16 — Electricity & magnetism Flashcards
1 charge
Charge of 6.25 x 10^18 protons
Each charge estimated 1.60 x 10^-19C
Ohm’s law
States that the current passing thru a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions remains constant.
Electric current
Measure of the rate of flow of electric charge
Electric circuit
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.
Emf
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
Electron flow
Movement of electrons in the electrical circuit
Potential difference
The pd across a component in a circuit is the work done by the source in driving a unit charge across the electrical component.
Resistance
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
Resistor
A resistor is a conductor or an insulator that has high resistance. It is used in circuit to control the amount of current.
Why the battery goes flat
Electrochemical reactions in batt hv depleted the materials that produce these reactions
EMF vs pd
EMF: the work done to move each unit charge thru complete circuit
Pd: the work done to drive each unit charge across the components
EMF: present even when no current is drawn from source
Pd: across any electrical component is 0 in absence of current
Examples of variable resistors
Rheostat
Potentiometer
Resistance vs resistivity
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
Why resistance of most metals increases linearly w tempt (ohmic conductors)
As tempt increases, metallic ions vibrate more vigorously abt fixed positions -> increasing no. Of collisions betw free electrons and metallic ions opposing electron flow
Ohmic characteristics
- linear rs betw pd V and current
- Passes thru origin (I/V graph)
- 1/m(gradient) = resistance
Examples of non-ohmic conductors
Diodes -> semiconductor diode
Negative tempt coefficient (NTC) thermistors
Filament lamp
Filament lamp
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
Semiconductor diode
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.
Determining the resistance of an unknown resistor of low value
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)
Explain why the current in lamp Q (series) is larger than the current in lamp P(parallel).
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.
Explain why lamp Q (series) has a different resistance from lamp P (parallel) although they r identical lamps
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.
Explain why a current smaller than xA over a fixed resistor is not obtained.
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.
State how figure shows that the filament lamp does not obey Ohm’s law.
The current flowing thru the filament lamp does not increase proportionally w the pd. Thus, it does not obey Ohm’s law.
State why does the filament lamp not obey Ohm’s law
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.
