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Exam 2 Flashcards

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1
Q

Voltage (potential difference)

A

A difference in electric potential (electric potential energy) between 2 points. V depends on where the potential is chosen to be zero. Zero can be chosen arbitrarily because only the differences in potential can be measured. Zero is usually ground

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2
Q

Voltmeter

A

A voltmeter is an instrument that measures the difference in electrical potential between two points in an electric circuit. It must be connected parallel to the device whose voltage it’s measuring (connected externally, and is parallel to wire). Symbolized by a V, has a high resistance. Objects in parallel experience the same potential difference (voltage)

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3
Q

Ammeter

A

An ammeter measures the electric current in a circuit. In order for an ammeter to measure a device’s current, it must be connected in series to that device. This is necessary because objects in series experience the same current. They must not be connected to a voltage source. Symbolized by an A, has a low resistance

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4
Q

Equivalent resistance

A

The single resistance that would draw the same current as the combination of resistors. In series, adding resistance to a circuit will decrease the current, so Req is the net resistance or the sum of resistances. In parallel, adding resistance will affect one branch, but not the rest of the circuit

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5
Q

3 lightbulbs are connected in series, and 3 in parallel. Which circuit’s lightbulbs will be brighter?

A

The lightbulbs connected in parallel. This is because each branch in the circuit gets the same amount of voltage, so each gets the same amount of current. The lightbulbs connected in series share the same amount of voltage across the 3 bulbs. Therefore, each bulb gets less current because there is more resistance.

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6
Q

Source of electromotive force (emf)

A

A device that transforms one type of energy (chemical, mechanical, or light) into electric energy. The device can be a battery or electric generator. The potential difference between the terminals of the source, when no current flows to an external circuit, is called the emf of the source.

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7
Q

Internal resistance (r)

A

There is always some hindrance to completely free charge flow. A battery has some internal resistance because electrons can’t always flow freely within it. Unless stated otherwise, assume internal resistance is negligible.

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8
Q

Is a battery a source of constant current?

A

No, the current varies according to the resistance in the circuit

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9
Q

Is a battery a source of constant voltage?

A

The voltage is nearly constant. Sometimes, a voltage drop will occur if an activity requires a large current. This is because the chemical reactions in the battery can’t supply charge fast enough to maintain a full emf.

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10
Q

Terminal voltage

A

The change in voltage between the two terminals of a battery (voltage at positive terminal a minus negative terminal b). Unless stated otherwise, assume terminal voltage is the battery’s voltage

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11
Q

Series circuits

A

When resistors are connected end to end, along a single path. The same current passes through each resistor (if it didn’t, charge either wouldn’t be conserved or would be accumulating somewhere). Resistance values are added together to get equivalent resistance. When resistance is added, the current will decrease.

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12
Q

Parallel circuits

A

When the current splits into separate branches or paths. When one device is disconnected, the rest of the circuit is not impacted. The current values are added together because charge is conserved, but the voltages are all equal. The equivalent resistance is less than the net resistance, as the current has more paths to follow, creating less resistance. Lighting in houses and in cars is wired in parallel.

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13
Q

Current

A

Any flow of charge

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14
Q

A circuit contains 3 lightbulbs. There is a parallel circuit (lightbulbs B and C) connected in series with lightbulb A. How do the brightness of the bulbs compare?

A

The current passing through bulb A must split into 2 equal parts when it reaches the junction between B and C (B and C will receive half of A’s current). Therefore, B and C will be equally bright, but less bright than A.

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15
Q

When do we use Kirchhoff’s rules?

A

When the circuit is too complicated to combine resistances

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16
Q

Kirchhoff’s junction rule

A

States that at any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction. Whatever charge is going into the circuit must be coming out

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17
Q

Kirchhoff’s loop rule

A

States that the sum of the charges in potential around any closed loop of a circuit must be zero. This is because potential energy peaks at the top of the “hill” in a loop, but is then converted to kinetic energy and is back at zero at the starting point.

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18
Q

When using Kirchhoff’s rules, how do you label currents?

A

Positive current moves away from the positive terminal of a battery. If the direction of a current isn’t obvious, you can choose a direction arbitrarily. The answer will be negative if the direction you chose was incorrect

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19
Q

Which side of a battery is positive?

A

The side with the longer line

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20
Q

As a current goes through a resistor, how does the potential change?

A

It goes from higher to lower potential

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21
Q

When EMFs are in series, opposite direction, what is the total voltage?

A

The difference between the two EMFs, but the lower voltage battery is charged. In this case, the two batteries are mirror images of each other.

