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Physics > Topic 5 - Electric currents > Flashcards

Topic 5 - Electric currents Flashcards

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1
Q

What is electric potential difference?

A

The electric potential energy transferred as a unit charge moves between two points

Unit: V or JC–1

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2
Q

What is the change in potential energy when a charge moves between two points at different potentials?

A

W = Vq

or for an electron:

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3
Q

What is the electronvolt?

A

The kinetic energy that would be gained by an electron if it was accelerated by a potential difference of one volt

Unit: eV

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4
Q

What is the relationship between the energy transferred in joules and the energy transferred in electronvolts?

A

energy transferred (J) = energy transferred (eV) X 1.6 X 10–19

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5
Q

What is electric current?

A

Force per unit length between parallel current-carrying conductors

Unit: A or Cs–1

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6
Q

What is resistance?

A

The ratio of the potential difference across a conductor to the current through it

Unit: Ω

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7
Q

What is a resistor?

A

A component that has been made to have a certain resistance

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8
Q

Why does resistance exist?

A

Because in a material, any electrons through it have to pass atom or ions vibrating in their path

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9
Q

What is the relationship between the resistance of a metal wire and its length and cross-sectional area?

A
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10
Q

What is resistivity?

A

A specific resistivity value given for different materials

Symbol: ρ

Unit: Ωm (ohm metre)

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11
Q

How is resistance calculated with the help of resistivity, length and cross-sectional area?

A
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12
Q

What usually affects resistance and resistivity?

A

Temperature

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13
Q

What is Ohm’s law?

A

The current through a metallic conductor is proportioonal to the potential difference across it, if the temperature is constant

I ∝V

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14
Q

What is the difference between an ohmic and non-ohmic resistor?

A

If the current through an electrical component is proportional to the p.d. across it, it is described as being ohmic because it obeys Ohm’s law

All other components are non-ohmic

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15
Q

What are the expression for power dissipation in resistors?

A
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16
Q

What is electromotive force (emf)?

A

The total energy transferred in the source per unit charge passing through it (not really a force)

Symbol: ε

Unit: V

ε = IR + Ir = I(R + r)

r = internal resistance

17
Q

What is internal resistance (r)?

A

Cells and batteries and other sources of electrical energy are not perfect conductors of energy and have small internal resistance of their own

18
Q

What is the voltage and resistance in resistor in series?

A
19
Q

What is the current and resistance in resistors in parallel?

A
20
Q

When are resistors in series?

A

When they are connected one after another, so that the same current flows through them all

21
Q

When are resistors parallel?

A

When they are positioned side by side and the current divides equally to all of them

22
Q

What is an ideal ammeter?

A

One that has an infinitely low resistance so that it does not reduce the current that it is trying to measure

23
Q

What is an ideal voltmeter?

A

One that has an infinitely high resistance so that current does not flow through it.

24
Q

What is a potential divider?

A
  • For resistors in series, the potential difference will be shared in the same ratio as the resitances
  • When this kind of arrangement is used deliberatly for the control of p.d.s, it is called a potential divider
25
Q

What are three types of potential dividers?

A
  1. Light-dependent resistors (LDRs)
  2. Temperature dependent resistors (thermistors
  3. Strain gauges
26
Q

How does an LDR function and what is it used for?

A
  • The resistance decreases when light intensity increases
  • As a result, the voltage across it decreases too
  • Can be used to control an electronic switch which turns off lights when the light intensity rises to a certain level
27
Q

How does a thermistor function and what is it used for?

A
  • The resistance decreases when temperature increases
  • When temperature drops below a certain value, the circuit turns on or when it rises above a certain value it turns off
  • Used in thermostats
28
Q

How does a strain gauge function?

A
  • Resistance is affected when a change of shape occurs
  • Used to detect when a structure undergoes a small change of shape
  • When strained, the resistance increases a little

Physics (16 decks)

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