MCAT Physical : Electricity and Magnetism

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #31 : Electricity And Magnetism

Consider a conducting rod. Which of the following is true regarding the relationship between the electrical conductivity, the resistance, and the length of the rod?

Possible Answers:

The electrical conductivity increases when both the length and the resistance of the rod decrease

The electrical conductivity increases when the length of the rod increases and the resistance of the rod decreases

The electrical conductivity increases when both the length and the resistance of the rod increase

The electrical conductivity increases when the length of the rod decreases and the resistance of the rod increases

Correct answer:

The electrical conductivity increases when the length of the rod increases and the resistance of the rod decreases

Explanation:

The electrical conductivity is the reciprocal of electrical resistivity; therefore, an increase in electrical resistivity will lead to a decrease in electrical conductivity, and vice versa.

Recall the definition of electrical resistivity:

Here,  is resistance,  is cross-sectional area, and  is the length of the rod. This equation reveals that an increase in resistance and area will increase resistivity, whereas an increase in length will decrease resistivity. Since conductivity is the reciprocal of resistivity, increasing resistance and area will decrease conductivity, whereas increasing the length will increase conductivity.

 

Example Question #32 : Electricity And Magnetism

A researcher has two rods: rod A and rod B. Rod A has the same geometrical properties as rod B, but has a larger resistance. What can you conclude about the relative rates of charge flow in the two rods?

Possible Answers:

Rod B has a higher rate of charge flow than rod A because rod B has the higher electrical conductivity

Rod B has a higher rate of charge flow than rod A because rod B has the higher electrical resistivity

Rod A has a higher rate of charge flow than rod B because rod A has the higher electrical conductivity

Rod A has a higher rate of charge flow than rod B because rod A has the higher electrical resistivity

Correct answer:

Rod B has a higher rate of charge flow than rod A because rod B has the higher electrical conductivity

Explanation:

The question is asking about the rates of charge flow. Recall that the current is defined as the amount of charge flowing through a point in a given time; therefore, we are looking for the amount of current flowing through the two rods. Current flow is higher for a material with higher electrical conductivity. This means that we need to find the rod with the higher electrical conductivity.

Since there are no easy equations to find conductivity, we need to find the resistivity first. Electrical conductivity is the reciprocal of electrical resistivity (measure of the ability of a material to resist current flow); therefore, an increase in resistivity leads to a decrease in conductivity, and vice versa. Resistivity is defined as:

Here,  is the resistance,  is the cross-sectional area, and  is the length of the rod. The question states that the two rods have the same geometrical properties; therefore, the area and the length of the rods are the same. However, the resistance of rod A is higher. This means that the resistivity is higher and conductivity, consequently, is lower for rod A. Since it has a lower conductivity, rod A has a lower charge flow rate. 

 

Example Question #31 : Electricity And Magnetism

Which of the following is false regarding a battery?

Possible Answers:

Recharging a battery involves a nonspontaneous chemical reaction

A battery houses two electrodes: the anode and the cathode 

A battery converts chemical energy into electrical energy

Electrons are reactants at an anode, and are products at a cathode

Correct answer:

A battery converts chemical energy into electrical energy

Explanation:

Batteries are a series of electrochemical cells that use the electrical energy associated with the movement of electrons to perform other processes. The electrical energy is not itself usable; instead, batteries convert the electrical energy into other forms of energy, such as chemical energy, that can be used to drive a process. The electrical energy is derived from the movement of electrons associated with redox reactions. 

Batteries contain one or more electrochemical cells. Each electrochemical cell houses a cathode and an anode, which facilitate a redox reaction. The cathode is the site of the reduction half-reaction, whereas the anode is the site of the oxidation half-reaction. Reduction is the process by which reactants gain electrons and become more negative; oxidation is the process by which reactants lose electrons and become more positive. As such, electrons are products at an anode and reactants at a cathode. The redox reaction proceeds in the spontaneous reaction direction when the battery is being used, but proceeds in the nonspontaneous reaction direction when the battery is being recharged after use. Recall that nonspontaneous reactions require energy. When you are charging a battery, you are supplying energy in the form of voltage to drive the nonspontaneous reaction. On the other hand, when you are discharging (or using) the battery, the spontaneous redox reaction is supplying energy to drive another process.

