Question 1

A 1200 W electric heater operates on a 120 V supply. What is the current drawn by the heater? Use $$P = IV.$$

Accepted Answer

$$5\ \mathrm{A}$$

Question 2

Two resistors, $$6\ \Omega$$ and $$12\ \Omega$$, are connected in parallel across a $$12\ \mathrm{V}$$ battery. What is the current through the $$12\ \Omega$$ resistor? Use $$I = V/R.$$

Accepted Answer

$$0.5\ \mathrm{A}$$

Question 3

What happens to the equivalent resistance when more resistors are added in parallel to a circuit?

Accepted Answer

It becomes smaller.

Question 4

If the resistance $$R in an$$ $$RC$$ circuit doubles and the capacitance $$C$$ halves, what happens to the time constant $$	au$$? Recall $$	au = RC.$$

Accepted Answer

It stays the same.

Question 5

A heater uses $$18\ \mathrm{kW} of $$power at $$240\ \mathrm{V}. $$What is its resistance? Use $$P = V^2$/R.$$

Accepted Answer

$$3.2\ \Omega$$

Question 6

What is the effect of inserting a dielectric material between the plates of a capacitor?

Accepted Answer

It increases the capacitance.

Question 7

A $$500\ \mathrm{W}$$ device runs $$6$$ hours per day. If electricity costs $$\0.12$ per $$\mathrm{kWh}$$, what is the annual cost to operate the device? (Assume 365$ days in a year.)

Accepted Answer

$$\130$

Question 8

A $$12\ \Omega$$ and a $$6\ \Omega$$ resistor are connected in parallel. What is the equivalent resistance $$R_{eq}$$? Use $$1/R_{eq} = 1/R_1$ + 1/R_2$.

Accepted Answer

$$4\ \Omega$$

Question 9

A $$240\ \mathrm{V}$$ heater is rated at $$18\ \mathrm{kW}. $$What current does it draw? Use $$P = IV.$$

Accepted Answer

$$75\ \mathrm{A}$$

Question 10

If you add a resistor in parallel to an existing resistor, what happens to the total current supplied by the voltage source (assuming the voltage remains constant)?

Accepted Answer

It increases.

Question 11

A $$1200\ \mathrm{W}$$ device is connected to a $$120\ \mathrm{V}$$ supply. What is the resistance of the device? Use $$P = V^2$/R.$$

Accepted Answer

$$12\ \Omega$$

Question 12

A $$6\ \Omega$$ and a $$12\ \Omega$$ resistor are connected in parallel across a $$12\ \mathrm{V}$$ battery. What is the total power dissipated by both resistors? Use $$P = V^2$/R$$ for each and sum.

Accepted Answer

$$36\ \mathrm{W}$$

Question 13

If the time constant $$	au of an$$ $$RC$$ circuit is $$5\ \mathrm{s}$$, and the resistance is doubled while the capacitance is halved, what is the new time constant?

Accepted Answer

$$5\ \mathrm{s}$$

Question 14

A $$500\ \mathrm{W}$$ device is used for $$6$$ hours per day. How many kilowatt-hours does it consume in one year?

Accepted Answer

$$1095\ \mathrm{kWh}$$

Question 15

Which of the following best explains why inserting a dielectric increases the capacitance of a parallel-plate capacitor?

Accepted Answer

The dielectric reduces the electric field, allowing more charge to be stored for the same voltage.

Question 16

A $$12\ \Omega$$ resistor is connected in parallel with a $$6\ \Omega$$ resistor. If the voltage across them is $$12\ \mathrm{V}$$, what is the current through the $$6\ \Omega$$ resistor?

Accepted Answer

$$2\ \mathrm{A}$$

Question 17

If the cost of electricity increases to $$\0.15$ per $$\mathrm{kWh}$$, what would be the new annual cost to run a 500$$\ \mathrm{W}$$ device for 6$ hours per day?

Accepted Answer

$$\164.25$

Question 18

A $$1200\ \mathrm{W}$$ device is connected to a $$120\ \mathrm{V}$$ supply. If the voltage drops to $$100\ \mathrm{V}$$, what happens to the power consumed (assuming resistance stays constant)?

Accepted Answer

It decreases.

Question 19

Which of the following is a correct expression for the total current in a parallel circuit with two resistors $$R_1$$ and $$R_2$$ connected across a voltage $$V$$?

Accepted Answer

$$I = \dfrac{V}{R_1$} + \dfrac{V}{R_2$}$$

Physics Study Guide: Electricity and Circuits

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