A Level Physics: Electricity

What is Current?

The rate of flow of charge

I\=

Q/t

Which way does conventional current flow in a circuit?

Positive to negative

Which way do electrons flow around a circuit?

Negative to positive

What is the unit for charge?

Coulomb

What is a coulomb?

The amount of charge that passes in 1 second if the current is 1 ampere (A)

How do you attach an ammeter?

In series

Why do you attach an ammeter in series?

So the current through the ammeter is the same as the current through the component

What is potential difference?

The work done per unit charge moved

V\=

W/Q

How do you attach a voltmeter?

In parallel

Why do you attach a voltmeter this way?

Because p.d. across components in parallel is the same

What is a volt?

The potential difference needed for 1 joule of energy ro move 1 coulomb of charge through the component

What is resistance?

How much current you get for a particular p.d.

R\=

V/I

What is an Ohm?

The resistance of 1 volt if 1 amp flows through it

What is Ohm's law?

The current through an ohmic conductor is directly proportional to the p.d. across it

What conditions are necessary for Ohm's law to be true?

Physical conditions, such as temperature and light level, remain constant

What should you assume the resistance of a voltmeter is?

infinite

What should you assume the resistance of an ammeter is?

none

Describe the I/V graph for an ohmic conductor

A straight line through the origin

How do you find the resistance on an I/V graph?

V/I at that point

The resistance of an I/V with a steep gradient is ...

... low

The resistance of an V/I with a steep gradient is ...

... high

Describe the I/V graph for a filament lamp.

A curve that starts steep, but gets shallower as the voltage rises.

Describe the V/I graph for a filament lamp.

A curve that starts shallow, but gets steeper as the current and voltage increase

Why is the I/V graph for a filament lamp different to that of an ohmic conductor

The current through the lamp increases its temperature and as the temperature of a metal increases, so does the resistance

Why aren't semiconducters as good at conducting electricty as metals?

They have much fewer charge carriers available

Why can a semiconducter be used as a sensor?

If energy is supplied to it, more charge carriers are released so they can detect changes in their environment

What is a thermistor?

A resistor with a resistance that depends on its temperature

How are resistance and temperature related in a NTC thermistor

As temperature increases, resistance decreases

Describe the I/V graph for a NTC thermistor.

Starts shallow, but gets steeper as current increases. As current increases, so does the temperature, so potential difference and resistance decrease

Describe the V/I graph for a NTC thermistor.

Starts steep, but gets shallower as current increases

Warming the thermistor gives electrons ...

... enough energy to escape from their atoms, so the there will be more charge carriers and resistance will go down

In a diode, forward bias is...

... the direction in which current is allowed to flow

Most diodes require a threshold voltage of...

... 0.6V in the forward direction before they will conduct

In a diode, reverse bias resistance is...

... very high and the current is very small

What is this component?

Variable resistor

What is this component?

Thermistor

What is this component?

Diode

What is this component?

LED

What component is this I/V graph for?

Ohmic conductor

What component is this V/I graph for?

Ohmic conductor

What component is this I/V graph for?

Filament lamp

What component is this V/I graph for?

Filament lamp

What component is this I/V graph for?

NTC Thermistor

What component is this V/I graph for?

NTC Thermistor

What component is this I/V graph for?

Diode

What component is this V/I graph for?

Diode

The longer the wire...

... the more difficult to make a current flow, so the higher the resistance

The wider the area of the wire...

... the easier for a current to flow, so the lower the resistance

The higher the resistivity...

the higher the resistance

Resistivity depends on...

the material, temperature and light intensity

What is resistivity?

The resistance of a 1 m length with a 1 m^2 cross-sectional area

What is resistivity measured in?

Ωm

ρ\=

RA/l

How do you find the area of a wire?

Assume that the wire is cylindrical and use a micrometer to measure the diameter of the wire in 3 places and take an average. Half this and use the formula πr^2 to find the area.

How do you find the resistivity of a wire?

1. Find area 2. Clamp wire to ruler and connect to circuit where ruler reads 0 3. Attach flying lead 4. Record the L, V and I 5. Calculate resistance using R\=V/I 6. Repeat this and calculate an average resistance for the length 7. Repeat for multiple lengths 8. Plot R against L 9. Multiply the gradient by A

In an experiment to find resistivity, how can current cause random errors and invalid results?

The resistivity of a material depends on temperature which can increase with a flowing current

How can you keep the temperature, and therefore the resistivity, of a wire constant?

Only use small currents

Why do all materials waste some electrical energy?

All materials have some resistance, which means they heat up and some electrical energy is wasted as thermal energy

How can you lower the resistivity of a material?

Cooling it down

What is the transition temperature?

By cooling a material to this temperature, resistivity disappears and it becomes a superconductor

What is a superconductor?

A material with zero resistivity or resistance, so no electrical energy is wasted as heat

Using a magnetic field to start a current in a superconductor and then taking the magnet away will result in ...

... the current carrying on forever

What is the average transition temperature?

10 kelvin or -263 ºC

What are the main issues surrounding the creation of superconductors?

Getting materials down to their transition temperatures is very hard and expensive

Give 3 uses of superconductors.

1. Transmitting electricity without any power loss 2. Strong electromagnets that don't need a constant power source which can be used for medical applications and Maglev trains 3. Faster electric circuits

What is power?

The rate of transfer of energy

E \=

Pt

P \=

VI

What is a watt?

1 watt is 1 joule per second

What is the equation that links power, current and resistance?

P \= I^2R

What is the equation that links p.d., resistance and power

P \= V^2/R

Give 3 equations that link energy, p.d., current, resistance and time

E \= VIt \= tV^2/R \= I^2Rt

Where does resistance come from?

Electrons colliding with atoms and losing energy to other forms

What causes internal resistance?

Chemical energy in a battery causes electrons to move and they collide with atoms inside the battery where they lose energy.

What causes batteries and cells to warm up when they're used?

Internal resistance

What is load resistance?

The total resistance of all the components in the external circuit- excluding the internal resistance

What is electromotive force?

The electrical energy the battery produces for each coulomb of charge

What is terminal potential difference?

The energy transferred when one coulomb of charge flows through the load resistance

The terminal potential difference and would be the same as the e.m.f if ...

... there was no internal resistance

What is the energy wasted per coulomb overoming the internal resistance?

lost volts

Conservation of energy tells us that energy per coulomb supplied by the source \=

energy per coulomb transferrd in load resistance + energy per coulomb wasted in internal resistance

emf \=

E/Q

emf \=

V + v \= I(R + r)

(internal resistance formula) V \=

emf - v \= emf - Ir

The total emf of cells in series is...

... the sum total of their individual emfs

The total emf of identical cells in parallel is...

... the same as the emf of an individual cell

How do you find internal resistance?

"1. Change load resistance in order to vary I

2. Measure V for different values of I

3. Plot results on a graph

4. V \= -rI+emf so the gradient is -r"

What is the issue of using a voltmeter to measure the emf of a power source?

The voltmeter will have some internal resistance so the measured value will be slightly less than the real emf

What is Kirchoff's first law?

The total current entering a junction equals the total current leaving it

What is Kirchoff's second law?

The total emf around a series circuit equals the sum of the pds across each component

In a series circuit, the current is ...

... the same at all points

In a series circuit, the emf is ...

... split between components

In series, Rtotal \=

R1 + R2 + R3

In a parallel circuit, the current is ...

... split between components