16 — Electricity & magnetism

Question 1

Formulas

Answer 1

I = Q/t
(Current A = charge C/ time taken s)

Q = no. Of charge x charge on each electron

EMF = W/Q

V = W/Q

R = V/I

R=kL/A

R = pl/A

Question 2

1 charge

Answer 2

Charge of 6.25 x 10^18 protons
Each charge estimated 1.60 x 10^-19C

Question 3

Ohm’s law

Answer 3

States that the current passing thru a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions remains constant.

Question 4

Electric current

Answer 4

Rate of flow of electric charge

Question 5

Electric circuit

Answer 5

A closed path connected w metal wires usually made from copper. It comprises a source such as battery, and a network of electrical components such as light bulb.

Question 6

Emf

Answer 6

The electromotive force of an electrical source is the work done by the source in driving a unit charge around a complete circuit

Scalar quantity

Question 7

Electron flow

Answer 7

Movement of electrons in the electrical circuit

Question 8

Potential difference

Answer 8

The pd across a component in a circuit is the work done per unit charge in driving charges thru the component.

Question 9

Resistance

Answer 9

Resistance R of an electrical component is the ratio of the potential difference V across the component to the current I flowing thru the component.

Old version: measure of the extent of difficulty for an electric current to pass thru a material

Question 10

Resistor

Answer 10

A resistor is a conductor or an insulator that has high resistance. It is used in circuit to control the amount of current.

Question 11

Why the battery goes flat

Answer 11

Electrochemical reactions in batt hv depleted the materials that produce these reactions

Question 12

EMF vs pd

Answer 12

EMF: the work done to move each unit charge thru complete circuit
Pd: the work done by each unit charge passing thru the components

EMF: present even when no current is drawn from source
Pd: across any electrical component is 0 in absence of current

Question 13

Examples of variable resistors

Answer 13

Rheostat
Potentiometer

Question 14

Resistance vs resistivity

Answer 14

Nature:
E: the degree of opposing force which an electric current experiences when it flows thru the component

Y: resistance of a material for a unit area per unit length

Dependency:
E: depends on
1. Shape
2. Tempt
3. Resistivity of material
-> longer, more collisions electron make w its particles, R directly proportionate to length
-> longer cross-sectional area, greater no. Of e- can flow thru, R inversely proportionate to area

Y: independent of shape n size of material, affected by
1. Length
2. Area
3. Resistivity of material (constant)
-> higher resistivity -> poorer conductor of electricity -> generate a lot of heat -> heating purposes

SI:
E: ohm
Y: ohm metre

E: resistance of most metals increases linearly w tempt

Question 15

Why resistance of most metals increases linearly w tempt (ohmic conductors)

Answer 15

As tempt increases, metallic ions vibrate more vigorously abt fixed positions -> increasing no. Of collisions betw free electrons and metallic ions opposing electron flow

Question 16

Ohmic characteristics

Answer 16

1. linear rs betw pd V and current
2. Passes thru origin (I/V graph)
3. 1/m(gradient) = resistance

Question 17

Examples of non-ohmic conductors

Answer 17

Diodes -> semiconductor diode
Negative tempt coefficient (NTC) thermistors
Filament lamp

Question 18

Filament lamp

Answer 18

I-V graph: mem
R increases at increasing rate
Desc: electric current increases -> energy transferred from cps of batt to i.s. Of filament increases -> increasing R of filament, limiting rate of increase of current

Question 19

Semiconductor diode

Answer 19

I-V graph: mem
Desc:
when +ve voltage applied to anode of diode, large current flows. R in forward direction
When current flows in reverse direction, current is v low, alm 0 bcos R in reverse direction is v high, alm infinite.

Recall: diode is a device that allows current to flow in only 1 direction.

Question 20

Circuit symbol of a diode and a thermistor

Answer 20

T: hockey thing
Diode: triangle

Question 21

Determining the resistance of an unknown resistor of low value

Answer 21

Apparatus:
Ammeter, voltmeter, cell, switch, fixed resistor, rheostat

Diagram:
Ammeter in series, voltmeter in parallel to resistor

Procedure:
1. Set up the apparatus according to the circuit diagram
2. Adjust the variable resistor to allow the smallest possible current to flow in the circuit.
3. Note the ammeter reading (I) and the voltmeter reading (V).
4. Adjust the variable resistor to allow a larger current to flow in the circuit, noting I and V readings
5. Repeat the above for 5 sets of I and V readings
6. Plot a graph of V against I and determine the gradient of the graph.

