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Electricity Flashcards

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1
Q

What is electric current? State its units.

A

The rate of flow of charge. it is measured in amperes (A)

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2
Q

What is potential difference?

A

The work done moving a unit charge between 2 points in a circuit

V = W/ Q

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3
Q

What is resistance? (2)

A

-How difficult it is for current to flow through an appliance.
-A component has a resistance of 1 ohm if 1 A flows through it when a p.d of 1V is applied across it.

R = V/I

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4
Q

What is meant by an ohmic conductor?

A

A conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant.

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5
Q

How can you measure the current in a circuit?

A

you can measure the current in a circuit with an ammeter connected in series with the component.

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6
Q

How do you measure potential difference across a component?

A

Using a voltmeter, connected in parallel across the component being measured.

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7
Q

What does the gradient of a current- potential difference graph represent?

A

Rate of change of current with respect to voltage.
This is not the same as 1/ R

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8
Q

Does a graph of current against voltage with a line straight through the origin of a graph extending to infinity positively and negatively represent an ohmic resistor?

A

Yes. As the line has a constant gradient and passes through the origin.
This shows that voltage is directly proportional to current

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9
Q

Describe a graph of current against voltage, representing an appliance with higher resistance.

A
  • An appliance with a higher resistance will have a line through the origin extending to infinity positively and negatively, however, the line will have a smaller angle to the x-axis both positively and negatively
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10
Q

How does a graph of current against voltage of a filament lamp look like?(3)

A

-The line goes through the origin extending to infinity, however, there are y-asymptotes both negatively and positively
- As the current increases the resistance also increases
- A big increase in the voltage produces only a small increase in current

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11
Q

Why does the current increasing on a filament lamp cause an increase in the resistance?

A

-As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude.
- This impedes the movement of electrons through the lamp as they collide with the ions (resistance has increased)

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12
Q

What is a diode?

A

A diode is an appliance that only allows current to flow in one direction.
In a graph of current against voltage, the line has an asymptote extending towards the negative x-axis, passes through the origin and further extending to infinity in the positive area of the x-axis

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13
Q

Unless stated in a question, should you assume that voltmeters to have zero resistance or infinite resistance?

A
  • You should assume they have infinite resistance.
  • Current takes the path of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow through the appliance
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14
Q

Why should the ammeter be assumed to have zero resistance unless stated otherwise?

A

This assumption means that there would be 0 potential difference across the ammeter and no energy is lost across it; it does not affect the circuit.

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15
Q

What is a Light Dependent Resistor (LDR)?

A
  • A semiconductor that is sensitive to light.
  • As the light intensity increases, its resistance decreases
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16
Q

How does a thermistor work?

A

Similar to an LDR but, as the temperature increases, the resistance decreases ( This means thermistors have a negative temperature coefficient)

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17
Q

What is resistivity?

A

The resistance of a 1m cylinder with a cross sectional are of 1m^2.
Resistivity is an inherent property of a material.

P’ = RA/L
P’ = resistivity (ohm meters)
R = resistance (ohms)
A = cross-sectional area (m^2)
L = length (m)

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18
Q

Describe an experiment to determine the resistivity of a metal.

A
  1. Measure the diameter of the wire with a micrometer. Then calculate the cross-sectional area = Pi (d/2)^2
  2. Set up the circuit as shown.
  3. Vary the wire length and record the voltage and current for each length
  4. Use R = V/I to work out the resistance
  5. Plot a graph of resistance against wire length
  6. The gradient = resistivity/ cross-sectional area
  7. So resistivity = gradient x cross-sectional area
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19
Q

What is a superconductor?

A

A material that has a resistivity of zero at or below a critical temperature.
The critical temperature is an inherent property of the material

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20
Q

How do you find the total resistance in a series circuit?

A

R -total = R-1 + R-2 + R-3 +…
Add the individual resistances of each component

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21
Q

If 6 cells, each of voltage 5V, are arranged in parallel what is the voltage in the circuit?

A

5V, meaning that the voltage in a parallel circuit is the same as the potential difference

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22
Q

If 6 cells, each of voltage 5V, are arranged in series what voltage is provided to the circuit?

A

30V
(5V x 6)
meaning that the total voltage in the circuit is the product of the potential difference and the cells

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23
Q

How does the current vary between each component of a series circuit?

A

The current does not vary as it is the same through all of the components.

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24
Q

Is the current in parallel components the same?

A

No, each branch of a parallel circuit can have different currents through them according to Kirchhoff’s first law.

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25
Q

What is Kirchhoff’s first law?

A

All of the current going into a junction is equal to the current leaving the junction.

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26
Q

What is Kirchhoff’s second law?

A

For any path (loop) of a circuit, the sum of all of the potential differences must equal to the total e.m.f of the circuit.

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27
Q

In a series circuit, if two cells are connected negative to negative, would their e.m.f add up or cancel out?

A

They will cancel out. The total e.m.f would be equal to:

p.d-total = p.d-1 - p.d-2

28
Q

What is power?

A

The rate of energy transfer.

29
Q

Give an equation for power in terms of current and voltage.

A

Power = Current x Voltage

30
Q

What is the purpose of a potential divider?

A
  1. To provide variable potential difference, or
  2. To provide a constant specific potential difference
31
Q

A circuit is set up with a cell providing a voltage of 12V to 2 resistors of 6 ohms and 7 ohms respectively, in series. What is the voltage across the 7 ohm resistor?

