Electricity

Question 1

Define Current

Answer 1

Electric current is the rate of flow of charge. S.I. unit the ampere, A.

Question 2

State the difference between the direction of conventional current and electron flow.

Answer 2

Conventional current flows from the positive terminal of a cell or battery around the circuit to the
negative terminal, whereas negatively charged electrons flow from the negative terminal around the
circuit to the positive terminal, i.e. in the opposite direction to the conventional current.

Question 3

Explain what is meant by the term Number density of charge carriers, n

Answer 3

The number density of charge carriers is the number of free charge carriers (free electrons, in
metals) per unit volume. SI unit m-3

Question 4

Explain why the term ions describes particles that form a crystal in a metal better than the term
atoms.

Answer 4

Atoms have lost electrons to a “free electron sea” and have now become positively charged ions.

Question 5

Describe what is meant by the term drift velocity of free electrons in a metal wire.

Answer 5

The drift velocity of free electrons in a metal wire is the average displacement of the free electrons
along the wire per second.

Question 6

Compare the difference in the movement of free electrons in a metal before and after applying a
potential difference across it.

Answer 6

Before a p.d. is applied free electrons move randomly in all direction at very high speeds (106 ms-1
)
with a zero mean velocity. When a p.d. is applied free electrons are accelerated by the p.d. and now
have a drift velocity of small magnitude (10-3
-10-4 ms-1
).

Question 7

Explain in terms of free electrons how energy is transferred in a conductor when a potential
difference is a applied across it.

Answer 7

Free electrons collide with the positive metal ions in the lattice. The kinetic energy of the free
electrons is transferred to vibrational energy of the lattice, or kinetic energy of the free electrons is
transferred to thermal energy in the metal.

Question 8

Explain in terms of free electrons why there is no energy transfer in superconductors.

Answer 8

In superconductors there are no collisions between free electrons and metal ions and as such there is
no energy transfer.

Question 9

Describe what is a superconductor.

Answer 9

A superconductor is a material with zero resistance.

Question 10

Describe what is the transition (critical) temperature.

Answer 10

Transition temperature is the temperature at which the resistance of a material suddenly drops to
zero and stays at zero below that temperature.

Question 11

Explain in terms of free electrons how electrical resistance arises in metal conductors.

Answer 11

When a P.D is applied, free electrons are accelerated and collide with positive ions in the metal
lattice.

Question 12

Explain in terms of free electrons how temperature affects electrical resistance in metal
conductors.

Answer 12

The greater the temperature, the greater the vibrational energy of the positive ions in the metal
lattice (or positive ions in the metal lattice vibrate with larger amplitude). This produces a higher
rate (or frequency) of collisions with free electrons.

Question 13

Explain in terms of free electrons how potential difference can change the temperature of a
metal conductor (Joule heating).

Answer 13

With greater potential difference, free electrons gain higher kinetic energy. Free electrons transfer
kinetic energy to vibrational energy of the lattice, or kinetic energy of the free electrons is
transferred to thermal energy in the metal.

Question 14

Explain in terms of free electrons how potential difference can change the electrical resistance of
a metal conductor (Joule heating).

Answer 14

This produces a higher rate (or frequency) of collisions with free electrons and as such limits their
drift velocity, giving rise effectively to higher resistance.

Question 15

Explain in terms of free electrons the shape of the I-V characteristic of a filament lamp (Joule
heating).

Answer 15

As potential different increases, more energy is transferred from kinetic energy in the free electrons
to thermal in the metal lattice. This causes metal ions to vibrate with a greater amplitude, limiting
the drift velocity of free electrons. Because drift velocity is directly proportional to the current (and
number density of free electrons stays constant) this effect limits the rate of increase of current with
potential difference. This is why resistance increases as potential difference increases.

Question 16

Explain how the I-V graph for a lamp (or LED) shows that it does not obey Ohm’s law

Answer 16

For an ohmic conductor current is proportional to p.d. and so the I-V graph is a straight line
through (0,0). The graph line for a lamp (or LED) in not straight and so a lamp (or LED) does not
obey ohm’s law.

Question 17

Explain how and why the resistance of a filament lamp varies with the current flowing
through it.

