Download Electricity: Circuits, Current, and Charge - High School Physics and more Cheat Sheet Physics in PDF only on Docsity!
Circuits, Current and Charge
An electrical circuit is a closed loop that contains a power source such as a cell and something for the electrons to flow through like a wire. Component is referred to any parts of an electrical circuit. Components Symbols Function Switch (open) • Controls the flows of electricity.
- Can be closed (when open) allowing electricity to flow.
- Or open which will turn off the circuits. Switch (closed) Cell (^) • Provides electrical energy to a circuit. (Source of potential difference) Battery Diode (^) • Allows current to flow in one direction only. Voltmeter (metre) (^) • A voltmeter can be used to measure the potential difference produced by a cell or battery.
- (Added in parallel) Ammeter (metre) (^) • An Ammeter can be used to measure the current, or the flow of charge through the circuit, measured in amps.
- (Connected in series)
Bulb/filament lamp (^) • An electrical current heat the filament in a bulb so that it gives out light. Fuse (^) • A fuse melts to break the circuit if there is too much current. LED (light emitting diode) (^) • Emits light when current flows through (in one direction).
- In other word transform electrical energy directly into light energy. E.g., led alarm or bulb. Resistors (resistor) (^) • Can be fixed to providing certain numbers of ohms worth of resistance. Variable resistors (resistor) (^) • Used to varies or differs the size of current in a circuit
- Modify the amount of resistance Thermistor (resistor) (^) • A resistor whose resistance is dependent on temperature
- Its resistance increases when the temperature falls and decreases when the temperature rises. (Heating sensor) LDR (light depended resistor) (resistor)
- A resistor whose resistance is dependent on light intensity
- Resistance decreases as light level rises and increases when light level falls (street light) Electrons can move ( flow ) around circuits – this is important, because electricity is the movement of charged particles (such as electrons) Electrons are negative and so they move:
- from the positive terminal to the negative terminal Key point The charged particles don’t have to be electrons – but the particular charged particles that move through metal are always electrons.
What causes resistance?
- Sometimes the electrons collide with the ions in the lattice which slows them down.
- This causes resistance: the ions are resisting the flow of electrons through the metal.
- Resistance is the opposite of conductance – so a more conductive material has less resistance and vice versa.
- The copper wire has a lower resistance than the iron wire.
- If potential difference stays the same, then current will be smaller in the iron wire, because resistance is greater. Key point A conductor is a material that allows charge (usually electrons) to flow through. E.g., Copper. And Aluminium. The only non-metal conductor is graphite, not all metals are good conductor. A material that doesn’t conduct electricity is called an insulator (plastic). Key point The reason an ammeter can be placed anywhere in a series circuit is because current is the same everywhere (in a series circuit).
KEY INFORMATION
- When we attach a cell to the conductor the delocalised electrons will begin to move in one direction.
- 1 mA (milliamps) = 1 x 10-^3 A
Resistors and Ohms Law
Ohmic Law: Ohm's Law states that the current through a resistor is directly proportional to the voltage across it, as long as the resistance and temperature remain constant. Therefore, if resistance is constant, an increase in voltage will result in a proportional increase in current. We only get straight line in wires and resistors because in these circuits’ resistance stays constant. Battery is attached the opposite way so the voltage and current are negative. If the resistor in circuit is small then a small potential difference will be needed to drive a large current.
Non-Ohmic Conductors
Non ohmic conductors do not obey ohmic law such as Filament lamp, diodes, LDRs, and thermistors. In a filament lamp as the voltage increases the temperature increases causing the resistance to increase, which leads current to decrease, meaning that voltage and current are indirectly proportional. Diode (Do not obey ohmic law) In diode current and voltage are different in the forward direction and reverse direction. Diode is a component that allows current to flow in 1 direction which is why they only show current when the voltage is positive as they have high resistance in the reverse direction. (no current flows through that direction) At higher voltage resistance quickly drops and current begins to flow Key point Ohmic conductors obeys ohmic law Key point If there is less resistance than less voltage/potential difference is needed. Key point Length: the longer the wire – higher resistance Thickness: the thicker the wire – the less resistance Temperature: The higher the temperature the more resistance. Filament lamp are light bulbs which contains a very thin filament metal. As the current flows through the filament the wire heats up until it’s so hot that it commits light. (Filament lamp produces more heat than light making them less efficient).
Mains Electricity
Alternating Current and Direct Current
A direct current (d.c.) supply;
- Has a direct potential difference that is always positive or always negative making the current direction constant/same.
- It is supplied by cells and batteries An alternating current (a.c.) supply:
- Has an alternating potential difference that is one that alternates from positive to negative making the current direction alternate – Polarity of P.D. doesn’t change.
- Is the type of current used in mains electricity
- Mains electricity in the UK is 230V and changes direction 50 times a second that is it has a frequency of 50Hz.
- The mains supply uses three-core cable that i9s the cable contains 3 wires.