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22
Q

When EMFs are in series, same direction, what is the total voltage?

A

Total voltage is the sum of the separate voltages

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23
Q

RC circuits

A

Circuits that contain capacitors and resistors. With these circuits, we are interested in how variables like voltage and charge change over time

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24
Q

When happens to charge a capacitor when the switch of an RC circuit is closed?

A

Current begins to flow through the circuit immediately. Electrons flow from the negative terminal of the battery through the resistor and accumulate on the upper plate of the capacitor. And electrons will flow into the positive terminal of the battery, leaving a positive charge on the other plate of the capacitor. As the charge is accumulating in the capacitor, the potential difference across it increases and the current is reduced until the voltage of the capacitor equals the emf of the battery. Then, there is no further current flow and no potential difference across the resistor

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25
Time constant
A measure of how quickly the capacitor becomes charged. With a small resistance, the time constant is smaller and the capacitor is charged more quickly
26
Exponential decay
In an RC circuit, the current in the circuit and the voltage across the resistor are decaying exponentially. The current is largest when the switch of the circuit is closed, and then it decreases over time
27
In an RC circuit, when does maximum charge occur in a capacitor?
When do further current flows. To find max current, you can use Q= CE
28
When the switch on a circuit is closed, what happens when the capacitor discharges?
In this case, there is no battery. When the switch is closed, charge flows through the resistor from one side of the capacitor to the other side, until the capacitor is fully discharged. The voltage decays exponentially.
29
How can electric current damage the human body?
It can heat tissue and cause burns, or stimulate nerves and muscles. The severity of a shock depends on the magnitude of the current, how long it acts, and which part of the body it passes through. A shock is more damaging if it passes through the heart or brain
30
Which part of the body has high resistance to current?
The living tissues of the body offer low resistance because it has ions that conduct very well. However, the outer (dead) layer of the skin offers high resistance when dry. Resistance is lowered when the skin is wet, and a high current can be deadly.
31
Why is it more dangerous to be exposed to a current while barefoot, or while standing in a bathtub?
If barefoot, resistance is less and current will pass through you to ground more easily- thick soled rubber shoes can prevent this. Being wet lowers resistance, but the water in a bathtub is in contact with the drain pipe, which is typically metal. Non-metal pipes would be protective.
32
Why would it be dangerous to touch a live wire in one hand and a sink faucet with the other?
A sink faucet is connected to ground via a metal pipe, or even by water in a non-metal pipe. The current is more dangerous in this situation because it would pass through the heart and lungs. Removing metal jewelry, especially rings (the skin can be wet underneath), wearing thick-soled rubber shoes, and not using your other hand would be protective
33
How would someone come into contact with a live wire?
By touching a bare wire whose insulation has worn off, or from a bare wire in a appliance you're tinkering with (that hasn't been unplugged), or if a wire in a device breaks or loses its insulation and comes into contact with the case.
34
Ammeter
Measures current. The current in a circuit passes through the ammeter, so the ammeter should have low resistance so as not to affect the current. The ammeter should be connected in series since the current is the same everywhere in a series circuit.
35
Voltmeter
Measures voltage. The resistance of a voltmeter should be very large, since it shouldn't affect the voltage across the area of the circuit it's measuring. It should be connected in parallel because voltage is the same everywhere in a parallel circuit
36
What magnitude of current can be dangerous?
1 mA can be felt and cause pain. 10 mA can cause severe muscle contractions, and the person might not be able to let go of the wire. Death can result from the paralysis of the respiratory system. a current of 80-100 mA can cause ventricular fibrillation if it passes through the torso, and it can cause death if sustained
37
Leakage current
Current along an unintended path. Leakage currents from electrical devices are generally very small. However, patients with implanted electrodes (like for a pacemaker) are in greater danger when touching electrical devices due to the absence of their protective skin layer and because electrodes have been inserted into the heart
38
Hitch hiker rule
Used to find the direction of a magnetic field around a wire. Line up thumb with the direction of the current, and the direction of fingers indicates the direction of the magnetic field. These rules assume a positive current, so if the current is negative, then find the direction and flip it 180 degrees
39
How do you find the direction of a magnetic force on a wire? (right hand rule)
1. Point the fingers in the direction of the current 2. Rotate your palm in direction of magnetic field (B) 3. The way your thumb is pointing is the direction of magnetic force This is only true for positively charged moving particles, flip the direction 180 for negative particles
40
Poles