Table:
Current (A) | Voltage (V)

Sketch of graph (V against I)

Question 22

Explain why the current in lamp Q (series) is larger than the current in lamp P(parallel).

Answer 22

As lamp P is parallel to resistor R, the total current is split into each pathway. The current in lamp Q is the sum of the current in resistor R and the current in lamp P, thus the current of lamp Q is larger than the current in lamp P.

Question 23

Explain why lamp Q (series) has a different resistance from lamp P (parallel) although they r identical lamps

Answer 23

Filament lamps are non-ohmic conductors. Since lamp Q is connected in series, Q’s resistance is smaller than P thus a larger current flows thru Q than P, which is connected in parallel. As more current flows into the filament, the rate of energy transfer increases, increasing the amount of energy in the internal store. Q’s resistance increases due to the increase in tempt of the filament.

Question 24

Explain why a current smaller than xA over a fixed resistor is not obtained.

Answer 24

The variable resistor is made of a coil of resistance wire that has a max length. A contact is placed along the length to provide a varying resistance in the circuit. There is a maximum length and hence a max resistance it can provide. When the max length is involved, the current will be of the smallest value of xA and cannot provide more resistance to reduce the current further.

Question 25

State how figure shows that the filament lamp does not obey Ohm’s law.

Answer 25

The current flowing thru the filament lamp does not increase proportionally w the pd. Thus, it does not obey Ohm’s law.

Question 26

State why does the filament lamp not obey Ohm’s law

Answer 26

As the current increases, the energy in the internal store of the filament increases, and thus its tempt increases. As tempt increases, the resistance of the filament lamp increases. Thus, it does not obey Ohm’s law.

Question 27

Describing graph involving resistance, pd and current

Answer 27

As the voltage increases from x to y, the resistance of the diode remains high and unchanged/decreases/increases and/hence the current remains at/increases/decreases.

Question 28

Explain why the student does not move the slider of the diode from max voltage to min voltage.

Answer 28

The diode stops working when the voltage across it is xV. If the slider moves from A to B, voltage across diode starts from the max hence the diode would overheat and stop working at the start of the experiment.

Question 29

Define resistivity

Answer 29

Ability of a material to resist the flow of current

resistance of a material for a unit area per unit length

Question 30

Define ohmic conductor

Answer 30

A type of conductor in which the potential difference across it is directly proportional to the current flow through it if the tempt remains constant.

Question 31

Describe the difference between electron flow and conventional current

Answer 31

In the electron flow, the electric current is caused by the movement of electrons from the negative terminal of an electrical source to the positive terminal. In conventional current, electric current is due to the flow of positive charges from the positive terminal of an electrical source to the negative terminal.

Question 32

Why battery tends to deplete faster in parallel circuit than in series circuit

Answer 32

Current splits across diff parallel branches -> current thru each cell = V/R -> when more parallel branches added, current is overdrawn from power source -> power depletes quickly

Question 33

What is the assumption made when calculating the resistance of a metallic conductor?

Answer 33

Temperature is kept constant and the metallic conductor is ohmic.

Question 34

Describe 2 differences betw the I-V characteristic graph of a piece of metal wire at constant temperature and the graph for the green LED.

Answer 34

I-V graph for a metal is a straight line graph passing through origin. Green LED is not, and will be an exponential graph.
When pd is negative, there is a “negative” current flow in the opposite direction, hence straight line graph of metal wire extends pass origin for negative pd. For green LED, there is no current flow for negative pd.

Question 35

What is 1 ampere?

Answer 35

1 coulomb of charge flowing around a complete circuit in 1 second.

Question 36

Define 1 coulomb of charge

Answer 36

One coulomb is the quantity of charges that passes any point in an electric circuit per second when a current of one ampere is flowing.