Summary Physics ch 5: circuits
1 Charge and voltage
Charging objects
- If you rub a woollen cloth on a PVS pipe, the pipe will attract paper shreds.
- We can say that the PVC tube has become electrically or statically charged
- A charged object mostly loses its charge quickly. The more water vapour is in the air, the more
quickly the object discharges.
Positive and negative charges
- When you rub two object against each other, one becomes positively charged and one negative.
- Two positive or negative objects repel each other.
o Two objects of the opposite charge attract each other.
o One object has a positive charge and one a negative charge.
Electrons
- An uncharged object contains exactly the same amount of positive charge and negative charge.
o Such an object is neutral.
- You can give an object an electrical charge by rubbing them with a cloth.
- The negative particles will ‘jump’ from the cloth to the object and the other way around.
o These particles are called electrons.
- An object’s positive charge is also due to particles, these particles are
called protons.
o Protons can’t move, their position is solid.
The electrons are the one who move.
Discharging a voltage
- If two objects, who are opposite charged, are connected. The electrons from the
negative charged object will move towards the positively charged object. Making the
two objects neutral again.
- In the picture, there is a voltage between A and B.
- The current is passed really quickly.
- Two objects can also be discharged by sparks between them.
- These discharges are not dangerous because the current is really low.
, Charge and current
- When 1 amp flows through a wire, 6.2 x 10 18 electrons flow through a cross-section of that wire
each second.
- The unit of charge, the coulomb is based on the amp and the second.
- A current of 1 amp transports 1 coulomb of electrical charge in 1 second.
- The total charge that flows trough a wire depends on the current and time
Q=Ixt
- Q is the charge in coulombs (C)
- I is the current in Amps (A)
- t is the time in seconds (s)
2 Resistance
Calculating the resistance
- The ‘voltage across a wire’ means the voltage between the two ends of the
wire.
- Some wires need a high voltage to ‘force’ a small current through the wire.
o A wire like this has a high resistance
- There are some wires where a low voltage create a large current.
o These wires have a low resistance: it is easy for current to flow through
- You can define the resistance of a circuit component using the voltage (across
the component) and the current (through the component).
R = U/I
- R is the resistance in Ohms (Ω)
- U is the voltage in volts (V)
- I is the current in amps (A)
Ohm’s law
- If the wire is made out of a metal called constantan, the U and I is directly proportional:
o If the voltage doubles, the resistance doubles.
- This rule is called Ohm’s law
- When a wire’s resistance has a constant value, this is called an ohmic resistance.
o When you divide the voltage by the current, the resistance is always the same
1 Charge and voltage
Charging objects
- If you rub a woollen cloth on a PVS pipe, the pipe will attract paper shreds.
- We can say that the PVC tube has become electrically or statically charged
- A charged object mostly loses its charge quickly. The more water vapour is in the air, the more
quickly the object discharges.
Positive and negative charges
- When you rub two object against each other, one becomes positively charged and one negative.
- Two positive or negative objects repel each other.
o Two objects of the opposite charge attract each other.
o One object has a positive charge and one a negative charge.
Electrons
- An uncharged object contains exactly the same amount of positive charge and negative charge.
o Such an object is neutral.
- You can give an object an electrical charge by rubbing them with a cloth.
- The negative particles will ‘jump’ from the cloth to the object and the other way around.
o These particles are called electrons.
- An object’s positive charge is also due to particles, these particles are
called protons.
o Protons can’t move, their position is solid.
The electrons are the one who move.
Discharging a voltage
- If two objects, who are opposite charged, are connected. The electrons from the
negative charged object will move towards the positively charged object. Making the
two objects neutral again.
- In the picture, there is a voltage between A and B.
- The current is passed really quickly.
- Two objects can also be discharged by sparks between them.
- These discharges are not dangerous because the current is really low.
, Charge and current
- When 1 amp flows through a wire, 6.2 x 10 18 electrons flow through a cross-section of that wire
each second.
- The unit of charge, the coulomb is based on the amp and the second.
- A current of 1 amp transports 1 coulomb of electrical charge in 1 second.
- The total charge that flows trough a wire depends on the current and time
Q=Ixt
- Q is the charge in coulombs (C)
- I is the current in Amps (A)
- t is the time in seconds (s)
2 Resistance
Calculating the resistance
- The ‘voltage across a wire’ means the voltage between the two ends of the
wire.
- Some wires need a high voltage to ‘force’ a small current through the wire.
o A wire like this has a high resistance
- There are some wires where a low voltage create a large current.
o These wires have a low resistance: it is easy for current to flow through
- You can define the resistance of a circuit component using the voltage (across
the component) and the current (through the component).
R = U/I
- R is the resistance in Ohms (Ω)
- U is the voltage in volts (V)
- I is the current in amps (A)
Ohm’s law
- If the wire is made out of a metal called constantan, the U and I is directly proportional:
o If the voltage doubles, the resistance doubles.
- This rule is called Ohm’s law
- When a wire’s resistance has a constant value, this is called an ohmic resistance.
o When you divide the voltage by the current, the resistance is always the same