The ends of a magnet where the magnetic effect is strongest. The north pole is the end of the magnet that points toward geographic north, and the south pole points toward the south.
41
Attractive forces between magnets
Like poles repel, opposite poles attract. However, magnetic poles are not the same thing as electric charge. You can't isolate a magnetic pole like you would be able to isolate a charge, as a single magnetic pole has never been observed.
42
Ferromagnetic
Materials that show strong magnetic effects, like iron, nickel, cobalt, and others
43
Magnetic field
The force one magnet exerts on another is an interaction between one magnet and the magnetic field of another.
44
Magnetic field lines
They are drawn so that: 1. The direction of the magnetic field is tangent to a field line at any point 2. The number of lines per unit area is proportional to the strength of the magnetic field
45
Direction of a magnetic field
Defined as the direction that the north pole of a compass needle would point if placed at the location. Therefore, magnetic field lines always point out from the north pole and in toward the south pole of the magnet.
46
Earth's magnetic field
The Earth's north pole is actually a magnetic south pole, because the north ends of magnets are attracted to it.
47
Uniform magnetic field
These magnetic fields don't change in magnitude or direction from one point to another, which is difficult to produce. The field between two flat parallel pole pieces is nearly uniform if the area of the pole faces is large compared to their separation.
48
How are electricity and magnetism connected?
An electric current produces a magnetic field. A compass needle placed near a straight section of current-carrying wire experiences a force, so a needle will align tangent to a circle around the wire
49
What determines magnetic force on a current-carrying wire?
The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation. The direction of the force is given by the right hand rule
50
In the magnetic force on a wire formula, what does theta represent?
The angle between B (magnetic field) and current directions
51
Unit for B
Tesla (T), sometimes Gauss (G)
52
What happens to moving charged particles when they pass through a magnetic field?
They experience a force, even when the moving charges are not in a wire. However, the force will be zero if the particle moves parallel to the field lines
53
What is the direction of the force in charged particles passing through a magnetic field?
The force is perpendicular to the magnetic field (B) and the velocity (V) of the particle
54
A charged particle is moving in a plane perpendicular to a uniform magnetic field. What is its path?
The particle moves in a circle. This is because the force is always acting perpendicular to velocity, making the particle move in a circle. The magnitude of v does not change, but the particle is accelerating as the direction changes. For example, a positive charge moving to the right that experiences a downward force at every point in the field will move in a clockwise direction
55
What path do magnetic field lines take around a long, straight wire?
They form circles, with the wire in the center
56
In a wire, how is magnetic field related to current and distance from the wire?
Magnetic field is proportional to current and inversely proportional to distance from the wire (r).
57
Direction of the force between 2 wires
Parallel currents in the same direction attract each other, currents in opposite directions repel. This is due to the directions of the forces between the wires
58
Solenoid
A long coil of wire consisting of many loops of wire. The current in each loop produces a magnetic field, and the magnetic field can be large because it's the sum of the current in each loop
59
How do solenoids act like magnets?
One end of a solenoid can be considered the north pole, the other can be considered the south pole. Magnetic field lines leave the north pole of the magnet
60
Electromagnet
When a piece of iron is placed inside a solenoid. This increases the magnetic field because the iron becomes a magnet. The resulting magnetic field is the sum of the field due to the current and the field due to the iron. Electromagnets are used in motors and generators
61
Why is Ampere's law important?
It represents the relationship between current and magnetic field in a wire of any shape. The other equations only are used to represent the relationship in a long straight wire. The sum of the magnetic field must be made over a closed path, and the total net current is enclosed by this closed path.
62
Induced current
An electric current produced by a changing magnetic field. When the magnetic field through a coil changes, a current occurs as if there's an emf in the coil. Therefore, a changing magnetic field induces an emf.
63
Electromagnetic induction
If a magnet is moved quickly into a coil of wire, a current is induced in the wire. If a magnet is quickly removed, a current is induced in the opposite direction (magnetic field through the coil decreases). Relative motion is required to induce an emf- if the magnetic is held steady and the wire is moved toward or away from it, an emf is still induced. Only one component needs to move compared to the other component, which one doesn't matter
64
Induced emf in a U shaped conductor
A U-shaped conductor has a uniform magnetic field that is perpendicular to the area surrounded by the conductor. If a rod on top of it is moved at speed v to the right, the induced current will be clockwise to counter the increasing flux
65
Motional emf
An emf induced on a conductor moving in a magnetic field.
66
A changing magnetic flux produces
An electric field
67
Generator
Transforms mechanical energy into electrical energy- the opposite of what a motor does
68
AC generator
