# Electricity

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• Created by: LouiseG
• Created on: 03-05-16 11:26
What are charge carriers in a metal?
Delocalised electrons. Charged carriers are the charged particles that move through a substance when a pd is applied across it
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What is current?
Rate of flow of charge; I (A) = Q (C) / t (s)
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In a semiconductor; the number of charge carriers _(a)_ with an increase in temperature; thus the _(b)_ decreases. A pure semiconductor is thus an _(c)_ semiconductor as it conducts due to liberated electrons.
(a) increases (b) resistance (c) intrinsic
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How many electrons are in a columb of current?
6.25 x 10 ^ 18 (which is 1/1.6x10^-19 , where 1.6x10^-19 is the electron charge)
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What is work done per unit charge?
Potential difference / voltage
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What is emf?
The "ideal" source voltage; or the electrical energy produced per unit charge passing through the source (produced as opposed to delivered, which is the potential difference).
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Why is emf never really equal to the total voltage (Sum of potential drops around the circuit)?
Because the cell will often have an internal resistance and the wires connecting the circuit will have some resistance too; so there is "lost volts" which are converted to heat.
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Give the power equation.
P=IV
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How is resistance defined?
The pd across a component/the current through it
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Give Ohm's Law.
The pd across a metallic conductor is proportional to the current through it; provided physical conditions do not change (e.g. temperature).
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Resistance is the constant. So, what equation relates V, I and R?
V = IR (V is proportional to I, with a constant R)
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Hence what shape will a V/I graph have and what is the gradient?
A straight line that passes the origin, with a gradient of R or 1/R depending on which value is on which axes (V on y - gradient is R)
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What name do we give to conductors that obey this?
Ohmic conductors.
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R is proportional to a wire's (a) and inversely proportional to its (b)
(a) Length (b) Cross sectional area (Imagine a corridor- there is more "resistance" trying to get down a long, thin corridor full of people)
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Give the resistivity equation and work out its units.
p (resistivity) = R(resistance) x A(cross sectional area)/L(Length) ; Ohm-metres
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What is a superconductor?
A conductor with 0 ohms resistance at and below a critical temperature.
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Give two uses.
Generating strong EM fields in an MRI scanner; to accelerate particles at CERN
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What component has a lower resistance at high temperatures?
Thermistor
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What is an LDR?
A Light Dependent Resistor, which has a lower resistance in bright light
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Why does the I/V graph of a filament lamp curve off?
Because it has higher resistance at high temperature as more atom ions vibrate and collide with current electrons, thus providing higher resistance
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Why, when a bulb is first turned on, does the current spike and why is it most likely to blow as it is turned on?
In the first instant the bulb's resistance is very low as it has a low temperature, so a high current flows. This quickly raises the temperature and resistance; which can cause the bulb to overheat and blow
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At any junction in a circuit, the total current entering...
... is equal to the total current leaving the junction. (Conservation of charge)
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Thus, in series, all components have...
... the same current through them
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The potential difference across components in parallel is...
.. the same (no matter how many branches) as each electron entering each branch has the same amount of energy.
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For any complete loop in a circuit, the sum of the potential drops...
... equals the source emf(s). Thus a circuit with a parallel combination can be seen as two having separate "loops", thus explaining again why both have the same pd.
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Give the equation for the sum of resistors in series.
R tot = R1 + R2 + R3.....
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Give the equation for the sum of resistors in parallel.
1/Rtot = 1/R1 + 1/R2 + 1/R3 ...
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Combine V=IR and P=IV to find two new equations for power, with resistance
P= V^2 / R and P= I^2 x R
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Give the EMF equation.
E = I(R + r) ; where R is the total resistance in the ciruit and r is the internal resistance of the cells
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If you connect a voltmeter across a cell and alter the current using a variable resistor; on a V/I graph what will be (a) The y intercept (b) gradient
(a) EMF (maximum V with no resistance) (b) negative internal resistance. Compare y=mx +c to V= Emf - Ir
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Power delivered to load is maximum when the load resistance is equal to what?
The internal resistance.
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If n cells of emf e are connected in parallel, what is the emf supplied to the circuit?
e, as they all have current I/n and electrons can travel through one cell only.
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What do we assume the resistance is of a diode in reverse bias?
Infinate
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What is a potential divider?
Two or more resistors in series with each other with a fixed source of potential difference which is divided between the components
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The ratio of voltages is equal to what?
The ratio of resistances, so V2/V1 = R2/R1
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You want to design a circuit that has a light come on when it gets dark. Should Vout be across the LDR or fixed resistor?
The LDR. As it gets darker the resistance increases of the LDR so it has a larger potential difference across it
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## Other cards in this set

### Card 2

What is current?

#### Back

Rate of flow of charge; I (A) = Q (C) / t (s)

### Card 3

#### Front

In a semiconductor; the number of charge carriers _(a)_ with an increase in temperature; thus the _(b)_ decreases. A pure semiconductor is thus an _(c)_ semiconductor as it conducts due to liberated electrons.

### Card 4

#### Front

How many electrons are in a columb of current?

### Card 5

#### Front

What is work done per unit charge?

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