16: Current of Electricity Flashcards
Define electric current (I) and state its formula and SI unit.
The rate of flow of electric charge Q.
I = Q (C)/t (s)
SI unit: ampere (A)
What device is used to measure current?
Ammeter.
What is the resistance of an ideal ammeter?
Zero.
How is an ammeter connected to a circuit? (series/parallel)
Series, with its positive terminal connected to the positive terminal of the cell.
What are the directions of conventional current and electon flow respectively?
Conventional current flows from the positive to negative terminals of a cell, and vice-versa for electron flow.
Define electromotive force (ε) and state its formula and SI unit.
The work done by a source in driving a unit charge around a complete circuit.
ε = W (J)/Q (C)
SI unit: volts (V) or joule per coulomb (J C ^-1)
What device is used to measure emf?
Voltmeter.
What is the resistance of an ideal voltmeter?
Infinite.
How is a voltmeter connected to a circuit? (series/parallel)
Parallel, with its positive terminal connected to the positive terminal of the cell.
Define potential difference (V) and state its formula and SI unit.
The work done in a circuit to drive a unit charge through a component.
V = W (J)/Q (C)
SI unit: volts (V)
Define resistance (R) and state its formula and SI unit.
The ratio of the p.d. to the current flowing through a component.
R = V/I (A)
SI unit: ohm (Ω)
What is the purpose of a resistor?
Control the size of the current flowing in a circuit
What are variable resistors also known as?
Rheostats.
What is the relationship between the resistance, length, and cross-sectional area of a wire?
R (Ω) ∝ l (m)/A (m^2)
R = ρ(l/A)
(ρ is resistivity)
Resistance is proportional to length and inversely proportional to area.
What is the formula for calculating resistivity?
ρ (Ωm) = RA/l