A

Total Resistance = 6+7 = 13 ohms

(7/13) x 12 = 6.5V

32
Q

What is e.m.f?

A

Electromotive force: the electrical energy transferred by a power supply per unit charge.

33
Q

Rearrange the equation p.d = I (R+r) into the form y = mx + c

A

P.d = IR + Ir
P.d = V + Ir

so the graph of V against I
V = -rI + p.d
Gradient = -r and y- intercept = p.d

34
Q

What are the two applications of superconductors?

A
  1. Power cables, which would reduce energy loss through heating to zero during transmission.
  2. Strong magnetic fields, which would not require a constant power source, These could used in maglev trains, where there would be no friction between the train and rail, and in certain medical applications
35
Q

what is the total resistance of a parallel arrangement of 3 ohm, 7 ohm and 9 ohm resistors?

A

1/ R -total = 1/3 + 1/7 + 1/9
1/ R - total = 37/ 63
R - total = 63/ 37 = 1.7 ohm

36
Q

What is a force field?

A

A region in which a non- contact force is experienced by a corresponding interacting particle.

37
Q

What is an electric field?

A

A region in which a charged particle will experience a non-contact force.

38
Q

what is electric field strength?

A

The force per unit charge experienced by a charged particle when placed at that point in the field.

39
Q

State the equation used to calculate the force a charge experiences in an electric field.

A

F = EQ
F - Force
E- Electric field strength
Q - Charge

40
Q

What law determines the magnitude of the electric force between two charges?

A

Coulomb’s Law

41
Q

State Coulomb’s law in words.

A

The force between two charges is directly proportional to the product of their charges and inversely proportional to the square of their separation.

42
Q

Give Coulomb’s law in equation form.

A

F = k [Qq’ / r^{2

where k = 8.99 x 10^{19} [fix this please]

43
Q

Describe the electric field pattern around a positive point charge.

A

A radial field, acting outwards.

44
Q

Describe the electric field pattern around a negative point charge.

A

A radial field, acting inwards.

45
Q

Describe the uniform electric field between parallel plates.

A

https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3De8L3F6gnteE&psig=AOvVaw0C3_Ki_57cx2N1WN_WAqec&ust=1703325425123000&source=images&cd=vfe&opi=89978449&ved=0CBAQjRxqFwoTCICk8_LjooMDFQAAAAAdAAAAABAO

46
Q

What equation is used to calculate the electric field between parallel plates?

A

E = V / d

47
Q

What is electric potential?

A

The amount of energy required to move a positive test charge from infinity to a given point in an electric field

48
Q

How do you calculate electric potential for a radial field?

A

F = [kQ]/ r

49
Q

What are equipotentials?

A

Equipotential are lines along which the electric potential remains the same

50
Q

How do you calculate the work done in moving a charge along an equipotential?

A

No work is done when moving a charge along an equipotential since the electric potential does not change

51
Q

What is a magnetic field?

A

A magnetic field is a region in which a magnetic pole will experience a non-contact force

52
Q

In which direction do magnetic field lines point?

A

From North to South

53
Q

What is magnetic flux density?

A

Magnetic flux density is a measure of the strength of a magnetic field.
It can be viewed as the number of magnetic field lines that pass through a given area

54
Q

What three factors determine the force exerted on a charge moving through a magnetic field?

A
  1. The magnetic flux density
  2. The charge of the particle
  3. The velocity of the particle perpendicular to the field
55
Q

State the equation used to calculate the force exerted on a charge moving through a magnetic field.

A

F = Bqvsin (theta)

56
Q

What is the relationship between the direction of a charge’s motion and the direction of the magnetic force it experiences?

A

The force is always perpendicular to the charge’s motion

57
Q

Describe and explain the path taken by a charge in a magnetic field.

A

The charge will move in a circular path. This is because the magnetic force always acts perpendicular to the charge’s motion and so acts as a centripetal force

58
Q

What is produced by a current-carrying wire?

A

A magnetic field is produced in concentric circles around a wire when a current passes through it.

59
Q

What occurs when a current-carrying wire is placed in a magnetic field?

A

The wire will experience a force due to the permanent magnetic field interacting with the wire’s magnetic field.

60
Q

What three factors affect the force experienced by a current-carrying wire placed in a magnetic field?

A
  1. The magnetic flux density of the field
  2. The current passing through the wire
  3. The length of the wire
61
Q

State the equation used to calculate the force experienced by a current-carrying wire placed in a magnetic field.

A

F = BIL sin (theta)

62
Q

What is Fleming’s left-hand rule used for?

A

To determine the direction of the force experienced by a current-carrying wire or moving charge in a magnetic field.

63
Q

What does the thumb represent when using Fleming’s left-hand rule?

A

The thumb represents the direction of the force

64
Q

What does the first finger represent when using Fleming’s left-hand rule?

A

The direction of the field.

65
Q

What does the second finger represent when using Fleming’s left-hand rule for a moving charge?

A

The direction that a positive charge would move. This means that if it is a negative charge, you must point your second finger on the opposite direction to its motion.

66
Q

What does the second finger represent when using Fleming’s left- hand rule for a current-carrying wire?

A

The direction of conventional current flow.

PHYSICS (9 decks)

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