current in the coil
47
Q

State Faradays Law

A

Faraday’s Law of electromagnetic induction states that the magnitude of the induced e.m.f in any closed circuit is directly proportional to the rate of change of the magnetic flux through the circuit.

48
Q

State Lenz law

A

Lenz’s Law states that the direction of the induced e.m.f. (and hence the direction of the induced current) in a closed circuit is such that its magnetic effect always opposes the change causing it. .

49
Q

State the factors that affect the direction and magnitude of the deflection of the galvanometer needle:

A
  • the speed of the magnet
  • the strength of the magnet
  • the number of turns of the coil
  • the direction of motion of the magnet with respect to the coil
  • the polarity of the magnet
  • the orientation of the magnet with respect to the Earth’s poles
50
Q

State when voltage of a.c generator is maximum and zero.

A
  • The output voltage ismaximumwhen the coil becomeshorizontal. At this position, the rate of change of magnetic flux through the coil is thelargest.
  • The output voltage iszerowhen the coil becomesvertical. At this position, the rate of change of magnetic flux through the coil is thesmallest.
51
Q

Why is structure of transformer core made of soft iron

A
  • The transformer core is made of soft iron as it can beeasily magnetised and demagnetised (can link changing magnetic field between the coils )
  • The iron core helps to ‘concentrate’ the magnetic field, increasing magnetic flux linkage between the magnet and the coil. Hence, increasing the induced e.m.f
52
Q

What is alternating voltage

A

alternating voltage is when the positive and negative terminals of electrical source switches at regular intervals

53
Q

State function of split ring commutator and how it ensures to continous rotation.

A

Function of a split-ring commutator is to reverse the direction of the current in the coil every half a revolution so that the coil continues to turn in the same direction. This occurs whenever the commutator changes contact from one brush to the other ensuring that the forces on each coil will act in the opposite directions.

54
Q

Function of slip rings

A

The slip rings to provide an electrical contact between carbon brushes and the coil

55
Q

How can magnitude of induced currents in a.c generator be increased?

A
  • increasing the number of turns in the coil (this will increase the magnitude of the output voltage);
  • using stronger permanent magnets (this will increase the magnitude of the output voltage);
  • increasing the frequency of rotation (this will increase the magnitude and the frequency of the output voltage);
  • winding the coil around a soft iron core to increase the magnetic flux linking the coil (this will increase the magnitude of the output voltage).
56
Q

How does current in the primary coil produce an output current in the secondary coil?

A
  • The alternating current connected to the primary coil produces a changing magnetic field in the soft iron core.
  • The changing magnetic field is linked to the secondary coil through the soft iron core.
  • An e.m.f. is induced in the secondary coil due to the changing magnetic field and the change in magnetic flux through coil according to Faraday’s law of electromagnetic induction.
  • Since the secondary coil is a closed circuit, induced current flows in the secondary coil.
57
Q

Why is the transformer core made out of laminated thin sheets.

A
  • The transformer core is madeoflaminated thin sheets as reduces energy transfer to the internal store of the core.
    • The lamination increases the resistance of the soft iron core and prevents unwanted induced current (also known as eddy currents) from flowing in the core which increases the temperature of core
58
Q

Why are safety devices connected to live wire?

A

All safety devices are always connected to live wire to disconnect appliances from the live wire and electrical mains preventing electrocution if touched or fire hazards

59
Q

How does a longer length and smaller cross sectional area affect resistance of wire in terms of electrons?

A
  • Longer the length of wire, more collisions the electrons will make with its atom, the higher the resistance
  • Smaller cross-sectional area, the smaller number of electrons can flow through it, the higher the resistance
60
Q

Explain why the coil turns when there is current flowing in the coil in a motor

A

Current flowing in the coil produces a magnetic field around the coil. The magnetic field around the coil interacts with the external magnetic field of the magnet and produces a force.

61
Q

Why do filament lamps not obey Ohms Law?

A

Resistance of lamps increases when these lamp heats up therefore resistance is not constant

62
Q

How are thunders formed?

A
  • Thunderclouds are charged by friction between water molecules in the thunderclouds and air molecules in the atmosphere.
  • Negative charges accumulate at the bottom of the clouds repelling the electrons near the surface of the Earth. Making suurface of earth positively charged.
  • When the accumulation of charges is large, the air particles nearby are ionised providing a conducting path to the earth.
  • This thunder can damage buildings, cars and harm people
63
Q

How can electrostatic electricity damage equipment

A
  • Excessive charges may build up on electronic equipment by friction.
  • Electronic equipment, such as computer boards and hard drives, can be easily damaged by electrostatic discharge. Such equipment are usually packed in antistatic packaging.
64
Q

What is the formula for power lost to wire and power to lamp

A
  • P=I^2R
  • P=V^2/R
65
Q

Why is fuse connected to live and not neutral wire?

A

The fuse as connected on the ‘live’ wire so that when it blows due to excessive current,the appliance will be disconnected before current reaches the appliance

66
Q

In what situation will a fuse blow in a electrical appliance wiring?

A
  • Live wire touches earth wire or nuetral wire
  • Live wire touches metal casing
  • As long it is a short circuit.

Physics (7 decks)

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