electricity

Define electric current.

The flow of charge per unit time (Q/T)

Define potential difference

The energy transferred per unit charge between two points in a circuit (W/Q)

Define resistance (R).

Measure of how difficult it is for charge carriers to pass through a component (V/I)

States ohms law

For an ohmic conductor, current is directly proportional to the potential difference across it, given that physical conditions (e.g. temp) are kept constant

Define ohmic conductor.

A component that follows ohms law (current is proportional to potential difference, provided physical conditions remain constant)

So graphs of component will be straight line through

If current and voltage are below zero on a graph, what does this mean?

The direction has been reversed

Explain the graph for a semiconductor diode (non ohmic conductor) component .

In the forward bias (V>0) there is high resistance until current is past the threshold voltage-0.7- (min voltage needed to allow current to to flow) where current increases rapidly for small increases in voltage- shown as a steep curved line

In the reverse bias (V>0) there is infinite resistance- graph remains flat on the negative x-axis, showing no significant current flows

What is the value of the threshold voltage

0.7V

Describe what happens to a filament lamp as this components metal wire is heats up

Component contains a length, which heats up as currently increases, therefore resistance increases as current increases (physical conditions were changed)

At low currents the metal wire will not heat up significantly so ohms law is obeyed

But as current increases in either direction, the graph curves due to increasing resistance

For V-I graphs which unit is the x-axis?

I= x axis

V= y axis

What is the resistance for any component on an I-V graph?

R= 1/gradient

Steeper line= higher gradient= lower resistance

Flatter line= lower gradient= higher resistance

What is the resistance for any component on a V-I graph>

Gradient=R (rise/run=V/I)

Steeper line= higher resistance

If line is horizontal on an I-V graph that means there is a lot of Push (V) but no result (I)= high R

Explain why an ideal voltmeter show have infinite resistance

To ensure that it does not draw any current from the circuit, which would change the potential difference across the component being measured

Explain why an ideal ammeter should have zero resistance

Because it would increase total resistance due to I=V/R therefore ammeter would measure a lower current

Explain a non ideal ammeter.

It has small resistance, so total circuit resistance increases and the current decreases- systematic error

Explain a non ideal voltmeter

It has high (but not infinite) resistance, some current flows through t, meaning measure p.d. Is lower than it should be (V=IR)

Define resistivity.

Measure of a materials opposition to the flow of electric current

Whats the difference between resistance and resitivity?

Resitance depends on the object shape

Whilst resistivity is a constant for the material

What are environmental factors of resistivity? (Resitivity can change)

Temperature- as temp increases resistivity increases

• conductors- more frequency collisions as temp increases- gain more kinetic energy. More collisions= more resitance

• Semiconductor (thermistor or diode)- as temp increases electrons are freed from their atoms. Increasing number of charge carriers available to flow.More charge carriers= low resitivity

What is an application of a thermistor

In circuits?

A temperature sensor - which triggers an event to occur once temp reaches a certain value

Define superconductor.

A material which, below critical temperature (certain temp), has zero resistivity

Critical temp depending on material its made out of

If resitivity drops to zero does that mean resistance always drops to zero?

Yes

What are some (2) applications of superconductors?

1. Power cables- reduces energy loss through heating to zero during transmission

2. Strong magnetic fields-

Define power (P)

Power is the energy Transferred over time (P=E/T)

In a series circuit: (2)

1. The current is the same everywhere in the circuit

2. The battery p.d. is shared across all elements in the circuit, therefore the total sum of voltages across all elements is equal o the supply p.d. (KIRCHOFF’S SECOND LAW)

3. Total resistance is the sum of all resistances, more resistance added to series circuit= current decreases (I=V/R)

If 2 identical cells are joined in parallel, what does this mean?

The cells are on different branches

In a parallel circuit (2)

1. The sum of the currents in each parallel set of branches is equal to the total current

2. The potential difference across each branch is the same

3. Total resistance decreases as you add more branches -because you are giving current more branches to travel through

Define a DC (direct current) circuit.

Current that flows in one direction only

Define a AC (alternating current) circuit

Current that constantly changes direction and magnitude

What is the UK mains frequency and (RMS) voltage

Frequency= 50Hz

(RMS) voltage= 230V

Describe the DC circuit on a current-time graph

A flat horizontal line

Describe the current- time graph of a AC circuit

A sine wave

Are charge and energy always conserved in a DC circuit?

Yes

Define Kirchoff’s first law.

The total current flowing into a junction is equal to the current flowing out of that junction- shows no charge is lost at any point in the circuit

Define Kirchoff’s second law.

The sum of all the voltages in a series circuit is equal to the batter voltage- shows no energy is lost at any point in a circuit (V=IR)

Define a potential divider

Is a circuit with several resistors in series connected across a voltage source, used to produce a required fraction of the source potential difference, which remains constant

Explain the potential divider.

A potential divider uses the relationship between resistors to "set" the voltage to the exact amount (the required fraction) that a specific component needs to function safely.

Is V out the same across all components, if so why?

Yes, Kirchoff’s second law

What is a thermistor?

As temperature, resistance decreases

(Opposite of metal wire)

How do thermistors work

1. Made of a semiconductor, as temp increases, the atoms of the semiconductor gain energy

2. This energy release more electrons from the lattice, increasing number of charge carriers available to flow

3. increase in available charge carriers (which transport charge=current) decreases resistance

Define light decent resistor (LDR).

As light energy/ intensity (Photons) increases, resistance decrease

How do LDR’s work?

1. It’s a semiconductor, when light shines on LDR, the material absorbs photons

2. This energy is transferred to the electrons in the material, releasing them from their atoms

3. Which increase amount if charge carriers available to flow Massive

4. With more charge carriers, the current increases for a given voltage , therefore resistance decreases

What is internal resistance (r) caused by?

Electrons colliding with atoms inside the battery therefore energy is lost before electrons even leave the battery

Define electromotive force (emf).

The energy transferred by a cell per coulomb of charge that passes through it

Define terminal p.d. (V)

The voltage that makes it out of the battery and is available to be used by rest of circuit

(V of components that are not the battery)

Define lost volts (v)

P.d. Across resistor, r (Internal resistance)- this value is equal to the energy wasted by the cell per coulomb of charge