Example Question #1 : Other Electrical Principles

In a battery, the __________ is the site of the reduction half-reaction and the __________ is the electrode where the oxidation number of the reactant increases.

Possible Answers:

cathode . . . anode

cathode . . . cathode

anode . . . cathode

anode . . . anode

Correct answer:

cathode . . . anode

Explanation:

An electrochemical cell in a battery contains two electrodes: the anode and the cathode. Anodes are the site of oxidation half-reactions and cathodes are the site of reduction half-reactions. Recall that oxidation involves loss of electrons from reactants. The reactants at the anode become more positive, and there is an increase in their oxidation number. Reduction, on the other hand, involves a gain of electrons and a decrease in the reactant oxidation number at the cathode. 

Example Question #35 : Electricity And Magnetism

current flows through a wire with a resistance of . Find the power generated by this current.

Possible Answers:

Correct answer:

Explanation:

Electric power is expressed in a number of ways. We will need to use the relationship that involves current and resistance:

We are given the current and resistance, allowing us to calculate the power.

Example Question #1 : Circuits

A capacitor attached to a 50 V battery aquires a charge of 10 C over a period of 5 seconds. How much energy is required to charge the capacitor?

Possible Answers:

1250 J

500 J

50 J

250 J

Correct answer:

1250 J

Explanation:

The power consumed by a capacitor is given by

\dpi{100} \small P=\frac{1}{2}qV

After finding power, we can calculate the energy used as we are told that the capacitor took 5 seconds to charge. The units of power are watts which can be further broken down into joules per second. By multiplying the time taken to charge the capacitor by the power, we can find the total energy required to charge the capacitor.

Example Question #37 : Electricity And Magnetism

A low resistance light bulb and a high resistance light bulb are connected in parallel with each other. This parallel combination is in series with a  battery. Which bulb is brighter?

Possible Answers:

Only the high resistance bulb will be lit

Both bulbs are equally bright

Only the low resistance bulb will be lit

The low resistance bulb

The high resistance bulb

Correct answer:

The low resistance bulb

Explanation:

The brightness of each bulb depends on its power, or how much energy it dissipates per unit time. This can be calculated using the equation:

If the bulbs are in parallel with each other, the bulb with lower resistance will receive more current, since current will tend toward the path of least resistance. Current is squared in the power equation, so it is of greater importance in determining the power. The bulb with greater current and lower resistance will thus have more power and will shine more brightly.

Example Question #38 : Electricity And Magnetism

During the cold winter months, some gloves have the ability to provide extra warmth due to an internal heating source. A simplified circuit, similar to those in electric gloves, is comprised of a 9V battery with no internal resistance and three resistors as shown in the image below. 

Screen_shot_2013-10-09_at_11.08.15_pm

How much power does the circuit provide?

Possible Answers:

Correct answer:

Explanation:

This question asks us about the power of the circuit, meaning the amount of energy per unit time. The power equation is P = IV. V = IR can be substituted in to allow P to be calculated from a number of parameters.

P = IV = I2R = V2/R

To solve for P, we first need the current supplied by the battery.

We can use the formula V = IR because we have the voltage drop across the circuit (9V) and can calculate the equivalent resistance.

By taking the inverse of the equation, we can see that RA4 is equal to 2Ω.

Req = RA4 + R1 = 2Ω + 2Ω = 4Ω

Now, using V=IR, we can solve for the current.

V = IR

I = V/R = 9V/4Ω = 2.25A

Now, we can use the current we calculated and the voltage that is dissipated across the circuit to calculate the power.

P = IV = (2.25 A)(9V) = 20.25W

Example Question #39 : Electricity And Magnetism

What is the resistance in a wire carrying a voltage of  and with a power of ?

Possible Answers:

Correct answer:

Explanation:

The formula we can use here is the power formula that involved both resistance and voltage:

We are given the voltage and power, allowing us to solve for the resistance.

Example Question #40 : Electricity And Magnetism

Household voltage in the United States is , supplied by an alternating current (AC). What is the maximum voltage at the outlet?

Possible Answers:

Correct answer:

Explanation:

Alternative current (AC) voltage is governed by the RMS (root-mean square) voltage law. This means that the RMS voltage of the circuit will be , but the actual voltage will fluctuate between two values. These values are determined by the equation:

Using the values from our question, we can find the maximum voltage value.

 

 

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