Sec 4 Physics Current of Electricity topical questions

Define electric current and state its unit.

State the direction of conventional current in the external circuit, and how it compares with the actual electron drift.

A current of 0.50 A flows through a lamp for 2.0 minutes. Calculate the charge that passes.

Define e.m.f. and state its unit.

A resistor has 4.5 V across it and carries 0.30 A. Calculate its resistance.

Describe the I–V graphs of (a) an ohmic conductor at constant temperature, (b) a filament lamp, and (c) a diode.

1. (a)Ohmic conductor.[2]
2. (b)Filament lamp.[2]
3. (c)Diode.[2]

Wire P has length 1.0 m and resistance 6.0 Ω. Wire Q is the same material with twice the length and twice the diameter.

1. (a)Determine the factor by which the cross-sectional area of Q differs from P.[2]
2. (b)Calculate the resistance of Q.[2]
3. (c)State one more factor, besides length and area, that affects the resistance of a wire.[2]

A 12 V battery drives 250 C of charge through a starter motor in 5.0 s.

1. (a)Calculate the electrical energy converted in the motor.[2]
2. (b)Calculate the average current.[2]
3. (c)State whether the current in the connecting leads is smaller, equal or larger than 50 A at points nearer the battery, with a reason.[2]

Distinguish between the e.m.f. of a battery and the potential difference across a lamp in the same circuit, and state the correct meter connections for measuring each.

1. (a)Define both quantities and how they differ.[4]
2. (b)State the meter and its connection for each measurement.[2]

A filament lamp is tested and gives these readings: V = 2.0, 4.0, 6.0, 8.0 V with I = 0.30, 0.52, 0.68, 0.80 A.

1. (a)Calculate the resistance at each voltage.[4]
2. (b)Describe and explain the trend.[3]
3. (c)State how the I–V graph of the lamp would look compared with an ohmic resistor.[1]

A student measures the current through a diode for forward voltages 0.2, 0.4, 0.6, 0.8 V and finds negligible current until about 0.7 V, after which the current climbs steeply. Reversed, even 5.0 V produces almost no current.

1. (a)Sketch (in words) the diode’s I–V graph.[3]
2. (b)Explain why the diode is described as a one-way valve.[2]
3. (c)State one practical use of a diode in a circuit.[3]

A heating element for a small heater and its connecting leads must be chosen from: thick copper wire, thin copper wire, thick nichrome wire, thin nichrome wire. (Nichrome is an alloy with much higher resistivity than copper.)

1. (a)Choose the best connecting lead and justify it with resistance reasoning.[3]
2. (b)Choose the best heating element and justify it.[3]
3. (c)State what happens to the element’s resistance as it glows hot, and the consequence for the current it draws.[2]

A student determines the resistance of a coil by plotting its I–V graph, taking readings quickly and switching off between readings.

1. (a)Explain why the coil should not carry current for long between readings.[3]
2. (b)Describe the graph expected if the coil behaves ohmically, and how R is found from it.[3]
3. (c)The graph instead curves slightly downward (gradient falls). What does this reveal about the coil?[2]