a) The power dissipated as heat in a resistor in a circuit is given by P = VI. Show that this may also be expressed as P=I2R and P=RV2. [1]
b) A student goes out to purchase an electric heater for his flat. The salesman says that, to get more heat, he should purchase a heater with a high resistance because P=I2R, but the student thinks that a low resistance would be best, because P=RV2. Explain who is correct. [2]
c) Copper is a better conductor than iron. Equal lengths of copper and iron wire, of the same diameter, are connected first in parallel, and then in series. A potential difference is applied across the ends of each arrangement in turn, and the p.d. is gradually increased from a small value until, in each case, one of the wires begins to glow. Explain this, and state which wire will glow first in each case.
Case 1 [parallel]
Case 2 [series] [4]
d) A surge suppressor is a device for preventing sudden excessive flows of current in a circuit. It is made of a material whose conducting properties are such that the current flowing through it is directly proportional to the fourth power of the potential difference across it. If the suppressor dissipates energy at a rate of 6 W when the applied potential difference is 230 V, what is the power dissipated when the potential rises to 1200 V? [3]