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Physics > 14 Electric Current > Flashcards

14 Electric Current Flashcards

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1
Q

When does an electric current exist?

A

When charge particles move

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2
Q

Define electric current.

A

Electric current is defined as the rate of flow of charge.

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3
Q

State the SI unit of electric current.

A

Ampere A (one of the seven base units of physical quantities)

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4
Q

Distinguish between average electric current I and instantaneous current I.

A

The average electric current I is equal to the net charge that passes through the area per unit time where I ave = Change in Q (amount of charge passing through the surface in time t)/ time t while the instantaneous current I is defined as the differential limit of the average current = dQ/dt.

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5
Q

What is an electric charge?

A

Electric charge is a fundamental property of matter which causes a charged object to experience a force when placed in an electric field.

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6
Q

State the formula to calculate charge Q.

A

When a constant current flows through a cross-section of a conductor for a duration of t, the amount of
electrical charge Q passing through it is given by Q = I*t

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7
Q

State the SI unit for electric charge.

A

coulomb, C

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8
Q

Define one coulomb / 1C.

A

One coulomb is defined as the amount of electrical charge that passes through a point in one second when there is a constant current of one amphere.

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9
Q

State the units for one coulomb.

A

I C = 1 As

1 mAh = 3.6C

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10
Q

Define the direction of current.

A

By convention, the direction of current is defined as the direction of flow of positive charge, regardless of the actual sign of the charged particles in motion.

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11
Q

What are moving charged particles often referred to as?

A

Mobile or free charge carriers

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12
Q

Define drift velocity, V D.

A

When there is an electric current in a conductor, the free charge carriers in the conductor move with an average velocity in the order of 10^-4 m/s. This is called the drift velocity.

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13
Q

Explain why drift velocity is an average velocity.

A

Electrons do not move in a straight line through the conductor, they will change directions and its speeds as it moves through the conductor due to collisions with other electrons or ions within the conductor. Hence, the drift velocity VD is an average velocity.

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14
Q

Derive the equation for a current-carrying conductor which relates current, the number of charge carriers per unit volume, cross-sectional area of the conductor, drift velocity and charge of a charge carrier.

A

Pg 5 of notes

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15
Q

Explain why a light turns on almost immediately when the swtich is turned on even though drift velocity is so slow.

A

The electric field travels through the wire with a speed close to that of light. Once the light is switched on, the electrons experience the electric field almost instantaneously and begin their bulk movement immediately.

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16
Q

Define potential difference V.

A

The potential difference V between two points in a circuit is defined as the amount of electrical energy per unit charge that is converted to other forms of energy when charge passes from one point to another.

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17
Q

State the SI unit of potential difference.

A

volt, V or J/C

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18
Q

State the formula used to calculate potential difference V.

A

V = W/Q

where W is the electrical energy converted to other forms of energy in J and Q is the net amount of charge that passes from one point to another in C.

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19
Q

Define one volt / 1V.

A

One volt is the potential difference between two points in a circuit when one joule of electrical energy is converted to other forms of energy as one coulomb of charge passes from one point to another.

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20
Q

How can the electrical energy that is converted to other forms in a part of a circuit be calculated?

A

If a charge flows in a part of a circuit across which there is a potential difference V, the electrical energy converted to other forms of energy in that part of the circuit is given by W = QV

21
Q

Define the electromotive force of a source.

A

The electromotive force (e.m.f.) of a source is defined as the amount of electrical energy per unit charge that is converted from other forms of energy when the charge passes through the source.

22
Q

Explain why a source of electromotive force is needed.

A

When electric current flows through conductors in a close circuit, electrical energy is continuously dissipated as heat or other forms of energy. In order to sustain this constant flow of electric current, a source of electromotive force is needed. They can be likened to a “charge pump” in which electrical energy is converted FROM chemical, mechanical or other forms of energy.

23
Q

State the formula used to calculate the electromotive force (e.m.f.) of a source.

A

E = W/Q

24
Q

State the SI unit of electromotive force (e.m.f.) of a source.

A

volt, V (similar to potential difference due to energy per unit charge)

25
Q

What is an ideal battery?

A

An ideal battery maintains a constant p.d between its terminals to ensure a constant current flow.

26
Q

Distinguish between potential difference V and electromotive force (e.m.f.) of a source.

A

Potential difference between two points in a circuit is defined as the amount of electrical energy that is converted to other forms of energy when a unit charge passes from one point to another (V = W/Q)

Electromotive force of a source is defined as the amount of electrical energy that is converted from other forms of energy when the source drives unit charge through a complete circuit (E = W/Q where W is the energy converted from other forms of energy to electrical energy)

27
Q

Define resistance.

A

The resistance of a circuit component is defined as the ratio of the potential difference across it to the current flowing through it.

