- Which unit is used to measure resistance in a circuit?
- Ω
- W
- V
- A
Resistance is measured in ohms (Ω). Watts (W) measure power, volts (V) measure potential difference, and amperes (A) measure current.
- Which of the following is the correct expression of Ohm’s law?
- \(I = \dfrac{R}{V}\)
- \(R = \dfrac{P}{V}\)
- \(I = \dfrac{P}{V}\)
- \(I = \dfrac{V}{R}\)
Ohm’s law states that current equals voltage divided by resistance:
\(I=\dfrac{V}{R}\)
The other expressions mix in power or invert the relationship.
- An operating lamp draws a current of 0.4 amperes. What is the amount of charge passing through the lamp in 10 seconds?
- 0.045 C
- 4.0 C
- 5.0 C
- 6.24 C
Charge is current (\(I\)) times time (\(t\)):
\(Q=It=0.4\ \text{A}\times10\ \text{s}=4.0\ \text{C}\)
- To increase the brightness of a desk lamp, a student replaces a 50 W light bulb with a 100 W light bulb. Compared to the 50 W light bulb, the 100 W light bulb has…
- Less resistance and draws more current
- Less resistance and draws less current
- More resistance and draws more current
- More resistance and draws less current
At the same supply voltage, \(P=\tfrac{V^2}{R}\). Higher power means lower resistance.
Also, \(I=\tfrac{P}{V}\), so the 100 W bulb draws more current.
- An electric dryer consumes \(6.0 \times 10^6\) joules of energy when operating at 220 volts for 30 minutes. During operation, the dryer draws a current of approximately how many amperes?
- 10
- 15
- 20
- 25
Use \(E=VIt\):
\(I=\dfrac{E}{Vt}\) \(=\dfrac{6.0\times10^6}{220\times(30\times60)}\approx15\ \text{A}\)
- When 8.0 eV photons strike a photoemissive surface, the maximum kinetic energy of ejected photoelectrons is 6.0 eV. What is the work function of the photoemissive surface?
- 0.01 eV
- 1.0 eV
- 2.0 eV
- 3.0 eV
The photoelectric equation is \(K_{\max}=hf-\phi\). Thus:
\(\phi=8.0-6.0=2.0\ \text{eV}\)
- What is created when a high resistance is connected in series with the internal coil of a galvanometer?
- An ammeter
- A motor
- A generator
- A voltmeter
Adding a large series resistor makes the instrument respond to voltage (voltmeter). An ammeter uses a low-resistance shunt in parallel instead.
- In a transformer, two coils are wound around a common iron core. What is required for the transformer to operate properly?
- More turns in the secondary coil than in the primary coil
- More turns in the primary coil than in the secondary coil
- A direct current source connecting to the secondary coil
- An alternating current source connecting to the primary coil
Transformers require a changing magnetic flux. An AC source on the primary produces the time-varying flux that induces voltage in the secondary.
- Which device can be used to increase voltage from a source of direct current?
- Generator
- Electroscope
- Induction coil
- Mass spectrometer
An induction coil (with an interrupter) takes DC and creates rapid current changes in a primary, inducing a higher voltage in the secondary. A transformer alone needs AC.
- A transformer on a power pole steps down the voltage from 10,800 volts to 120 volts. If the secondary coil contains 360 turns, how many turns are found on the primary coil?
- 603
- 900
- 15,000
- 32,400
Use \(\dfrac{V_p}{V_s}=\dfrac{N_p}{N_s}\). Here, \(\tfrac{10,800}{120}=90\), so:
\(N_p=90\times360=32,400\)
- An electric motor draws 150 amperes of current while operating at 240 volts. What is the power rating of this motor?
- \(2.1 \times 10^4 \text{ W}\)
- \(2.7 \times 10^5 \text{ W}\)
- \(3.6 \times 10^4 \text{ W}\)
- \(4.1 \times 10^4 \text{ W}\)
Power is:
\(P=VI=240\times150\) \(=36,000\ \text{W}=3.6\times10^{4}\ \text{W}\)
- What is the potential difference across a 2.0 ohm resistor that draws 2.0 coulombs of charge per second?
- 1.0 V
- 2.0 V
- 3.0 V
- 4.0 V
Current is charge per time:
\(I=\dfrac{Q}{t}=2.0\ \text{A}\)
\(V=IR=2.0\times2.0=4.0\ \text{V}\)
- If a 15 ohm resistor is connected in parallel with a 30 ohm resistor, what is the equivalent resistance?
- 5 Ω
- 7 Ω
- 10 Ω
- 15 Ω
For two in parallel:
\(\dfrac{1}{R_\text{eq}}=\dfrac{1}{15}+\dfrac{1}{30}=\dfrac{3}{30}=\dfrac{1}{10}\)
This means \(R_\text{eq}=10\ \Omega\).
- A metal wire has length \(L\) and cross-sectional area \(A\). Which of the following is the resistance of the wire directly proportional to?
- \(\dfrac{L}{A}\)
- \(\dfrac{A}{L}\)
- \(L+A\)
- \(L \times A\)
For a uniform wire, \(R=\rho\tfrac{L}{A}\).
Thus, holding material constant (ρ), resistance increases with \(L\) and decreases with \(A\).
- A wire carries a current of 2 amperes. How many electrons pass a given point in this wire in one second?
- \(1.3 \times 10^{18}\)
- \(2.0 \times 10^{18}\)
- \(1.3 \times 10^{19}\)
- \(2.0 \times 10^{19}\)
Current is charge per second. One electron has \(e=1.602\times10^{-19}\ \text{C}\), so:
\(N=\dfrac{I}{e}=\dfrac{2}{1.602\times10^{-19}}\) \(\approx1.25\times10^{19} \text{ electrons/sec}\)
This is closest to \(1.3\times10^{19}\).