COE&DC

Question 1

Electric Current

Answer 1

EC through an area is the rate of flow of the electric charges through the cross sectional area of the surface

Question 2

Direction of electric current

Answer 2

the direction of EC/Conventional Current is the same direction as the flow of positive charges

Question 3

EMF E vs Potential Difference V

Answer 3

EMF is defined as the energy converted per unit electric charge (from other forms of energy) to electrical energy in driving a charge round a complete circuit while the potential difference V, between 2 points is the energy converted per unit electric charge from electrical energy to other forms of energy when a charge is moved between 2 points

Question 4

Resistance meaning vs Resistivity

Answer 4

Ratio of the potential difference V across the device to the current I flowing through it (basically the formula in words) and is property of a device that indicates the degree to which it obstructs the current flow

Resistivity is a property of the material in the device obstructing the current flow

Question 5

Ohms Law

Answer 5

states that a steady current flowing through a metallic conductor is proportional to the potential difference across it, provided the temperature and other physical conditions are constant

Question 6

what is the resistance of uniform conductor affected by

Answer 6

prop to L, inv prop to A, dependent on property of material known as p, proportional to temp of conductor

Question 7

ideal ammeter and voltmeter conditions

Answer 7

ideal ammeter 0 resistance, ideal voltmeter infinite resistance
ammeter placed in series, voltmeter placed in parallel

Question 8

{The equivalent resistance of a parallel network is smaller than even the smallest resistor in the
network}

what is brightness determined by

Answer 8

brightness is determined by power I2R

excessive power blows fuse

Question 9

2 conservation principles for DC Circuits

Answer 9

1) Because charge is always conserved, the sum of currents entering any
junction in an electric circuit is always equal to the sum of currents leaving that junction,

2) Because energy is always conserved, in any closed loop in an electric circuit,
sum of e.m.f.s in a loop equals the potential drops across any components.

Question 10

functions of LDR and Thermistors

Answer 10

as light intensity decreases and temperature increases, both their resistance decreases, so can use as lighting which switch on automatically in the evening when intensity falls below a certain amount or temperature sensor in fire alarm

Question 11

what is a balanced length ? and assumption

Answer 11

the point where the galvanometer shows null deflection (no current flows through it and functions as ideal voltmeter)

and driver cell has negligible internal resistance

Question 12

advantages and disadvantages of using potentiometer to measure voltage instead of voltmeter

Answer 12

it draws no current and functions as idea voltmeter and voltmeter tends to show a reading that is less than actual pd but it is slow in operation and cannot be used to measure rapidly changing voltages and the resistance wire may not be uniform due to wear and tear

Question 13

increasing length of balanced length

Answer 13

reduce emf of driver cell, add resistors in circuit to driver cell (all these reduces the PD per unit length across the potentiometer wire, so need longer length for same pd across secondary circuit)

Question 14

disadvantage of using high voltage and advantage

Answer 14

V=IR, since R is the same, I is higher then more vulnerable to electric shocks

but power efficiency (less I needed for same P so less i2r loss) and higher power rating - supply larger load