A generator consists of many loops of wire wound on an armature that can rotate in a magnetic field. The axle is turned by some mechanical means (falling water, car motor belt), and an emf is induced in the rotating coil. Therefore, an electric current is the output of a generator. We can use the hitch hiker rule to determine the direction of the current
69
How is a dc generator different from an ac generator?
The slip rings are replaced by split ring communicators
70
Brushes in generator
Each brush is fixed and presses against a continuous slip ring that rotated with the armature. After one half of a turn, each one half ring changes its connection over to the other brush. Wire b will be were wire a was before the half turn, so the current is alternating
71
How is emf generated in a generator?
Generated due to the force on charges of the different segments on the wire loop
72
In the rotating loop of a generator, what function does emf represent?
The emf is sinusoidal. Emf is at maximum when the coil is perpendicular to the magnetic field. It is at minimum when the coil is parallel to the field
73
Transformer
A device for increasing or decreasing an ac voltage- found in phone chargers. It consists of two coils of wire called primary and secondary coils. In a transformer, nearly all magnetic flux produced by the current in the primary coil also passes through the secondary coil
74
How does voltage travel through coils in a transformer?
When an AC voltage is applied to the primary coil, the changing magnetic field it produces will induce an ac voltage of the same frequency in the secondary coil. However, the voltage will be different according to the number of turns in each coil. Faraday's law can be used to find the emf in the secondary coil and the transformer equation can be used to find the emf in the primary coil.
75
Step up transformer
If the secondary coil contains more loops than the primary coil. This means that the secondary voltage is greater than the primary voltage. The voltage is proportional to the number of turns
76
Step down transformer
If the primary coil has more loops than the secondary coil
77
Conservation of energy in a transformer
The power input essentially equals the power output in a transformer. Very little energy is lost to heat.
78
Change of current in an LR circuit
Current grows when the circuit is connected to a battery (logarithmic graph) and decays when the battery is out of the circuit (negative exponential graph).
79
Time constant
e is the time constant of an LR circuit in the curve for current as a function of time
80
LR circuit
An inductor with inductance L and resistance R
81
EMF vs terminal voltage
The electromotive force is the voltage difference that exists across a battery when electric current does not flow through the battery while the terminal voltage is the actual voltage difference that exists within a battery when electric current does flow through the electric circuit.
82
When is there no magnetic force?
When magnetic field (B) and velocity are parallel or opposite, or when velocity is zero
83
How to find direction of induced current (3)
1. Find direction of magnetic flux- which direction are the field lines going? Remember they point from north to south 2. Find the direction of the change in flux 3. Align thumb with the change in flux and flip 180. Fingers point in the direction of current.
84
What type of function describes how fast a a capacitor discharges?
An exponential function
85
A resistor and a capacitor are used in series to control the timing in a pacemaker circuit. How should capacitance and resistance change to double the heart rate when the patient is exercising?
The capacitor needs to discharge faster, so resistance should be decreased
86
How does net capacitance change when capacitors are connected in series or in parallel?
Connecting capacitors in series increases the plate separation distance, decreasing the net capacitance. Connecting them in parallel increases the plate area, which increases the net capacitance
87
If ammeters and voltmeters are not to significantly alter the quantities they are measuring
The resistance of the ammeter should be much lower, and the resistance of a voltmeter should be much higher, than those of the circuit being measured. The ammeter is placed in series with the circuit and therefore should have a small resistance, so there is minimal voltage drop across the ammeter. The voltmeter is placed in parallel with the circuit. It should have a large resistance so that minimal current from the circuit passes through the voltmeter instead of passing through the circuit.
88
When a charged particle moves parallel in the direction of a magnetic field, the particle travels in a
Straight line. The charged particle only experiences a force when it has a component of velocity perpendicular to the magnetic field. When it moves parallel to the field, it follows a straight line at constant speed.
89
As a proton moves through space, it creates
An electric field and a magnetic field. Electric fields are created by charged objects whether the charges are moving or not, and magnetic fields are created by moving charged objects.
90
Does a stationary charged particle experience a force in a magnetic field?
A stationary charged particle does not experience a force in a magnetic field. Therefore, the particle must be moving to experience a force. The force is a maximum when the particle is moving perpendicular to the field, not parallel to the field. Since the force is perpendicular to the motion of the particle, it acts as a centripetal force, changing the particle’s direction but not its kinetic energy. That is, since the force is perpendicular to the motion, it does no work on the particle.

Physics 2 (4 decks)

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