28
Q

State the formula used to calculate electrical resistance.

A

R = V/I

29
Q

State the SI unit for resistance.

A

ohm, Ω or V/A

30
Q

Define one ohm.

A

One ohm is the resistance of a circuit component when a potential difference of one volt across it causes a current of one ampere to flow through it.

31
Q

State ohm’s law.

A

Ohm’s law states that the potential difference across a conductor is proportional to the electric current passing through it, provided that its temperature remains constant.

32
Q

What are ohmic materials?

A

Materials that obey Ohm’s law. Experimentally, it has been found that if the temperature of a material conductor is kept constant, its resistance remains constant.

33
Q

What are non-ohmic materials?

A

Materials that do not obey Ohm’s law.

34
Q

What is the constant of proportionality in Ohm’s law?

A

Resistance of the conductor (V=IR)

35
Q

State the formula used to calculate the resistance of a uniform conductor at a given temperature (conductor that is made of a single type of material).

A

Since the resistance of a uniform conductor is directionally proportional to its length l and inversely proportional to its cross-sectional area A, R = roe l/A where the constant of proportionality roe is called the resistivity of the material.

36
Q

State the SI unit for resistivity.

A

ohm metre

37
Q

Distinguish between resistance and resistivity.

A

Resistance is the property of an object while resistivity is the property of the material.

38
Q

Describe the I-V characteristic for an ohmic conductor.

A

It is a straight line graph through the origin where any increase in potential difference produces a proportionate increase in current and the ratio V/I is constant. Thus, resistance is constant. (Temperature can be kept constant by immersing the conductor in a constant temperature bath)

(Note: Resistance is the ratio of V and I, not the reciprocal of the gradient of the I-V graph.)

39
Q

What is a diode?

A

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

40
Q

Draw the circuit symbol for a diode.

A

Pg 12 of notes.

41
Q

Describe the current flow through a diode when it is in reverse-bias and forward-bias respectively.

A

When it is in reverse-bias, the current through a diode is very small as the resistance is very high (I = a few microamperes). When it is in forward-bias, the current through a diode increases very rapidly when the potential difference V is greater than 0.7V as for every increase in V, current increases to a large extent and thus, resistance is very low.

42
Q

Describe the effect of temperature on non-ohmic conductors.

A

As current increases, more charge carriers move through the conductor per unit time,resulting in more collisions with the lattice ions. This increases the temperature of the conductor, which affects its resistivity in two ways.

  1. Electrons gain enough energy to break free, which results in an increase in the number density of charge carriers, increasing conductivity.
  2. Lattice ions gain thermal energy and start to vibrate faster with greater amplitude. This causes more collisions between the charge carriers and the lattice ions which in slow slows down the movement of the charge carriers, increasing resistivity.
43
Q

Describe how the resistivities and conductivities of non-ohmic conductors - filament lamps change due to an increase in their temperatures.

A

In non-ohmic conductors such as filament lamps,

  1. Electrons are already free and mobile at room temperature. As temperature increases, there is no appreciable increase in the number of conducting free electrons.
  2. Frequency of collision between lattice ions and electrons increases and the movement of charge carriers slow down.

As a result, resistivity increases more than conductivity.

As current increases, temperature increases, resistivity and hence resistance increases, causing the ratio of V/I to increase and the I-V curve becomes more gentle.

44
Q

Describe how the resistivities and conductivities of non-ohmic conductors - thermistors (NTC) change due to an increase in their temperatures.

A

In non-ohmic conductors such as thermistors, these semiconductors are in general poor conductors at relatively low temperatures due to the small number of free electrons compared to that in metals.

  1. As temperature increases, more electrons acquire energy to break free from their atoms and the number of charge carriers increase significantly.
  2. The frequency of collision between lattice ions and electrons increases and the movement of charge carriers slow down.

As a result, conductivity increases more than resistivity.

As current increases, temperature increases, resistivity and hence resistance decreases, causing the ratio of V/I to decrease and the I-V curve gets more steep.

45
Q

Define electrical power P.

A

Since energy is conserved, a change in electrical potential energy suggests that there is a conversion of energy from one form to another. The rate at which the conversion takes place is that power P.

46
Q

State the formula used to calculate power P.

A

P = W/t = QV/t = IV

47
Q

State the units for power P.

A

The S.I. unit for power is the watt, W (1W = 1Js^-1)

48
Q

State the formula used to calculate the power supplied by an ideal source.

A

An ideal battery maintains a constant source of e.m.f. and the work done by the source is W = QE. Hence, the power supplied by the source P = W/t = QE/t = IE

49
Q

State the formula used to calculate the power dissipated by a resistor.

A

The potential difference across a resistor is V = IR.
The rate at which energy is dissipated across a resistor is P = IV or P = I^2R = V^2/R

Physics (14 decks)

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