chapter 17 current electricity Flashcards
what is conventional current
direction opposite to electron flow = ‘movement’ of positive charges
how does current flow
starts from the positive terminal to the negative terminal
what is electric current
rate of flow of electric charge
I = Q/t (I = current in A, Q = charge in C, t = time in s)
why does bulb remain unlit when there is an extra wire
wire provides alternative path of negligible (little) resistance for current to flow = ammeter shows reading
current does not flow through bulb instead = remains unlit
why does a bulb remain unlit when switch is open
break in circuit = current cannot flow = bulb is unlit = ammeter shows no reading
what is electromotive force (emf)
work done by electrical energy SOURCE in driving a unit charge around a complete circuit
E = W/Q (E = emf in V, W = amount of non electrical energy converted to electrical energy in J, Q = amount of charge in C)
what is potential difference
work done to drive an unit charge through a COMPONENT (eg a bulb or resistor)
V = W/Q
how to measure emf of source
connect voltmeter in parallel with source eg dry cell
positive and negative terminals of voltmeter must be connected to positive and negative terminal of cell respectively
how to measure pd of component
connect voltmeter in parallel with component
what is resistance of a component
ratio of potential difference across the component to the current flowing through the component
R = V/I (R is in ohm)
what are resistors
conductor in a circuit that is used to control the size of the current flowing in a circuit
types of resistors
fixed resistor: has a fixed resistance
variable resistor: resistance can be varied - eg rheostat
how to measure current in circuit
connect ammeter in series with circuit
what is ohms law
current passing through a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions remain constant
gradient of graph = resistance
since R = V/I, I = V/R, I = (1/R)(V), I = kV (where k is a constant - gradient)
what are non ohmic conductors
conductors that do not obey ohms law = current flowing through non ohmic conductors do not increase proportionally with pd
what affects resistance
- Cross sectional area: CSA increases, R decreases
- Length: length increases, R increases
- Temperature: temperature of conductor increases, R increases
- resistivity (in ohm meter) of a material: resisitivity decreases, R decreases
formula: R = resitivity x length/CSA