Answer 17

• The current passing through a filament lamp produces a heating effect: P = I2R
• As the p.d. across the filament lamp increases the current increases.
• This leads to more frequent collisions between free electrons and lattice ions
• This leads to an increase in the amplitude of vibrations of the lattice ions.
• Therefore, as the current increases the temperature of the filament lamp increases.
• Therefore, as the temperature increases the resistance of the filament lamp increases.
• Therefore, as the resistance increases the rate of increase of current with p.d. decreases and
  so the I-V graph line becomes less steep.

Question 18

Explain how the I-V characteristics show that the resistance (or resistivity) of a thermistor
(or semiconductor material) decreases with temperature.

Answer 18

• The current produces a heating effect: P = I^2 R
• As the p.d. across the thermistor increases the current increases.
• Therefore, as the current increases the temperature of the thermistor increases.
• As the temperature increases the resistance of the thermistor decreases because the number
  density of charge carriers increases.
• Therefore, as the resistance decreases the rate of increase of current with p.d. increases and
  so the I-V graph line becomes steeper.

Question 19

Define electromotive force

Answer 19

The e.m.f. of any source of electrical energy is defined as the energy transferred from other forms
into electrical energy per unit charge which passes through it and around a complete circuit. S.I.
unit the volt, V.

Question 20

Define Potential difference

Answer 20

The potential difference across a component (resistor, lamp, etc.) is the electrical energy transferred
into other forms per unit charge passed through the component. S.I. unit the volt, V.

Question 21

Define Resistance

Answer 21

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

Question 22

Explain the meaning of the term Internal Resistance

Answer 22

Some energy is transferred into thermal energy in driving charge through a cell (or battery). The cell
behaves as if it has an internal resistance. Consequently, there is a decrease in the voltage measured
across the cell terminals when a current flows through it. S.I. unit the ohm, .

Question 23

In the equation V=E-Ir explain what V and Ir represent (for E, see Emf above):

Answer 23

V is the terminal p.d., or the energy per unit charge delivered to the external circuit. Ir is the p.d.
across the internal resistance or the energy per unit charge dissipated (or lost) in the internal
resistance.

Question 24

State Ohm’s Law

Answer 24

The current through an ohmic conductor is proportional to the p.d. applied across it, provided that
the temperature and other physical conditions remain constant. This means that the I-V graph is a
straight line passing through (0,0).

Question 25

Define resistivity of a material

Answer 25

The resistivity,  , of a material, at a given temperature, is given by the equation: rho = A/L x R where R = resistance of a sample of the material L = Length of material A = Cross-sectional area of material

Question 26

State Kirchhoff’s 1st law

Answer 26

The sum of the currents flowing out of a junction of conductors is equal to the sum of the currents flowing into the junction. Kirchhoff’s first law is in effect a statement of conservation of charge.

Question 27

State Kirchhoff’s 2nd law

Answer 27

In any closed conducting loop, the sum of the e.m.f’s is equal to the sum of the p.d.’s. Kirchhoff’s second law is in effect a statement of conservation of energy.

Question 28

Define Power

Answer 28

Power is work done (or energy transferred) per unit time. Or Power is the rate of transfer of energy. S.I. unit the watt, W.

Question 29

Define electrical power

Answer 29

The rate at which electrical energy is converted or transferred into other forms.

Question 30

Explain why the p.d. across the terminals of a cell (or battery), through which a current flows is not the same as the e.m.f.

Answer 30

A cell (or battery) has an internal resistance. Some of the e.m.f. is across the internal resistance leaving a smaller p.d. across the terminals of the cell.

Question 31

Describe briefly two advantages of using a data logger to monitor the light intensity falling on a Light Dependent Resistor (LDR).

Answer 31

A data logger can: - record continuously for a very long time scale of observation. - record very short time scale signals at intervals. - automatically record and remote sense. - feed data directly to computer for analysis.

Question 32

Explain what is meant by charge is quantised.

Answer 32

The net charge on a particle, or an object, is a multiple of e, 1.6 ×10-19C.

Question 33

Explain why a potential divider circuit is more suitable for obtaining the I-V characteristics of and LED than a variable resistor circuit.

Answer 33

For a potential divider the voltage range is from zero to maximum possible (the supply voltage)  For a variable resistor circuit, the resistance variation is small.  For a variable resistor circuit, the voltage variation across LED is too small.

Question 34

Explain the purpose of a fuse in a circuit

Answer 34

- The fuse contains a piece of wire that melts at a relatively low temperature. - If the current passing through the fuse is too great, the wire heats up until it melts and breaks the circuit.