Section 5 - Electricity: 5.1 - Current 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 (amps).

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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?

A

How difficult it is for current to flow through an
appliance.

A component has a resistance of 1Ω if 1A 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 this graph represent an ohmic conductor?
(Insert graph)

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

Which graph represents an appliance with higher
resistance? (Insert graphs A and B)

A

B

A higher voltage is required in
B for the same change in
current in comparison to A.

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10
Q

What common appliance could this curve represent?
(Insert graph)

A

A filament lamp. As the
current increases the 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? What does the graph look like?

A

A diode is a appliance that only allows current to
flow in one direction.
(Insert graph)

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13
Q

Unless stated in the 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 you assume that an ammeter has 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 an 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).

17
Q

What is resistivity?

A

The resistance of a 1m cylinder with a cross
sectional area of 1m2

. Resistivity is an inherent

property of a material.

ρ = RA/L

ρ = resistivity (Ωm) R = resistance (Ω)
A = cross-sectional area (m2

) L = length (m)

18
Q

Describe an experiment to determine the resistivity
of a metal. Draw diagram.

A
  1. Measure the diameter of the wire with a micrometer.
    Then calculate the cross-sectional area = π(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
    (Insert diagram)
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.

20
Q

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

A

RTotal= R1+ R2 + R3 + …
Add the individual resistances of each component

21
Q

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

A

5V.

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)

23
Q

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

A

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

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.

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.

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 total emf
of the circuit.

27
Q

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

A

They will cancel out. The total emf would be equal to:
ε total= ε1 - ε2.

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
P = IV

30
Q

What is the purpose of a potential divider?

A

● To provide variable potential difference, or
● 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 6Ω and 7Ω respectively, in series. What is the voltage
across the 7Ω resistor?

A

Total resistance = 6 + 7 = 13Ω

(7/13) x 12 = 6.5V

32
Q

What is emf?

A

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

33
Q

Rearrange the equation Ɛ = I(R + r) into the form y = mx + c

A

Ɛ = IR + Ir
Ɛ = V + Ir

So for a graph of V against I
V = -rI + Ɛ
Gradient = -r and y-intercept = Ɛ

34
Q

What are two applications of superconductors?

A

● Power cables, which would reduce energy loss
through heating to zero during transmission.
● Strong magnetic fields, which would not require a
constant power source. These could be 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 3Ω,7Ω and 9Ω resistors?

A

1 / RT = 1⁄3 + 1/7 + 1/9
1 / RT = 37 / 63
RT = 63 / 37 = 1.7Ω