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110 Cards in this Set

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The __ the resistance, the __ the current.

Higher, lower

The __ the current, the __ the bulb in the circuit.

Higher, brighter

The __ the current, the __ a motor turns

Higher, faster

How do you calculate resistance?

Resistance = voltage / current

What does resistance of a wire depend on?

Its length

How does a variable resistor work?

By changing the length of wire in the circuit; the longer the wire the greater the resistance

What does the voltage current graph for a resistor look like?

A straight line passing through the origin. This shows that voltage is directly proportional to current.

What law does a resistor obey?

Ohms law

The __ the gradient of the voltage current graph, the __ the resistance

Steeper, higher

What does the voltage current graph for a bulb look like?

It is not a straight line. The gradient of the line increases as the current increases.

Does a bulb follow Ohms law?

No

When does the resistance of a filament bulb increase?

As the current increases

What happens when electrons collide with the atoms in the filament bulb?

It makes the atoms vibrate more. This leads to an increased number of collisions so the resistance increases and an increased temperature in the filament

What does the gradient of the voltage current graph look like for a fixed resistor?

The gradient of the graph is equal to resistance.

What does a voltage current graph look like for a filament lamp?

The gradient increases as resistance increases, so the resistance must be found using instantaneous values from the graph

What do some potential divider circuits have?

Fixed resistors, or one variable resistor

What is the output voltage in a potential divider circuit?

A fixed proportion of the supply voltage. The output voltage can be altered.

How do you workout output voltage?

Vout = R2 / R1 + R2 x V2

What happens when R2 is much larger than R1?

Vout =(squiqqly equals) Vin

What happens when R2 is much less than R1?

Vout =(squiggly equals) 0

When will some electrical components start working?

At a threshold voltage

What does using a variable resistor as one part of the potential divider allow?

The threshold voltage to be set

How can resistors be arranged?

In parallel, which will decrease the total resistance.

How do you work out total resistance?

1/Rt = 1/R1 + 1/R2 + 1/R3

What happens to the resistance of a light dependant resistor as the light intensity increases?

As light intensity increases, the resistance of an LDR decreases

How does temperature affect the resistance of a thermistor?

The resistance of a thermistor decreases as temperature increases.

When does the resistance of a resistor increase?

When temperature increases

What do street lights have?

LDRs in potential divider circuits

How does a streetlight work?

When it is dark, the resistance of the LDR is high, the output voltage is high and the street light switches on

What can a thermistor be used to switch on?

A heater when the temperature gets too low

What happens in a transistor?

A small base current can switch on a greater current through the collector

What is the formula for a transistor? (Or whatever this weird formula is for I dont know)

Ie = Ib + Ic

What can two transistors combine to make?

An AND gate

How do two transistors combine to make and AND gate? (3)

-A small input current will switch on the upper transistor


-A small input current will switch on the lower transistor


-A current at both points are needed to switch on the supply

What do transistor circuits always have?

A high value resistor in the base circuit to limit the current

How does an OR gate behave?

Like two switches in parallel. The output is high if x input or y input is high.

What do truth tables show?

How logic gates behave

What happens when there are several logic gates combined together?

Truth tables can be used to work out what happens in the system

Does it matter how many logic gates there are when a truth table is being worked out?

No

What are used in potential divider circuits to make the input to a logic gate high?

Switches, LDRs and thermistors

What happens is a variable resistor is used instead of a fixed resistor?

A threshold level can be set

How does a relay work? (3)

-When a current passes through the coil the iron armature is attracted


-The armature pivots and pushes an insulating bar against the central contact


-The central contact moves, opening the normally closed contacts and closing the normally open contacts

Current for logic gates is low but how can it switch on a larger current needed for motors and heaters?

By passing it through a relay

How is damage to logic gates avoided?

The relay isolates the logic circuit from the high current

What do you use the right hand grip rule for?

Finding the direction of the field around a wire

What is the field pattern due to a long coil of wire similar to?

That of a bar magnet

What happens when a wire is placed between the poles of a magnet?

The wire moves out of the gap when then current is switched on.

What happens in the current is reversed?

The wire moves in the opposite direction

What does flemings left hand rule allow you to do?

Predict the direction in which a motor turns

What happens when a current passes through a coil placed between the poles of a magnet?

There is force on either side of the coil

When does a motor spin faster? (3)

-Number of turns on the coil is increased


-The size of the current is increased


-The strength of the magnetic field is increased

What is the job of a commutator?

Make a motor continue to spin

When is the direction of the current in a commutator reversed?

Every half turn

Why is the direction of the current in a commutator reversed every half turn?

To make sure the force on the coil is always in the same direction

What do the magnets in practical magnets have?

Curved poles to give a radial magnetic field

What does a radial magnetic field increase?

The force on the coil in a commutator and keeps it constant as the motor turns

When happens when a wire is held stationary between the poles of a magnet?

