# Physics OCR, P5

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What happens when two objects are rubbed together?
they become charged because electrons are transferred from one object to the other
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What do objects with similar charges do?
they repel
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What do objects with opposite charges do?
they attract
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What charge to electrons have?
negative
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What is electric current?
the flow of charge
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What is electric current measured in?
amperes
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What do metal conductors in an electric circuit contain?
many charges that are free to move
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What happens when a circuit is made?
the battery causes the free charges to move and are not used up but flow in a continuous loop
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What is an electric current in metallic conductors?
a movement of free electrons that are present throughout such materials
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Why is an insulator different to a metal conductor?
in an insulator there are no charges free to move
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What causes a bigger current in a circuit?
the larger the voltage of the battery
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What do components in a circuit do? (e.g. resistors, lamps, motors)
resist the flow of charge through them
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What makes the current in a circuit smaller?
the larger the resistance
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Why can the resistance of connecting wires be ignored?
it is very small
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What happens when electrical charge flows through a component?
work is done by the power supply and energy is transferred from the power supply to the component and/or its surroundings
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What is power (watts, W)?
a measure of the rate at which an electrical power supply transfers energy to an appliance or device and/or its surroundings
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What is the equation for power?
power (W) = voltage (V) x current (A)
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When and why do resistors in a circuit get hotter?
when there is an electric current flowing through them and this is caused by collisions between the moving charges and stationary ions in the wire. This heating effect makes a lamp filament hot enough to glow
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What happens to the resistance of a LDR (light dependent resistor) when light intensity increases?
the resistance decreases and current flow increases
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What happens to the resistance of a thermistor when the temperature increases?
the resistance decreases and current flow increases
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What does the voltage across a battery do?
provides a measure of the 'push' of the battery on the charges in the circuit
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What is potential difference?
voltage
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What is the effect of potential difference and current when more batteries are added in a circuit series?
the potential difference and current increases
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What is the effect of potential difference and current when more batteries are added in a parallel circuit?
the total potential difference and current remain the same. Each battery supplies less current
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What happens when two or more components are connected in series to a battery?
current in each component is same, the p.d across the components add up to the p.d across the battery, the p.d largest across component with greatest resistance, change in resistance in one component results in change of p.d across all components
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What happens when two or more components are connected in parallel directly to a battery?
p.d across each component is equal to p.d across battery, current in each component is the same if only one component present, current from battery is sum of current in each parallel component, current largest in component with smallest resistance
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Why is the current largest through component with smallest resistance when components are connected in parallel to battery?
the same battery voltage causes a larger current to flow through a smaller resistance than through the bigger one
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How is mains electricity produced?
by generators
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How do generators produce a voltage?
by a process called electromagnetic induction
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What happens when a magnet is moving into a coil of wire?
a voltage is induced across the ends of the coil
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What happens when a magnet is moving out of the coil or the other pole of the magnet is moving into it?
there is a voltage induced in the opposite direction
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What happens if the ends of the coil are connected to make a closed circuit?
a current will flow round the circuit
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How can voltage in a neighbouring coil be induced?
when there is a changing magnetic field caused by the changes in the current in one coil of wire
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What are transformers used for?
to change the voltage of an alternating current
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What is the construction of a transformer?
primary and secondary coils of wire are wrapped around a soft iron core.
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What does a change in magnetic field in one coil caused by changes in the current do?
induce a voltage in the other coil
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What does the amount of voltage of a transformer depend on?
the number of turns on the primary and secondary coils.
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How is a voltage induced in a generator?
uses the principle of electromagnetic induction to generate electricity by rotating a magnet inside a coil
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How can the size of a voltage induced by a generator be increased?
by increasing the speed of rotation of the magnet or electromagnet, increasing the strength of its magnetic field, increasing the number of turns on the coil and placing an iron core inside the coil
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How does the induced voltage change across the generator during each rotation?
as the magnet rotates, the voltage induced in the coil changes direction and size. The current produced is an alternating current because it reverses its direction of flow every half turn of the magnet.
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When is a current a direct current?
when the current is always in the same direction
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What type of current is used in mains electricity? Why is it used?
an alternating current. An alternating current is used because it is easier to generate and is easier to distribute over long distances
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What can exert a force on a permanent magnet?
a current- carrying wire or coil
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What happens when a current-carrying wire is placed in a magnetic field whose lines of force are at right angles to the wire?
the current-carrying wire experiences a force at right angles to both the current direction and the lines of force of the field
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When does a current-carrying wire experience no force?
when the current-carrying wire is parallel to the lines of force of a magnetic field
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How can the motor effect result in a turning force?
when current flows through the col, it will cut the field lines in opposite directions on each side of the coil. This creates a pair of forces in opposite directions and cause the coil to rotate
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How can a commutator be used to produce continuous rotation of the coil?
a commutator is a rotary switch that turns with the coil, but the brushes remain fixed. It makes sure the current direction into the coil is switched.
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## Other cards in this set

### Card 2

#### Front

What do objects with similar charges do?

they repel

### Card 3

#### Front

What do objects with opposite charges do?

### Card 4

#### Front

What charge to electrons have?

### Card 5

#### Front

What is electric current?