O lvl Phy: Current Electricity

(a) State Ohm's Law. Under what conditions is it true? Sketch the V-I grap for a material that obeys Ohm's Law.

(b) The resistance of a filament is 1.2 kΩ. If it needs a current of 0.20A to light up, what is the potential difference across the lamp when it is in use?

(c) Draw graphs p.d. V against the current I for the following materials:
(i) A filament lamp
(ii) A thermistor
(iii) A semiconductor diode

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Answer:

(a) Ohm's Law states that the voltage across the ends of a conductor is directly proportional to the current flowing through it, provided that the temperature and other physical conditions are changed.

(b) potential difference = 1.2 k * 0.2 = 0.24 kV = 240 V

(c)
(i) Filament Lamp


(ii) Thermistor


(iii) Semiconductor Diode

O lvl Phy: Current Electricity

(a) Define electromotive force.

(b) Define potential difference.

(c) A potential difference of 12 V is causing electrons to flow through a wire so that 2.0 * 10²⁰ electrons pass a point in the wire in 1.0 minute. Calculate
(i) the charge which passes the point in 1.0 minute, given that the charge of each electron is 1.6 * 10^-19 C,
(ii) the electric current in the wire an
(iii) the resistance of the wire.

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Answer:

(a) Electromotive force of an electrical energy source is defined as the work done by the source in driving a unit charge around a complete circuit.

(b) Potential difference is the amount of energy of electrical energy converted to other forms of energy when 1 C of positive charge passes between 2 points.


(c)
(i) 1 electron --> 1.6 * 10^-19 C
2.0 * 10²⁰ electrons --> 1.6 * 10^-19 * 2.0 * 10²⁰
= 3.2 * 10
= 32 C

(ii) I = Q/T
= 32/60
= 0.533 A (3 s.f.)

(iii) R = V / I
= 12 / 0.533
= 22.5 Ω

O lvl Phy: Current Electricity

The following figures shows the V-I graph of two resistors X and Y.

(a) Which resistor has a higher resistance? Explain briefly.

(b) If X and Y are connected in series, in which region will the V-I graph of the combined resistor lie?

(c) If X and Y are connected in parallel, in which region will the V-I graph of the combined resistor lie?

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Answer:

(a) Resistor X has a higher resistance. This is because it has a higher gradient. Gradient of the graph is V/I. Since R = V/I, the gradient of the graph gives the resistance of the resistor. Thus, a higher gradient means a higher resistance (resistor X).

(b) The combined resistor would lie in region P. This is because the combined resistor will have a higher resistance than X or Y alone (in series). The graph should be of a higher gradient than resistor X, and thus it will lie in region P.

(c) The combined resistor would lie in region R. This is because the combined resistor will have a lowerer resistance than X or Y alone (in series). The graph should be of a lower gradient than resistor Y, and thus it will lie in region R.

O lvl Phy: Current Electricity

Each dry cell above has an electromotive force of 6 V. The ammeter, A, records a current of 4 A.

(a) Calculate the resistance, R, of the fixed resistor.
(b) Calculate the potential difference, V, across the fixed resistor.
(c) Calculate the total charge flowing across the fixed resistor in 10 s.
(d) Calculate the total work done on the charges in 10 s.

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Answer:

This is a simple standard question ;)
Treat it as conceptual question to test your concepts :D

(a) Resistance R of the fixed resistor = V / I
= (6+6-6) / 4 => The net emf is 6V because there's two batteries cancelling each other out.
= 1.5 Ω

(b) Potential difference across resistor R = emf = 6 V

(c) Using Q = IT,
Total charge = 4 A × 10 s = 40 C

(d) Using E = QV,
Total work done in 10 s = 40 C × 6 V
= 240 J