Electric circuits

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What is static electricity and what is it caused by?
Static electricity is just electricity that cannot move and a build-up is caused by friction.
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What happens when two insulating materials are rubbed together?
The electrons are scraped off one and dumped on the other. Electrons are negatively charged and therefore leaves a positive static charge on one and a negative charge on the other. The classic examples are polythene and acetate rods.
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Explain how static electricity is responsible for bad hair days.
Static builds up on your hair, giving each strand the same charge - so they repel each other.
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Explain how static electricity is responsible for attracting dust.
Dust particles are easily attracted to anything that is charged. Many objects around the house are made of insulating materials that get easily charged and attract dust particles.
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Explain how static electricity is responsible for clinging clothes and crackles.
When synthetic clothes are dragged over each other, electrons get scrapped off, leaving static charges on both parts. As the charges rearrange, sparks are formed.
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What is voltage?
Voltage is the driving force that pushes the current around.
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What is current?
Current is the flow of charge through an electric circuit.
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What is resistance?
Resistance opposes the flow of current.
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What is power?
Power is the rate of energy transfer.
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What is potential difference?
Potential difference is another name for voltage. It tells us how much energy is transferred to or from each unit of charge as it moves between two points.
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What is there to say about voltage-current graphs?
Voltage- current graphs shows how the current in a circuit varies as you change the voltage. The current through a component is proportional to the voltage across it when the resistance stays constant. The steeper the slope the lower the resistance.
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What causes the filament in the lamp to get hot when current passes through it?
When electrons move through a resistor, they collide with stationary positive ions. This makes the ions vibrate more, which causes an increase in temperature.
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Why are filaments in lamps designed to have a very high resistance?
So that it glows - when current passes through, the temperature increases so much that it glows.
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Describe how the resistance of an LDR varies with light intensity.
In bright light, the resistance falls but in darkness, the resistance is highest. This makes them useful for automatic night lights.
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Describe how the resistance of a thermistor varies with temperature.
In hot conditions, the resistance drops but in cool conditions the resistance goes up. This makes them useful for thermostats.
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What happens to the potential difference in series circuits?
The potential difference is shared (V = V1 + V2)
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What happens to the current in a series circuit?
Current is the same everywhere. It is determined by the total P.D and the total resistance. (A1 = A2 = A3)
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What happens to the resistance of a series circuit?
The total resistance is the sum of the individual resistances. (R = R1 + R2 + R3)
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What happens to the cell voltages of a series circuit?
Cell voltages add up. If you connect several cells in series, you get a bigger total voltage - because each charge passes through all cells and gets a 'push' from each cell in turn.
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What happens to the cell current in series circuits?
Cell current doesn't add up. Adding cells in series does not increase the current. The maximum current in the circuit will just be the same as if you had one cell in the circuit.
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What happens to the cell voltages in parallel circuits?
Cell voltages do not add up like series circuits in parallel. Each charge only goes through one cell.
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What happens to the cell current in parallel circuits?
Cells connected in parallel increase the total current in the circuit. But the current through each cell is less than in the rest of the circuit, because they join together to make the total current.
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What happens to the P.D is parallel circuits?
P.D is the same across all components. The P.D of each component is equal to the P.D of the battery. (V1 = V2 = V3)
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What happens to the current in parallel circuits?
(A = A1 + A2) The current flowing from the battery is the same as the current flowing back to it - there is nowhere else for the charge to go.
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What happens to the resistance in a parallel circuit?
The total resistance is always less than the branch with the samllest resistance. The resistance is lower because the charge has more than one branch to take - only some of the charge will flow along each branch.
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What are transformers used to change and what do they look like?
Transformers are used to change the size of the voltage - they use electromagnetic induction to 'step up' or 'step down' the voltage. They have two coils of wire, the primary and the secondary coils, wound round an iron core.
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What does a step-up transformer do?
Step-up transformers increase the voltage. They have more turns on the secondary coil than on the primary coil.
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What does a step-down transformer do?
Step-down transformers decrease the voltage. They have more turns on the primary coil than the secondary.
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Why don't transformers use D.C; only A.C?
If you supplied a direct current to the primary coil, you'd get nothing out of the secondary coil at all. There would still be a magnetic field in the iron core but it wouldn't be constantly changing, so there'd be no induction in the secondary coil.
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What do you need to induce a voltage?
A changing field - this is why transformers only use AC.
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What is a magnetic field?
A magnetic field is a region where magnetic materials (like iron and steel) and also wires carrying currents experience a force acting on them.
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Describe what happens in a transformer.
The primary coil produces a magnetic field which stays within the iron core. Because there is an AC in the primary coil, the magnetic field in the iron core is also constantly changing direction. This induces a voltage in the secondary coil.
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Why is AC used for mains supply?
Because it is easier to generate and simpler to distribute over long distances.
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What is electromagnetic induction?
You can create a voltage, in a conductor by moving a magnet in or near a coil of wire. This is called electromagnetic induction.
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What happens in electromagnetic induction?
As you move the magnet, the magnetic field through the coil changes - this change induces a voltage across the ends of the coil. If the ends of the wire are connected then a current will flow in the wire.
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Describe how the way in which you move the magnet affects the direction of voltage.
If you move the magnet into the coil the voltage is induced in the opposite direction from when you move it out of the coil. If you reverse the magnetic's north-south polarity (S-N) the voltage is induced in the opposite direction.
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What is the motor effect?
If a rectangular coil of wire carrying a current is placed in a uniform magnetic field, the force will cause it to turn. This is called the motor effect.
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Why does the motor rotate?
Because the coil is on a spindle and the forces act one up and one down, it rotates. The split-ring commutator is a clever way of swapping the contacts every half turn. This reverses the current direction every half turn-keeps the coil rotating conti
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Give four examples of anything that uses an electric motor.
A DVD player, domestic appliances, electric cars and trains and they also spin platters.
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Other cards in this set

Card 2

Front

What happens when two insulating materials are rubbed together?

Back

The electrons are scraped off one and dumped on the other. Electrons are negatively charged and therefore leaves a positive static charge on one and a negative charge on the other. The classic examples are polythene and acetate rods.

Card 3

Front

Explain how static electricity is responsible for bad hair days.

Back

Preview of the front of card 3

Card 4

Front

Explain how static electricity is responsible for attracting dust.

Back

Preview of the front of card 4

Card 5

Front

Explain how static electricity is responsible for clinging clothes and crackles.

Back

Preview of the front of card 5
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