- Each wire carries a different electrical potential and is colour coded:
- Live wire is brown – 230V , alternates between positive and negative
- Neutral wire is blue – at or close to the 0V earth potential
- Earth wire is green and yellow – 0V potential. During operation:
- The voltage causes current to flow through the live and neutral wires
- The live wire carries the alternating potential from the supply
- The neutral wire completes the circuit carrying away the current from live wire
- Current will only flow in the earth wire if there is a fault connecting it to a non-zero potential
- The earth wire is a safety wire which stops the current of an appliance becoming live. `
Dangers of Mains Electricity
- Mains electricity can be very dangerous – an electric shock from a mains supply can easily be fatal.
- Touching the live wire can create a large potential difference across the body and result in a large current flowing through the body.
- The live wire can be dangerous even if a switch in the circuit is open.
- For example, a tv might be switched meaning that the current wont flow, but still plugged in and switched on at the wall:
- The live wire between the wall and the switch on the tv is still at an alternating potential.
- All it needs a path for the electricity to flow.
- This path could be provided by a damaged cable exposing the live wire.
- If someone then touches the live wire, creating a potential difference from the live to the earth and causing current to flow, they will get an electric shock. Fuse Circuit Braker
- Thin piece of wire connected to the live wire.
- If there’s a surge :
- Large current flows through live wire and fuse.
- Fuse wire heats up, melts, and breaks the circuit.
- Available in different ratings based on current needs:
- Ex: 5A fuse for appliances with a 3A current, only breaks above 5A.
- Use fuse slightly above appliance rating.
- Pros : Simple, cheap, common in appliances.
- Cons : Permanently breaks after a surge; needs replacing.
- Breaks circuit during surges, not permanently damaged.
- Trips off and can be reset, no replacement needed.
- Downside : More expensive than fuses. Earthing Double Insulation
- Prevents electric shocks from loose live wires touching metal casings.
- Earth wire provides alternate pathway, diverting current away from the user.
- Prevents shocks by covering the appliance in plastic (non-conductive).
- No exposed metal, so no electric shock risk as plastic don’t conduct electricity.
- Double-insulated appliances don’t need an earth wire; only have live and neutral wires. Key Point Any connection between live and earth wire can be dangerous, because it creates a potential difference, causing current to flow.
Energy Transfer and Power
- Whenever a charge flows, it has to overcome the resistance of the circuit. This requires energy, therefore:
- Work is done when charge flows
- The amount of work done depends on the amount of charge that flows and the potential difference.
- The amount of energy transferred can also be found from the power of the appliance and how long it is used for, e.g., a 20W bulb uses 20J of energy in every second. Energy transferred = power x time E = Pt Energy transferred = charge flow x potential difference E = QV
- Electrical appliances are designed to cause energy transfers.
- The type and amount of energy transferred between stores depends on the appliance. Key Point The energy transferred by an appliance depends on the power and the time it is on for.
The National Grid
- The National Grid is a system of cables and transformers linking power station to consumers in order to transfer electrical powers.
- Each component of the grid has a particular function.
- Power station:
- The power station transfers the energy supply into electrical energy.
- Using a smaller number of large power stations is more efficient than building many small, local power stations, because large stations can be made more efficient.
- This is because most power plants use steam turbines, which are more efficient at higher steam temperatures, and the bigger the plant, the bigger the boiler, so the higher the steam temperature. Set-up transformers:
- The transformers increase the potential difference from the power station to the transmission cables.
- This reduces the current and, therefore, reduces the heating effect caused by current flowing in the transmission cables.
- Reducing the heating affect reduces energy loss to the surrounding due to resistance, so makes the transmission more efficient. Transmission cables:
- Transmission cables transfer the electricity using huge pylons. Step-down transformers:
- The transformers reduce the potential difference form the transmissions cables to a much lower value for domestic use (safety).
Electric Field
Any charged objects has its own electric field around it Charged particles
- Example: One with positive charge and one with negative charge.
- Electric fields around particles are shown with field lines. Field Lines:
- Represent influence; drawn with arrows from positive to negative.
- Emerge from positive particles and enter negative particles.
- Must be perpendicular (at right angles) to the particle’s surface.
- In exams, draw several lines (e.g., 8) to show the field. Field Strength :
- Strongest near the particle ; weakens with distance.
- Closer particles have stronger interactions. Interactions :
- Opposite charges attract each other (electrostatic forces of attraction).
- Like charges (e.g., two positives or two negatives) repel each other Electric Fields and Air :
- Normally, air is an electrical insulator with no charge.
- Strong electric fields (e.g., near highly charged metal) can ionize air particles (removing electrons, creating positive ions).
- Ionization allows air to conduct electricity.
- This process enables sparks to travel through air. Explain why the reading on the balance increase if charged rod is held against a negatively charged rod (without touching it) over a balance? (3 marks)
- there is an additional (downwards) force on the balance (increasing the mass reading)
- (because) the (held) rod is negatively charged (and rods with) negative charges repel each other Explain why balance had a zero error (1)
- Only the change in reading or mass is being observed