There is no current in the wire

When a wire is held stationary between the poles of a magnet, how do you induce a current? (2)

-If the wire is moved upwards a voltage is induced in the wire and a current passes


-The same thing happens if the magnet is moved downwards




(It is the relative movement of wire and field that is important)

When a wire is held stationary between the poles of a magnet, what happens if you move the wire down or the magnet up?

The induced voltage is reversed and the current is in the opposite direction

What does reversing the direction of a magnetic field do?

Reverses the induced voltage and current direction

What is a voltage induced?

Whenever a magnetic field changes

What does the size of induced voltage depend on?

The rate at which the magnetic field changes

When is voltage low?

If the wire is moved slowly

When is voltage high?

If the wire is move quickly

What happens in a power station generator?

The magnetic field inside the coil is produced by electromagnets.The electromagnetic is made from coils of wire (rotor coils) which are turned by the turbine

How is an alternating current produced in a power station generator?

It is produced in the stator coils which surround the rotor coils

What does increasing the speed of rotation of the electromagnet in a power station generator do?

It increases both the current induced in the stator coils and the frequency of the voltage generated

What does increasing the number of turns of the electromagnet in a power station generator do?

Increases the magnetic field and therefore a larger voltage is induced in the stator coils

What do some ac generators have?

A rotating coil between the poles of a magnet. As the coils rotates, the direction of the current in the coil reverses every half turn

What do slip rings do?

They are connected to the ends of the coil to allow the coil to spin without winding the wire around itself

What do brushes do?

They are contacts that touch the slip rings and complete the circuit

What does a transformer consist of?

Two coils of wire wound onto an iron core

What is the input ac voltage connected to in a transformer?

The primary coil

Where is the output ac voltage obtained from in a transformer?

The secondary coil

What do step down transformers have more of?

Turns on the primary coil

What do step up transformers have more of?

Turns on the secondary coil

What are isolating transformers used for?

Safety. The water and the steam in the bathroom could lead to electrocution or damage to the house wiring system if an ordinary socket is used.

What does the changing current in a primary coil in a transformer do?

Produce a magnetic field in the core

What does the changing magnetic fields in the core in a transformer do?

Induces a changing voltage in the secondary coil.

What does an isolating transformer have?

The same number of turns on both primary and secondary coils

What safety features do isolating transformers have? (2)

-The mains supply is hidden


-The output terminals are not live so there is not danger of electrocution if you touch them with hands

What happens when a current passes through a wire?

The wire gets hot

Why does the national grid lose energy to the surroundings?

The transformer coils and the overhead power lines get hot

What is power?

A measure of how fast energy is transferred

What does the electrical power supplied to the primary coil of a transformer depend on?

The input voltage and current

What happens in the transformer is 100% efficient?

input power = output power

What happens if you increase voltage in a step up transformer?

The current decreases by the same factor. Assuming the transformer is 100% efficient.

What circuits do you use for diode characteristics?

the same as you would use for a resistor or bulb

What do most diodes need before they start to work?

A voltage of about 0.6v

When is a diode forward biased?

When a diode is connected so that a current passes

When is a diode reverse biased?

When a diode is connected so that no current passes

What does an ac supply to a single diode produce?

A half wave rectified output

When is a resistor always used?

In a series with a diode to protect the diode

What does a diode consist of?

A piece of n type and a piece of p type semi conductor joined together

What does an n type semiconductor have?

An excess of electrons

What does a p type semiconductor have?

A shortage of electrons, the gaps are called holes

What happens if the positive terminal of the supply voltage is connected to the n type semi conductor?

The space widens and no current passes

What happens if the positive terminal of the supply voltage is connected to the p type semi conductor?

The space narrows, eventually disappears and a current passes

How can four diodes be arranged?

To make a bridge circuit

How can you make a bridge circuit output smoother?

The addition of a large capacitor

What happens when a dc supply is connected to a capacitor?

The capacitor becomes charged. The voltage across the capacitor increases until it is equal to the supply voltage.

What happens when a capacitor is connected to a resistor?

The voltage decreases as the capacitor disacharges

What happens in a bridge circuit during the positive half cycle?

The current passes from A to B to the external circuit, then to D then C and back to the AC supply.

What happens during the negative half cycle of a bridge circuit?

The current passes from C to B to the external circuit then to D then A and back to the AC supply

When is the DC output positive and negative.

The DC output is always positive at B and negative at D

What does the time it takes for a capacitor to discharge depend on?

The resistance and the capacitor

What do the chemicals in a battery do?

Continue to produce energy for a battery to use

When is a battery is no longer useful?

When the chemicals are used up

Why is there no continual energy source in a capacitor?

The capacitor stores electrical energy

What does a smoothing capacitor do?

Acts as a reservoir. When the DC voltage from the rectifier circuit falls, the capacitor supplies current to the output. The capacitor charges near the peak value of the varying DC.