Chapter 17: Current Electricity Flashcards
Define electric current and state the formula involving it.
Rate of flow of electric charge through a given cross-section of a conductor
I (A) = Q (charge, C)/ T (s)
How should an ammeter be connected in a circuit?
- Connected in series
- Current should flow through the positive terminal (red) and leave through the negative terminal (black)
Define electromotive force, and state the formula involving it.
The electromotive force of an electrical energy source (cell) is the work done by the source in driving a unit charge around a complete circuit.
ε (emf, V) = W (work done, J)/ Q (charge, C)
How does cell arrangement affect e.m.f.?
When cells are arranged in series, the resultant e.m.f. is the sum of the e.m.fs of all the cells. Hence, the cells last for a shorter time as the charges gain energy as they pass through each cell.
When cells are arranged in parallel, the resultant e.m.f. is equivalent to that of a single cell. Hence, the cells last for a longer time as the charges gain only a portion of the energy from the cell.
Define potential difference, and state the equation involving it.
The potential difference (p.d.) across a component in an electric circuit is the work done to drive a unit charge through the component. 1 volt is the potential difference across a component such that 1 joule of work is done in taking 1 coulomb of charge from one point to another.
V = W (J)/ Q (C)
How do we measure the e.m.f. of a dry cell?
The positive and negative terminals of the voltmeter must be connected to the positive and negative terminals of the dry cell respectively.
How do we measure the p.d. across a component (e.g. a bulb)?
The voltmeter must be connected in parallel with the component.
Define resistance, and state the formula involving it.
The resistance of a component is the ratio of the potential difference V across it to the current I flowing through it.
R (Ω) = V/ I (A)
State Ohm’s law.
The current passing through a conductor is directly proportional to the potential difference across its ends, provided that the physical conditions (e.g. temperature) remain constant.
I/V = R = constant. Thus, the resistance of metallic conductors remains constant under steady physical conditions.
Describe the I/V graph of an ohmic conductor.
1) Straight line that passes through the origin
2) Constant gradient that is equal to the inverse of the resistance R of the conductor (1/R = I/V)
Describe the I/V graph of a filament lamp.
1) Curve with decreasing gradient → resistance (V/I) increases with temperature
2) As current increases, the devices generate more heat and thus their temperatures increase
3) As temperature increases, the resistance of the filament lamp increases
Describe the I/V graph of a semiconductor diode.
1) When p.d. is applied in the forward direction, current flow is relatively large → resistance is low in the forward direction
2) When p.d. is applied in the reverse direction, there is almost no current flow → resistance is very high in the reverse direction
State the four physical conditions that the resistance of a conductor is dependent on, and state the formula involving resistivity.
Physical conditions:
1) Temperature
2) Length of conductor, l
3) Cross-sectional area, A
4) Type of material it is made of, ρ (resistivity, a fixed property of the wire’s material)
R = ρl/ A
ρ (a constant) = measured in Ω m
