Electricity and Magnetism Flashcards
Magnetic field lines tell you what?
- The relative strength of the magnetic field (closer=stronger)
- The direction of force on a north pole of an object when placed at that position
Electrical field
A force field that surrounds a charged object (objects experience a force due to its charge)
What are the three advantages of an electromagnet over a normal magnet?
- It can be turned off and on when the current is turned off and on.
- The direction of the magnetic field can be reversed when the current is reversed.
- The strength of the magnetic field can be varied by changing the strength of current.
Objects with opposite charges
Attract each other
Resistance (R)
A measure of how easily current can flow through an object. Measured in ohms.
Electrical charge
Unit - q, measured in coulombs C
In a series circuit, current is the… around the circuit
The same
Why do more turns on a solenoid lead to a stronger magnetic field?
Each turn of the wire interacts to create a stronger magnetic field.
What two ways can a charged object be discharged?
- Electrical discharge to the air
- Earthing
Electromagnetism
When a current flows through a wire, a magnetic field is created as the electrons move.
Electron flow
Electrons flow from the negative end of a power supply to the positive end
Current (I)
The rate of flow of electrical charge. (How many electrons flow past a point per second). Measured in amperes (A)
Parallel circuit
Connected side by side
Objects with like charges
Repel each other
Magnetic field
A region where magnetic forces can be detected.
Circuit breaker
A safety device that is designed to flip a switch when a set current is reached. This breaks the circuit and prevents current from flowing
Conductor
An object with free electrons. Eg - metal
Power (P)
How much electrical energy is converted to other types of energy per second. Measured in joules per second (js-1)or watts (W)
In a parallel circuit, the current.. around the circuit
Splits up
Conventional Current
Electrons flow from the positive end to the negative end. Use when working out magnetic fields.
The strength of electrical field is based on what?
- The distance from the charged object
- How charged the charged object is
Voltage (V)
A measure of how much energy electrons lose or gain around a circuit. Measured in volts (V).
Electrons can be lost or gained in two ways:
Charged by contact, charged by friction
What are the two ends of a bar magnet?
South pole and north pole
Insulator
An object with no free electrons. Example - plastic
Static electricity
Charge that builds up on an object and remains until discharged.
Series circuit
Connected in a line
Fuse
A safety device. It is a piece of wire that is designed to heat up and blow when a set current is reached to prevent a high current. It breaks the circuit.
Magnetic field strength
Symbol - B, units tesla, T
What does the strength of a magnetic field depend on?
- The distance from the magnet
- The strength of the magnet
When electrons are transferred by friction, what charge do they gain?
Equal but opposite charge. This is called charge separation by friction.
Only insulators can become charged by friction. Why can’t conductors?
Conductors have free electrons which can move around an neutralise the charge.
How does a Van de Graaf generator (hair) work?
When the person touches the positively charged globe, electrons flow from the person to the globe. This is because opposite charges attract. The person and their hair gain a positive charge. As like charges repel, the strand of hair repels each other.
Why does hair stick up on a slide?
Hair sticks up on a slide as electrons are transferred by friction between the slide and the person. The person becomes charged and their hair gains the same charge. Like charges repel and so their hair is repelled.
How does a Van de Graaf generator (spark) work?
When an uncharged object is held near the positively charged generator, the electrons in the uncharged object are attracted to the generator. They move near the generator and jump across to the generator as a spark.
Why do charged objects attract uncharged objects?
The charged object attracts or repels the electrons in the uncharged object. This creates a temporarily oppositely charged region of the object, meaning they are attracted as opposites attract.
When an object is charged by contact, what charge does it gain?
The same charge as the charged object
Only conductors can be charged by contact. Why can’t insulators?
There are no free electrons to be transferred.
How are charges on a conductor distributed?
Evenly spread across the surface of the conductor due to electron movement, more concentrated at the corners.
What is electrical discharge to the air?
Objects slowly lose or gain electrons to the air
What is earthing?
The earth is an infinite store of charge. When a charged object is connected by a conductor to the Earth, electrons flow to or from the Earth to discharge the object.
What do field lines on a force field show?
- The direction of the force on a positive charge at that position.
- The strength (the closer the field lines, the stronger the force field)
The strength of an electrical field decreases following what law?
Inverse square law (2x distance, 1/4 strength)
One ampere is equal to what?
One coloumb
Voltmeters have to be connected in ___ to the object they are measuring
Parallel
Ammeters have to be connected in ___ to the object they are measuring
Series
Where do electrons lose/gain energy in a circuit?
Lose energy in components, gain energy in the power source
If resistance increases, what happens to the voltage and current?
Current decreases, voltage increases
Why does Earth have a magnetic field?
Movement of liquid iron in the core
Do you use conventional current or electron flow to work out magnetic field direction?
Conventional current
What would happen if a solenoid was given an iron core?
It would become an electromagnet. The strength of the magnetic field would increase.
What is the difference between hard and soft iron?
Hard iron remains magnetised after the current is turned off, soft iron loses magnetism.
What is the charge of an electron?
1.6 x 10 ^ 19 coloumbs
What is lightning? (in terms of earthing)
Lightning is an extreme form of earthing. Storm clouds are negative. This causes the negative electrons in the ground to be repelled and the surface of the ground to become positive. The negative region of the storm cloud is attracted to the ground as opposites attract and jumps across as a bolt of lightning.
What does a bulb’s brightness depend on?
A bulb’s brightness depends on its voltage.
Can magnetic fields be combined?
If two magnetic fields are close enough and in the same direction, they can be combined. If they are in the opposite direction, the magnetic fields minus off each other.
How does a speaker work?
A permanent magnet sits on an electromagnet. When current flows, the magnets attract or repel each other. The movement of the magnets vibrates the paper cone which creates sound waves.
How do magnetic fields generate electricty?
If a magnet is moved near a wire, the magnet exerts a force on the electrons and causes them to move and gain energy. This energy can be converted to electrical energy.
